For list of authors, see Credits (Chapter 19).
This document is a specification by the PNG development group. It has been approved by a vote of the group. Future technical changes will require formal approval by a vote of the group. It is the intent of the group to maintain backward compatibility if possible. We will, however, correct any technical deficiencies discovered in the course of developing "beta" implementations.
Comments on this document can be sent to the MNG specification maintainers at one of the following addresses:
Distribution of this memo is unlimited.
At present, the latest version of this document is available on the World Wide Web from
ftp://swrinde.nde.swri.edu/pub/mng/documents/.
This document defines the MNG (Multiple-image Network Graphics) format. It also defines the MNG-LC (Low Complexity), MNG-VLC (Very Low Complexity), and JNG (JPEG Network Graphics) formats. These are proper subsets of MNG.
MNG is a multiple-image member of the PNG (Portable Network Graphics) format family. It can contain animations, slide shows, or complex still frames, comprised of multiple PNG or JNG single-image datastreams.
The MNG and JNG formats use the same chunk structure that is defined in the PNG specification, and they share other features of the PNG format. Any MNG decoder must be able to decode PNG and JNG datastreams.
The MNG format (but not MNG-LC or MNG-VLC) provides a mechanism for reusing image data without having to retransmit it. Multiple images can be composed into a "frame" and a group of images can be used as an animated "sprite" that moves from one location to another in subsequent frames. "Palette animations" are also possible. MNG can also store images in a highly compressible "Delta-PNG" format, defined herein.
A MNG frame normally contains a two-dimensional image or a two-dimensional layout of smaller images. It could also contain three-dimensional "voxel" data arranged as a series of two-dimensional planes (or tomographic slices), each plane being represented by a PNG or Delta-PNG datastream.
A Delta-PNG datastream defines an image in terms of a parent PNG or Delta-PNG image and the differences from that image. This provides a much more compact way of representing subsequent images than using a complete PNG datastream for each.
This document includes examples that demonstrate various capabilities of MNG. These include simple movies, composite frames, loops, fades, tiling, scrolling, storage of voxel data, and converting GIF animations to MNG format.
This specification defines the format of a MNG (Multiple-image Network Graphics) format. It also defines low-complexity and very-low-complexity versions (MNG-LC and MNG-VLC), and the JNG (JPEG Network Graphics) format, which are proper subsets of MNG.
Note: This specification depends on the PNG (Portable Network Graphics) [PNG] and the JPEG (Joint Photographic Experts Group) specifications. The PNG specification is available at the PNG web site,
http://www.libpng.org/pub/png/
MNG is a multiple-image member of the PNG format family that can contain
comprised of multiple PNG or JNG single-image datastreams.
Like PNG, a MNG datastream consists of an 8-byte signature, followed by a series of chunks. It begins with the MHDR chunk and ends with the MEND chunk. Each chunk consists of a 4-byte data length field, a 4-byte chunk type code (e.g., "MHDR"), data (unless the length is zero), and a CRC (cyclical redundancy check value).
A MNG datastream describes a sequence of zero or more single frames, each of which can be composed of zero or more embedded images or directives to show previously defined images.
The embedded images can be PNG, JNG, or Delta-PNG datastreams. MNG-LC and MNG-VLC datastreams do not contain JNG datastreams, but MNG-LC and MNG-VLC applications can be enhanced to recognize and process JNG datastreams as well.
A typical MNG datastream consists of:
MNG is fundamentally declarative; it describes the elements that go into an individual frame. It is up to the decoder to work out an efficient way of making the screen match the desired composition whenever a nonzero interframe delay occurs. Simple decoders can handle it as if it were procedural, compositing the images into the frame buffer in the order that they appear, but efficient decoders might do something different, as long as the final appearance of the frame is the same.
Images can be "concrete" or "abstract". The distinction allows decoders to use more efficient ways of manipulating images when it is not necessary to retain the image data in its original form or equivalent in order to show it properly on the target display system.
MNG is pronounced "Ming."
When a MNG datastream is stored in a file, it is recommended that ".mng" be used as the file suffix. In network applications, the Media Type "video/x-mng" can be used. Registration of the media type "video/mng" might be pursued at some future date.
The MNG datastream begins with an 8-byte signature containing
138 77 78 71 13 10 26 10 (decimal)
90 4d 4e 47 0d 0a 1a 0a (hexadecimal)
\212 M N G \r \n \032 \n (ASCII C notation)
which is similar to the PNG signature with "\212 M N G" instead of "\211 P N G" in bytes 0-3.
MNG does not yet accommodate sound or complex sequencing information, but these capabilities might be added at a later date, in a backward-compatible manner. These issues are being discussed in the mng-list@ccrc.wustl.edu mailing list.
Chunk structure (length, name, data, CRC) and the chunk-naming system are identical to those defined in the PNG specification. As in PNG, all integers that require more than one byte must be in network byte order.
The chunk copying rules for MNG employ the same mechanism as PNG, but with rules that are explained more fully (see below, Chapter 8). A MNG editor is not permitted to move unknown chunks across the SAVE and SEEK chunks, across any chunks that can cause images to be created or displayed, or into or out of a IHDR-IEND or similar sequence.
Note that decoders are not required to follow any decoding models described in this specification nor to follow the instructions in this specification, as long as they produce results identical to those that could be produced by a decoder that did use this model and did follow the instructions.
Each chunk of the MNG datastream or of any embedded object is an independent entity, i.e., no chunk is ever enclosed in the data segment of another chunk. MNG-compliant decoders are required to recognize and decode independent PNG or JNG datastreams.
Because the embedded objects making up a MNG are normally in PNG format, MNG shares the good features of PNG:
In addition:
See also the glossary in the PNG specification.
In MNG-VLC datastreams, each frame (except for the first, which also
includes the background layer) contains a single layer,
unless the framing rate (from the MHDR
ticks_per_second field) is zero. When the framing rate is zero,
the entire datastream describes a single frame.
When the layers of a frame do not cover the entire area defined by the width and height fields from the MHDR chunk, the layers are composited over the previous frame to obtain the new frame.
When the frame includes the background layer, and the background layer is transparent, the transparent background is composited against the outside world and the remaining layers are composited against the result to obtain the new frame.
Note that a layer can be completely empty if the image is entirely outside the clipping boundaries.
A layer can be thought of as a transparent rectangle with the same dimensions as the frame, with an image composited into it, or it can be thought of as a rectangle having the same dimensions (possibly zero) and location as those of the object after it has been located and clipped.
The layers in a MNG datastream are gathered into one or more subframes for convenience in applying frame parameters to a subset of the layers (see the definition of "subframe" below).
An embedded visible PNG or JNG datastream generates a
single layer,
even though it might be interlaced or progressive.
If the background
consists of both a background color and a background image, these
are combined into a single layer.
object_id is
an unsigned sixteen-bit number that serves as the identifier of a set of
object attributes. In MNG-LC
and MNG-VLC
only object 0 is permitted.
do_not_show flag to zero,
for
on-the-fly
display while the embedded image that defines it is being
cloned or
decoded.
do_not_show flag to zero.
An "object", which is identified by
an object_id, is an
image or it is a nonviewable entity that is created by the BASI
chunk. The object_id is an unsigned sixteen-bit number that
serves as the identifier of a set of object attributes.
An "image" is a viewable object.
Object 0 is a special object whose pixel data is not available for later use (see below).
An embedded object is:
Objects have object attributes that can be defined and modified by the contents of various MNG chunks. Decoders are responsible for keeping track of them. The simplest decoder might establish a 65,536-element array for each attribute, but real applications will undoubtedly use a more memory-efficient method. Object attributes include:
A nonzero object ceases to exist when it does not have the "frozen" attribute and
object_id replaces it
without an intervening DEFI chunk. In this case, the new object
inherits the set of object attributes from the previous object with the same
object_id.
Object 0 always exists.
do_not_show byte of the DEFI or CLON chunk
that introduced it. The "potential visibility" of viewable objects
can be changed by the SHOW chunk. When an embedded object is
"potentially visible," it can be displayed "on-the-fly"
as it is being
decoded. Later, the SHOW chunk can direct that a "potentially
visible" viewable object be displayed.
It is permitted to change the potential visibility of "frozen"
objects; if this is done, the potential visibility must be restored to
its "saved" condition by the encoder prior to the end of the
segment.
A nonviewable object becomes viewable immediately when the decoder receives a viewable object buffer or when an image delta makes it viewable, and if the object is potentially visible it can be displayed "on-the-fly" while the object buffer is being decoded or updated. Note that object 0 is only viewable while its embedded image is being decoded and displayed on-the-fly, after which it becomes nonviewable again because no object buffer is ever created for object 0.
An object buffer is created by the appearance of an embedded object
in the datastream, with a nonzero object_id, or by the
appearance of a CLON chunk that specifies a "full clone".
The contents of an object buffer can be modified by processing an image
delta or a PAST chunk.
Object buffers contain a 2D array of pixel data and can contain additional information. In addition, decoders are responsible for keeping track of some properties of the data in the object buffer:
Object 0 conceptually never has an object buffer. Decoding applications can create one for their own convenience, but such an object buffer must never be made available to the rest of the MNG datastream or be considered viewable after it has been processed.
When the "stored object buffers" flag (bit 9 of the simplicity
profile) is 0
and valid (i.e., bit 6 is 1 and bit 9 is 0), an object buffer need not be
created even when an embedded object with a nonzero object_id
appears, since the flag promises that the object buffer will never be used
again.
There is no requirement not to create an object buffer; no
harm will be done except for some unnecessary memory consumption.
When a partial clone is made of an object via the CLON chunk, the reference count for the object buffer is incremented, and no new object buffer is created.
When an object is discarded and it points to an object buffer that has a nonzero reference count, that reference count is decremented and the object buffer is also discarded if the resulting reference count is zero.
Object 0 is a special object that has a set of object attributes
that control its location, clipping, and visibility properties, and
also has a set of magnification factors and methods, but does not have
an object buffer. The object attributes and magnification data,
which can be modified by the DEFI, MOVE, CLIP,
and MAGN chunks, are applied to subsequent
embedded objects whose object_id is zero. The pixel data for
object 0 is available only for on-the-fly display and not available for later use.
If at the end of any segment the attribute values or magnification data
are different from the default/saved values, they become undefined when
a SEEK chunk appears.
This chapter describes chunks that can appear at the top level of a MNG datastream. Unless otherwise specified in the Delta-PNG chapter of this specification, they need not be recognized within a Delta-PNG datastream.
Chunk structure (length, name, data, CRC) and the chunk-naming system are identical to those defined in the PNG specification [PNG]. As in PNG, all integers that require more than one byte must be in network byte order.
Unlike PNG, fields can be omitted from some MNG chunks with a default value if omitted. This is permitted only when explicitly stated in the specification for the particular chunk. If a field is omitted, all the subsequent fields in the chunk must also be omitted and the chunk length must be shortened accordingly.
This section describes critical MNG control chunks that MNG-compliant decoders must recognize and process. "Processing" a chunk sometimes can consist of simply recognizing it and ignoring it. Some chunks have been declared to be critical only to prevent them from being relocated by MNG editors.
The MHDR chunk is always first in all MNG datastreams except for those that consist of a single PNG or JNG datastream with a PNG or JNG signature.
The MHDR chunk contains 28 bytes, none of which can be omitted:
Frame_width: 4 bytes (unsigned integer).
Frame_height: 4 bytes (unsigned integer).
Ticks_per_second: 4 bytes (unsigned integer).
Nominal_layer_count: 4 bytes (unsigned integer).
Nominal_frame_count: 4 bytes (unsigned integer).
Nominal_play_time: 4 bytes (unsigned integer).
Simplicity_profile: 4 bytes:(unsigned integer).
bit 0: Profile Validity
0: Absence of any features is unspecified.
All other bits of the simplicity profile
must be zero (i.e, all other even numbers
are invalid).
1: Absence of certain features is specified by
the remaining bits of the simplicity profile.
(must be 1 in MNG-LC and MNG-VLC
datastreams)
bit 1: Simple MNG features
0: Simple MNG features are absent.
1: Simple MNG features may be present.
(must be 0 in MNG-VLC datastreams)
bit 2: Complex MNG features
0: Complex MNG features are absent.
1: Complex MNG features may be present.
(must be 0 in MNG-LC and MNG-VLC datastreams)
bit 3: Internal transparency
0: Transparency is absent or can be ignored.
All images in the datastream are opaque or
can be rendered as opaque without affecting
the final appearance of any frame.
1: Transparency may be present.
bit 4: JNG
0: JNG and JDAA are absent.
1: JNG or JDAA may be present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 5: Delta-PNG
0: Delta-PNG is absent.
1: Delta-PNG may be present.
(must be 0 in MNG-LC and MNG-VLC datastreams)
bit 6: Validity flag for bits 7, 8, and 9
0: The absence of background transparency,
semitransparency, and stored object buffers
is unspecified; bits 7, 8, and 9 have no
meaning and must be 0.
1: The absence or possible presence of
background transparency is expressed by
bit 7, of semitransparency by bit 8, and of
stored object buffers by bit 9.
bit 7: Background transparency
0: Background transparency is absent (i.e., the
first layer fills the entire MNG frame with
opaque pixels).
1: Background transparency may be present.
bit 8: Semi-transparency
0: Semitransparency (i.e., an image with an
alpha channel that has values that are
neither 0 nor the maximum value) is absent.
1: Semitransparency may be present.
If bit 3 is zero this field has no meaning.
bit 9: Stored object buffers
0: Object buffers need not be stored.
1: Object buffers must be stored.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
If bit 2 is zero, this field has no meaning.
bits 10-15: Reserved bits
Reserved for public expansion. Must be zero in
this version.
bits 16-30: Private bits
Available for private or experimental expansion.
Undefined in this version and can be ignored.
bit 31: Reserved bit. Must be zero.
Decoders can ignore the "informative"
nominal_frame_count, nominal_layer_count,
nominal_play_time, and simplicity_profile fields.
The frame_width and frame_height fields
give the intended display size (measured in
pixels) and provide
default clipping boundaries
(see Recommendations for encoders, below).
It is strongly recommended that these be set to zero if
the MNG datastream contains no visible images.
The ticks_per_second field gives the
unit used by the FRAM chunk to specify interframe delay and timeout.
In MNG-VLC datastreams, it gives the framing rate.
It must be nonzero if the datastream contains a sequence of images.
When the datastream contains exactly one frame,
this field should be set to zero.
When this field is zero, the length of a tick is infinite, and
decoders will ignore any
attempt to define interframe delay, timeout, or any other variable that
depends on the length of a tick. If the frames are intended to be
displayed one at a time under user control, such as a slide show or
a multi-page FAX, the tick length can be set to any positive number
and a FRAM chunk can be used to set an infinite interframe
delay and a zero timeout. Unless the user intervenes, viewers will only
display the first frame in the datastream.
When ticks_per_second is nonzero,
and there is no other
information available about interframe delay,
viewers should display the sequence of frames
at the rate of one frame per tick.
If the frame count field contains a zero, the frame
count is unspecified. If it is nonzero, it contains the number
of frames that would be displayed, ignoring the
fPRI chunks and the
TERM chunk. If the frame count is greater
than (2^31)-1,
encoders should write (2^31)-1, representing an infinite
frame count.
In MNG-VLC datastreams, the frame count is the same as the number of
embedded images in the datastream (or one, the background layer, if there are
no embedded images).
If the nominal_layer_count field contains a zero, the layer
count is unspecified. If it is nonzero, it contains the number of
layers (including
all background layers)
in the datastream, ignoring any effects of the
fPRI chunks and the
TERM chunk.
If the layer count is greater than (2^31)-1, encoders
should
write (2^31)-1, representing an infinite layer count.
In MNG-VLC datastreams, the layer count is the number of embedded images,
plus one (for the background layer).
If the nominal_play_time field contains a zero, the
nominal play time is unspecified. Otherwise, it gives the play time,
in ticks, when the file is displayed ignoring the
fPRI chunks and the
TERM chunk.
Authors who write this field should choose a
value of ticks_per_second that will allow the nominal play time
to be expressed in a four-bit integer. If the nominal play time is greater
than (2^31)-1 ticks, encoders should write (2^31)-1,
representing an infinite nominal play time.
In MNG-VLC datastreams, the nominal play time is the same as the frame count,
except when the ticks_per_second field is zero, in which case the
nominal play time is also zero.
When bit 0 of the simplicity_profile field is zero, the
simplicity (or complexity) of the MNG datastream is unspecified,
and all bits of the simplicity profile must be zero.
The simplicity profile must be nonzero in
MNG-LC and MNG-VLC
datastreams.
If the simplicity profile is nonzero, it can be regarded as a 32-bit profile, with bit 0 (the least significant bit) being a "profile-validity" flag, bit 1 being a "simple MNG" flag, bit 2 being a "complex MNG" flag, bits 3, 7, and 8 being "transparency" flags, bit 4 being a "JNG" flag, bit 5 being a "Delta-PNG" flag, and bit 9 being a "stored object buffers" flag. Bit 6 is a "validity" flag for bits 7, 8, and 9, which were added at version 0.98 of this specification. These three flags mean nothing if bit 6 is zero.
If a bit is zero, the corresponding feature is guaranteed to be absent or if it is present there is no effect on the appearance of any frame if the feature is ignored. If a bit is one, the corresponding feature may be present in the MNG datastream.
Bits 10 through 15 of the simplicity profile are reserved for future MNG versions, and must be zero in this version.
Bits 16 through 30 are available for private test or experimental versions. The most significant bit (bit 31) must be zero.
When bit 1 is zero ("simple" MNG features are absent), the datastream does not contain the DEFI, FRAM, MAGN, or global PLTE and tRNS chunks, and filter method 64 is not used in any embedded PNG datastream.
When bit 2 is zero, the datastream does not contain any "complex MNG features". These are the BASI, CLON, DHDR/IEND, PAST, DISC, MOVE, CLIP, and SHOW chunks, or any chunk in a future version of this specification that defines or uses stored objects. If the DEFI chunk is present, it only defines object 0. If the BACK chunk is present, it does not define a background image. If the LOOP chunk is present, it has iteration_min=1. A MNG with a "complex MNG feature" (which has a simplicity profile that has bit 2 set to 1) may contain at least one of these chunks. A simple decoder can display "simple" MNGs (which have a simplicity profile with bit 2 set to 0) without having to store any objects or dealing with the SAVE/SEEK mechanism, and it can ignore the LOOP and ENDL chunks and execute all loops exactly once.
"Transparency is absent or can be ignored" means that either the MNG or PNG tRNS chunk is not present and no PNG or JNG image has an alpha channel, or if they are present they have no effect on the final appearance of any frame and can be ignored (e.g., if the only transparency in a MNG datastream appears in a thumbnail that is never displayed in a frame, or is in some pixels that are overlaid by opaque pixels before being displayed, the transparency bit should be set to zero).
"Semitransparency is absent" means that if the MNG or PNG tRNS chunk is present or if any PNG or JNG image has an alpha channel, they only contain the values 0 and the maximum (opaque) value. It also means that the JDAA chunk is not present. The "semitransparency" flag means nothing and must be 0 if bit 3 is 0 or bit 6 is 0.
"Background transparency is absent" means that the first layer of every segment fills the entire frame with opaque pixels, and that nothing following the first layer causes any frame to become transparent. Whatever is behind the first layer does not show through.
When "Background transparency" is present, the application is responsible for supplying a background color or image against which the MNG background layer is composited, and if the MNG is being displayed against a changing scene, the application should refresh the entire MNG frame against a new copy of the background layer whenever the application's background scene changes. The "background transparency" flag means nothing and must be 0 if bit 6 is 0. Note that bit 3 does not make any promises about background transparency.
The "stored object buffers" flag is only useful when bit 2 is nonzero (i.e., "complex MNG features" are present). This flag promises that even though such features are present, no chunk will ever use the information in an existing object buffer; therefore it is not necessary to store an object buffer for any object. A set of object attributes is necessary for each object, however. Therefore, the MOVE, CLIP, DISC, deterministic LOOP, partial CLON, and immediately-displayed BASI chunk are permissible. The "stored object buffers" flag means nothing if bit 2 is 0 or bit 6 is 0.
A MNG-LC (i.e., a "low-complexity MNG") datastream must have a simplicity profile with bit 0 equal to 1 and all other bits except possibly for bits 1, 3, 6, 7, and 8 ("simple MNG" MNG features and transparency) equal to zero. If bit 4 (JNG) is 1, the datastream is a "MNG-LC that might contain a JNG" datastream carrying an image or an alpha channel. MNG-LC decoders are allowed to reject such datastreams unless they have been enhanced with JNG capability.
A MNG-VLC (i.e., a "very low-complexity MNG") datastream must have a simplicity profile with bit 0 equal to 1 and all other bits except possibly for bits 3, 6, 7, and 8 (transparency) equal to 0. If bit 4 (JNG) is 1, the datastream is a "MNG-VLC with JNG" datastream. It might contain a JNG datastream carrying an image or an alpha channel. MNG-VLC decoders are allowed to reject such datastreams unless they have been enhanced with JNG capability.
Encoders that write a nonzero simplicity profile should endeavor to be accurate, so that decoders that process it will not unnecessarily reject datastreams or avoid possible optimizations. For example, the simplicity profile 351 (0x15f) indicates that JNG, critical transparency, semitransparency, and at least one "complex" MNG feature are all present, but Delta-PNG, stored object buffers, and background transparency are not. This example would not qualify as a MNG-LC or a MNG-VLC datastream because a "complex" MNG feature might be present. If the simplicity profile promises that certain features are absent, but they are actually present in the MNG datastream, the datastream is invalid.
The MEND chunk's data length is zero. It signifies the end of a MNG datastream.
The LOOP chunk provides a "shorthand" notation that can be used to avoid having to repeat identical chunks in a MNG datastream. The LOOP chunk can be ignored by MNG-LC and MNG-VLC decoders, along with the ENDL chunk. Its contents are the first two or more of the following fields. If any field is omitted, all remaining fields must also be omitted:
Nest_level: 1 byte (unsigned integer).
Iteration_count: 4 bytes (unsigned integer),
range [0..2^31-1].
Termination_condition:
1 byte (unsigned integer).
Must be omitted if termination_condition=0, which
means Deterministic, not cacheable, or if
iteration_count=0.
1: Decoder discretion, not cacheable.
2: User discretion, not cacheable.
3: External signal, not cacheable.
4: Deterministic, cacheable.
5: Decoder discretion, cacheable.
6: User discretion, cacheable.
7: External signal, cacheable.
Iteration_min: 4 bytes(unsigned integer). Must be present if
termination_condition is 3 or 7. If omitted, the
default value is 1.
Iteration_max: 4 bytes (unsigned integer). Must be present if
termination_condition is 3 or 7; must be omitted if
iteration_min is omitted; if omitted, the default
value is infinity.
Signal_number: 4 bytes (unsigned integer). Must be present if
termination_condition is 3 or 7. Must not be present
otherwise.
Additional
signal_number: 4 bytes. May be present only if termination_condition
is 3 or 7.
...etc...
Decoders must treat the chunks enclosed in a loop exactly as if they
had been repeatedly spelled out. Therefore, during the first iteration
of the loop, the parent objects for any Delta-PNG datastreams in the
loop are the images in existence prior to entering the LOOP
chunk, but in subsequent iterations these parent objects might have been
modified. The termination_condition field can be used to
inform decoders that it is safe to change the number of loop iterations.
Simple decoders can ignore all fields except for the
iteration_count.
When the LOOP chunk is present, an ENDL chunk
with the same nest_level must be present later in the MNG
datastream. Loops can be nested. Each inner loop must have a higher
value of nest_level than the loop that encloses it, though
not necessarily exactly one greater.
The termination condition specifies how the actual number of iterations is determined. It is very similar to the termination condition field of the FRAM chunk, and can take the same values:
termination_condition field
is omitted or has a value that is unrecognized by the decoder.
The loop terminates after exactly the number of iterations
specified by the iteration count. This value must be used if altering
the number of repetitions would mess up the MNG datastream, but can be
used merely to preserve the author's intent.
iteration_min nor more than
iteration_max. If the decoder has no reason to choose its
own value, it should use the iteration_count. One example of a
decoder wishing to choose its own value is a real-time streaming decoder
hovering at a loop while waiting for its input buffer to fill to a
comfortable level.
iteration_min and iteration_max limits. Some
decoders might not be able to interact with the user, and many decoders
will find that nested user-discretion loops present too great of a
user-interface challenge, so the <user-discretion> condition
will probably usually degenerate into the <decoder-discretion>
condition.
iteration_min nor more than iteration_max. The
exact number can be determined by the arrival of a signal whose number
matches one of the signal_number fields.
When the value of the termination_condition field is 4 or more, the
loop is guaranteed to be "cacheable", which means that every iteration
of the loop produces the same sequence of frames, and that all objects
and object buffers are left in the same condition at the end of each
iteration. Decoders can use this information to select a different
strategy for handling the loop, such as storing the composited frames
in a cache and replaying them rather than decoding them repeatedly.
The iteration_min and iteration_max
can be omitted. If the condition is <deterministic> the
values are not used. Otherwise,
defaults of 1 and <infinity> are used. The iteration_count,
iteration_min, and iteration_max can be any
non-negative integers or <infinity>, but they must satisfy
iteration_min <= iteration_count <= iteration_max.
Infinity is represented by 0x7fffffff. If all of the loops in
a MNG datastream have iteration_min=1, the datastream
can qualify as a "simple" MNG for the purpose of setting bits
1 and 2
of the "simplicity profile" to zero, unless there are other
reasons for setting them to one.
If iteration_count is zero,
the termination_condition, and the remaining fields must be
omitted, and the loop is done zero times.
Upon encountering a LOOP chunk whose iteration_count
is zero,
decoders simply skip chunks until the matching ENDL chunk is
found, and resume processing with the chunk immediately following it.
The signal_number can be omitted only if the termination
condition is not <external-signal>. There can be any number
of signal_number fields. Signal_number = 0 is
reserved to represent any input from a keyboard or pointing device,
and 1-255 are reserved to represent the corresponding character code,
received from a keyboard or simulated keyboard, and values 256-1023 are
reserved for future definition by this specification.
An infinite or just overly long loop could give the appearance of having locked up the machine. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by abandoning the MNG entirely or just proceeding to the next SEEK chunk (the SEEK chunk makes it safe for a viewer to resume processing after it has jumped out of the interior of a segment).
MNG editors that extract a series of PNG or JNG files from a MNG datastream
are expected to execute the loop only iteration_min times, when
the termination condition is not <deterministic>.
The ENDL chunk ends a loop that begins with the LOOP chunk. It contains a single one-byte field:
Nest_level: 1 byte (unsigned integer), range [0..255].
When the ENDL chunk is encountered, the loop iteration count is decremented, if it is not already zero. If the result is nonzero, processing resumes at the beginning of the loop. Otherwise processing resumes with the chunk immediately following the ENDL chunk.
When the ENDL chunk is present, a LOOP chunk with
the same nest_level must be present earlier in the MNG
datastream. See below.
Loops must be properly
nested: if a LOOP chunk with higher nest_level
appears inside a LOOP/ENDL pair, a matching ENDL chunk
must also appear to close it.
The SAVE and SEEK chunks are not permitted inside a LOOP-ENDL pair. To rerun an entire datastream that includes these chunks, use the TERM chunk instead. See below (Paragraph 4.2.11).
The chunks described in this section create objects and initialize their object attributes, or change their object attributes or the data in their object buffers. Some of them also may cause images to be immediately displayed.
The DEFI chunk sets the default set of object attributes
(object_id, do_not_show flag,
concrete_flag, location, and clipping boundaries) for
any subsequent images that are defined with IHDR-IEND,
BASI-IEND,
or JHDR-IEND datastreams.
If bit 1 of the MHDR simplicity profile is 0 and bit 0 is 1, the DEFI chunk must not be present.
The DEFI chunk contains 2, 3, 4, 12, or 28 bytes. If any field is omitted, all remaining fields must also be omitted.
Object_id: 2 bytes (unsigned integer) identifier to be given to the
objects that follow the DEFI chunk. This field must be
zero in MNG-LC files.
Do_not_show: 1 byte (unsigned integer)
0: Make the objects potentially visible.
1: Make the objects not potentially visible.
Concrete_flag: 1 byte (unsigned integer)
0: Make the objects "abstract" (image cannot be the
source for a Delta-PNG)
1: Make the objects "concrete" (object can be the
source for a Delta-PNG).
MNG-LC decoders can ignore this flag.
X_location: 4 bytes (signed integer).
The X_location and Y_location fields can be omitted as
a pair.
Y_location: 4 bytes (signed integer).
Left_cb: 4 bytes (signed integer). Left clipping boundary. The
left_cb, right_cb, top_cb, and bottom_cb fields can be
omitted as a group.
Right_cb: 4 bytes (signed integer).
Top_cb: 4 bytes (signed integer).
Bottom_cb: 4 bytes (signed integer).
If the object number for an object is nonzero, subsequent chunks can use this number to identify it.
When the object number for an object is zero, its object buffer
can be discarded immediately after it has been processed, and it can
be treated as an "abstract" image, regardless of the contents of the
concrete_flag field.
Negative values are permitted for the X and Y location and clipping boundaries. The left and top boundaries are inclusive, while the right and bottom boundaries are exclusive. The positive directions are downward and rightward from the frame origin (see Recommendations for encoders, below).
Multiple IHDR-IEND, JHDR-IEND, and
BASI-IEND objects can follow a single DEFI chunk.
When object_id is nonzero, the DEFI chunk values
remain in effect until another DEFI chunk or a SEEK
chunk appears, unless they are modified by SHOW, MOVE,
or CLIP chunks.
The object_id and concrete_flag
can only be changed by using another DEFI chunk. If no
DEFI chunk is in effect (either because there is none in
the datastream, or because a DISC or SEEK chunk has
caused it to be discarded),
the decoder must use the following default values:
Object_id = 0
Do_not_show = 0
Concrete_flag = 0
X location = 0
Y location = 0
Left_cb = 0
Right_cb = frame_width
Top_cb = 0
Bottom_cb = frame_height
The object attributes for all existing unfrozen objects except for object 0 become undefined when a SEEK chunk is encountered.
The object attributes for object 0 become undefined when a SEEK chunk is encountered, only if they have been reset to values other than these defaults. It is the encoder's responsibility to reset them explicitly to these values prior to the end of every segment in which they have been changed, or to include a full DEFI chunk prior to embedding object 0 in any segment.
These default values are also used to fill any fields that were omitted from
the DEFI chunk, when an object with the same object_id
has not been previously defined or a DISC or SEEK chunk has
caused it to be discarded.
An set of object attributes is created or an existing one is modified when
the DEFI chunk appears, but an object buffer is neither created
nor discarded.
If object_id is an identifier that already exists when
a DEFI chunk appears, the set of object attributes (except for
the pointer to the object buffer) is immediately replaced. The
contents of the object buffer do not change, however, until and unless
an IHDR, JHDR, BASI, or PAST chunk is
encountered. When one of these chunks appears, all of the contents
of the object buffer previously associated with the identifier are discarded
and the new data is stored in the object buffer.
Note that if the object has partial clones, the object buffer of the clones is naturally affected by the new data because it is shared, but the object attributes sets of the clones are not affected.
The PLTE chunk has the same format as a PNG PLTE chunk. It provides a global palette that is inherited by PNG datastreams that contain an empty PLTE chunk.
The tRNS chunk has the same format as a PNG tRNS chunk. It provides a global transparency array that is inherited along with the global palette by PNG datastreams that contain an empty PLTE chunk.
If a PNG datastream is present that does not contain an empty PLTE chunk, neither the global PLTE nor the global tRNS data is inherited by that datastream.
If the global PLTE chunk is not present, each indexed-color PNG in the datastream must supply its own PLTE (and tRNS, if it has transparency) chunks.
The global PLTE chunk is not permitted in MNG-VLC datastreams.
A PNG (Portable Network Graphics) datastream.
See the PNG specification [PNG] and the Extensions to the PNG Specification document [PNG-EXT] for the format of the PNG chunks.
The IHDR and IEND chunks and any chunks between them are written and decoded according to the PNG specification, except as extended in this section. These extensions do not apply to standalone PNG datastreams that have the PNG signature, but only to PNG datastreams that are embedded in a MNG datastream that begins with a MNG signature. Nor are they allowed in MNG-VLC datastreams.
64: Adaptive filtering with five basic types and intrapixel
differencing.
The intrapixel differencing transformation, which is a modification of a method previously used in the LOCO image format [LOCO], is
S0 = Red - Green (when color_type is 2 or 6) S1 = Green (when color_type is 2 or 6) S2 = Blue - Green (when color_type is 2 or 6) S3 = Alpha (when color_type is 6)
in which S0-S3 are the samples to be passed to the next stage of the filtering procedure.
The transformation is done in integer arithmetic in sufficient
precision to hold intermediate results, and the result is calculated
modulo 2^sample_depth.
Intrapixel differencing (subtracting the green
sample) is only done for color types 2 and 6, and only when the filter
method is 64. This filter method is not permitted in images with
color types other than 2 or 6.
Conceptually, the basic filtering is done after the intrapixel differencing transformation has been done for all pixels involved in the basic filter, although in practice the operations can be combined.
To recover the samples, the transformation is undone after undoing the basic filtering, by the inverse of the intrapixel differencing transformation, which inverse is
Red = S0 + S1 Green = S1 Blue = S2 + S1 Alpha = S3
As in the forward transformation, the inverse
transformation is done
in integer arithmetic in sufficient precision to hold intermediate
results and the result calculated modulo
2^sample_depth.
Applications that convert a MNG datastream to a series of PNG datastreams must convert any PNG datastream with the additional filter method 64 to a standard PNG datastream with a PNG filter method (currently 0 is the only valid filter method).
The extra filter method can also be used in PNG datastreams that is embedded in Delta-PNG and BASI datastreams.
It is suggested that encoders write a "nEED MNG-1.0" chunk if they use this feature, for the benefit of pre-MNG-1.0 decoders.
Applications must not write MNG-VLC datastreams or independent PNG datastreams (with either the .png or .mng file extension) with the new filter method, until and unless it should become officially approved for use in PNG datastreams.
Note that the top-level color space chunks are used only to supply missing color space information to subsequent embedded PNG or JNG datastreams. They do not have any effect on already-decoded objects.
If do_not_show is zero for the image when the IHDR
chunk is encountered, a viewer can choose to display the image while
it is being decoded, perhaps taking advantage of the PNG interlacing
method, or to display it after decoding is complete.
If object_id is zero, there is no need to store the
pixel data after decoding it and perhaps displaying it.
If concrete_flag=1 is 1 and object_id is nonzero,
the decoder must store the original pixel data losslessly, along
with data from other recognized PNG chunks, because it is possible
that a subsequent Delta-PNG datastream might want to modify it. If
concrete_flag is zero, the decoder can store the pixel data in any
form that it chooses.
If the "stored object buffers" flag in the simplicity profile is
valid and zero, there is no need to store the pixel data and other chunk
data after decoding and perhaps displaying the image.
If an object already exists with the same object_id, the
contents of its object buffer are replaced with the new data.
A JNG (JPEG Network Graphics) datastream.
See the JNG specification below (Chapter 5) for the format of the JNG datastream.
The JHDR and IEND chunks and any chunks between them are written and decoded according to the JNG specification.
The remaining discussion in the previous paragraph about PNG datastreams also applies to JNG datastreams.
MNG-LC and MNG-VLC applications are not expected to process JNG datastreams unless they have been enhanced with JNG capability.
The BASI chunk introduces a basis object that, while it might be incomplete, can serve as a parent object to which a delta image can be applied.
The first 13 bytes of the BASI chunk are identical to those of the IHDR chunk. The next 8 bytes, which can be omitted, provide sixteen-bit {red, green, blue, alpha} values that are used to fill the entire basis object when the IDAT chunk is not present, and a 1-byte "viewable" flag can also be present.
Width: 4 bytes (unsigned integer).
Height: 4 bytes (unsigned integer).
Sample_depth: 1 byte (unsigned integer) 1, 2, 4, 8, or 16.
Color_type: 1 byte (unsigned integer) 0: Gray, 2: RGB, 3: indexed
color, 4: Gray-alpha, 6: RGBA
Compression_method: 1 byte (unsigned integer).
0: zlib with deflate
Filter_method: 1 byte (unsigned integer).
0: five basic filter types.
64: intrapixel differencing and five basic filter
types.
Interlace_method: 1 byte (unsigned integer).
0: none, 1: Adam7
Red_sample or
gray_sample: 2 bytes (unsigned integer).
Green_sample: 2 bytes (unsigned integer).
Blue_sample: 2 bytes (unsigned integer).
Alpha_sample: 2 bytes (unsigned integer).
Viewable: 1 byte (unsigned integer).
0: Basis object is not viewable.
1: Basis object is viewable.
The sample depth, color type, compression method, and interlace method must be valid PNG types, and the width and height must be within the valid range for PNG datastreams. The filter method must be one of the filter methods allowed in PNG datastreams (currently only 0) or the additional filter method (64) allowed in PNG datastreams that are embedded in MNG datastreams.
The alpha_sample can be omitted if the viewable
field is also omitted. If so, and the color_type is one
that requires alpha, the alpha value corresponding to an opaque
pixel will be used. If the color samples are omitted, zeroes will
be used. If the viewable field is omitted, the object is not
viewable.
The decoder is responsible for converting the color and
alpha samples to the appropriate format and sample depth for the
specified color_type.
The color and alpha samples are written as four sixteen-bit samples
regardless of the color_type and sample_depth.
When the sample_depth is less than sixteen, the least
significant bits are used and the remaining bits must be zero
filled.
When color_type is 0 or 4, the green and blue
samples must be present but must be ignored by decoders.
When color_type is 0 or 2,
only the values 0 and 2^sample_depth should be written.
Any other alpha value must be interpreted as fully opaque.
When color_type is 3, the
decoder must generate a palette of length 2^sample_depth,
whose first entry contains the given {red_sample, green_sample,
blue_sample} triple, and whose remaining entries are filled with
zeroes. It must also generate an alpha array whose first entry is the
given alpha sample and the rest are opaque
(i.e., if the alpha sample is not opaque, it creates a one-entry tRNS
chunk containing the least significant byte of the given alpha sample).
The BASI datastream contains PNG chunks, but is not necessarily a PNG datastream. It can be incomplete or empty and it can deviate in certain ways from the PNG specification. It can serve as a parent object for a Delta-PNG datastream, which must supply the missing data or correct the other deviations before the image is displayed. The end of the datastream is denoted by an IEND chunk.
The permitted deviations from the PNG format in a BASI datastream are:
color_type is 3. If so, the subsequent Delta-PNG that uses this
as the parent object can supply a complete replacement PLTE chunk,
if the single-entry palette that is generated is not desired.
This deviation is only permitted when the object is concrete and not viewable.
The BASI chunk can be used to introduce such things as a library of iCCP chunks from which one or another can be selected for use with any single image, or it can be used to introduce a simple blank or colored rectangle that will be immediately displayed or into which other images will be pasted by means of the PAST chunk.
A BASI chunk appearing in a MNG datastream receives its
object_id, location, and potential visibility from the
preceding DEFI chunk, if one is present, or the default
values for DEFI, if one is not present. The
concrete_flag can be either 0 (abstract) or 1 (concrete),
depending on whether the basis image is intended for subsequent use by
a Delta-PNG datastream or not. When it is abstract, it must also be
viewable. When it is viewable, the
resulting object, after the pixel samples are filled in, must be identical
to an object that would have been obtained by decoding a legal
PNG datastream. If viewable is 1 and
do_not_show is 0, a viewer is expected to display
it immediately, as if it were decoding a PNG datastream.
If an object already exists with the same object_id, the
contents of its object buffer are replaced with the new data.
Top-level gAMA, sRGB, cHRM, bKGD, sBIT, pHYs, iCCP, and sPLT chunks are inherited by a BASI datastream in the same manner as by a PNG datastream.
No provision is made in this specification for storing a BASI datastream as a standalone file. A BASI datastream will normally be found as a component of a MNG datastream. Applications that need to store a BASI datastream separately should use a different file signature and filename extension. Better, they can wrap it in a MNG datastream consisting of the MNG signature, the MHDR chunk, the BASI datastream, and the MEND chunk.
Create a clone (a new copy) of an image, with a new
object_id. The CLON chunk contains 4, 5, 6, 7, or
16 bytes. If a field is omitted, all remaining fields must also be
omitted.
Source_id: 2 bytes (nonzero unsigned integer). Identifier of the
parent object to be cloned.
Clone_id: 2 bytes (nonzero unsigned integer). Identifier of the child
object that is created.
Clone_type: 1 byte (unsigned integer).
0: Full clone of the set of object attributes and the
object buffer.
1: Partial clone; only set of object attributes (the
location, clipping boundaries, and potential
visibility) are copied and a link is made to the
object buffer.
2: Renumber object (this is equivalent to
"CLON source_id clone_id 1
DISC source_id").
If this field is omitted, the clone_type defaults to zero
(full clone).
Do_not_show: 1 byte (unsigned integer).
0: Make the clone potentially visible and display it
immediately.
1: Make the clone not potentially visible.
When this field is omitted, the object retains the
potential visibility of the parent object.
Concrete_flag:
1 byte (unsigned integer).
0: Concrete_flag is the same as that of the parent
object.
1: Make the clone "abstract" (concrete_flag=0).
When this field is omitted, the object retains the
concrete flag of the parent object.
Loca_delta_type:
1 byte (unsigned integer)
0: Location data gives X_location and Y_location
directly.
1: New positions are determined by adding the location
data to the position of the parent object.
This field, together with the X_location and Y_location
fields, can be omitted as a group. When they are omitted,
the clone has the same location as the parent object.
X_location or delta_X_location:
4 bytes (signed integer).
Y_location or delta_Y_location:
4 bytes (signed integer).
The source_id must be an existing object identifier, and
the clone_id must not be an existing object identifier.
Negative values are permitted for the X and Y position. The positive directions are downward and rightward from the frame origin.
The clone is initially identical to the parent object except for the
location and potential visibility. It has the same clipping boundaries
as the parent object. Subsequent DHDR, SHOW,
CLON, CLIP, MOVE, PAST, and
DISC chunks can use the clone_id to identify it. If
the parent object is not a viewable image, neither is the clone.
Subsequent chunks can modify, show, or discard a full clone or modify its potential visibility, location and clipping boundaries without affecting the parent object. They can also modify, show, or discard the parent object or modify its set of object attributes without affecting the clone.
The concrete_flag byte must be zero or omitted when the
clone_type byte is nonzero.
If an object has partial clones, and the data in the object buffer of a parent object or any of its partial clones is modified, the parent object and all of its partial clones are changed. Decoders must take care that when the parent object or any partial clone is discarded, the object buffer is not discarded until the last remaining one of them is discarded. Only the location, potential visibility, and clipping boundaries can be changed independently for each partial clone.
If viewable is 1 and
do_not_show is 0, the resulting image is displayed immediately.
A Delta-PNG datastream.
See The Delta-PNG Format (Chapter 6),
below, for the
format of the Delta-PNG datastream. Any chunks between DHDR
and IEND are written and decoded according to the Delta-PNG
format. The object_id of the Delta-PNG DHDR
chunk must point to an existing parent object. The resulting image
is immediately displayed if its do_not_show is 0. The parent
object must be concrete (i.e., concrete_flag must be 1).
Paste an image or images identified by source_id,
or part of it, into an existing abstract image identified by
destination_id.
The PAST chunk contains a 2-byte destination_id
and 9 bytes giving a "target location", plus one or more 30-byte source
data sequences.
Destination_id: 2 bytes (unsigned integer).
Target_delta_type:
1 byte (unsigned integer).
0: Target_x and target_y are given directly.
1: Target_x and target_y are deltas from their
previous values in a PAST chunk with the same
destination_id.
2: Target_x and target_y are deltas from their
previous values in the previous PAST chunk
regardless of its destination_id.
Target_x: 4 bytes (signed integer), measured rightward from the
left edge of the destination image.
Target_y: 4 bytes (signed integer), measured downward from the
top edge of the destination image.
Source_id: 2 bytes (unsigned nonzero integer). An image to be
pasted in.
Composition_mode:
1 byte (unsigned integer).
0: Composite over.
1: Replace.
2: Composite under.
Orientation: 1 byte (unsigned integer).
The source image is flipped to another orientation.
0: Same as source image.
2: Flipped left-right, then up-down.
4: Flipped left-right.
6: Flipped up-down.
8: Tiled with source image. The upper left corner of
the assembly is positioned according to the
prescribed offsets.
Offset_origin: 1 byte (unsigned integer).
0: Offsets are measured from the {0,0} pixel in the
destination image.
1: Offsets are measured from the {target_x,target_y}
pixel in the destination image.
X_offset: 4 bytes (signed integer).
Y_offset: 4 bytes (signed integer).
Boundary_origin: 1 byte (unsigned integer).
0: PAST clipping boundaries are measured from the
{0,0} pixel in the destination image.
1: PAST clipping boundaries are measured from the
{target_x,target_y} pixel in the destination image.
Left_past_cb: 4 bytes (signed integer).
Right_past_cb: 4 bytes (signed integer).
Top_past_cb: 4 bytes (signed integer).
Bottom_past_cb: 4 bytes (signed integer).
...etc...
The destination image must have the "abstract" property
(concrete_flag=0). When destination_id=0, the
resulting image is "write-only" and therefore
only "composite-over"
(composition_mode=0) operations are permitted.
The source images can be "abstract" or "concrete"
and have any
color_type and sample_depth. They must
have the "viewable" property. The number of source images is
((chunk_length-11)/30).
The x_offset and y_offset distances and the
PAST clipping boundaries are measured, in pixels, positive
rightward and
downward from either the {0,0} pixel of the destination image
or the {target_x, target_y} position in the destination
image. They do not necessarily have to fall within the destination
image. Only those pixels of the source image that fall within the
destination image and also within the specified clipping boundaries
will be copied into the destination image. The coordinate
system for offsets and clipping is with respect to the upper lefthand
corner of the destination image, which is not necessarily the same
coordinate system used by the DEFI, MOVE and CLIP
chunks.
If the source image has been flipped or rotated, X_offset and
Y_offset give the location of its new upper left hand corner.
When it is tiled, the offsets give the location of the upper left hand
corner of the upper left tile, and tiling is done to the right and down.
The PAST left and top clipping boundaries are inclusive, while the
right and bottom clipping boundaries are exclusive
(see Recommendations for encoders, below).
When composition_mode=0, any non-opaque pixels in the
source image are combined with those of the destination image. If
the destination pixel is also non-opaque, the resulting pixel will be
non-opaque.
When composition_mode=1, all pixels simply replace those
in the destination image. This mode can be used to make a transparent
hole in an opaque image.
When composition_mode=2, any non-opaque pixels in the
destination image are combined with those of the source image. If the
source pixel is also non-opaque, the resulting pixel will be non-opaque.
The order of composition is the same as the order that the
source_ids appear in the list (but a decoder can do the
composition in any order it pleases, or all at once, provided that the
resulting destination image is the same as if it had actually performed
each composition in the specified order). Decoders must be careful when
the destination image equals the source image--the pixels to be drawn
are the ones that existed before the drawing operation began.
The clipping information from the DEFI, MOVE
or CLIP chunks associated with the
destination_id and the source_ids is not used in
the PAST operation (but if a decoder is simultaneously updating
and displaying the destination_id, the clipping boundaries
for the destination_id are used in the display
operation).
This chunk provides mandatory magnification factors for existing objects and/or for subsequent embedded images whose object id is 0.
The chunk contains 0 to 18 bytes. If any field is omitted, all remaining fields must also be omitted.
First_magnified_object_id:
2 bytes. If omitted, any previous MAGN chunk is
nullified.
Last_magnified_object_id:
2 bytes. If omitted, last object_id = first object_id.
X_method: 1 byte
0 or omitted: No magnification
1: Pixel replication of color and alpha samples.
2: Magnified intervals with linear interpolation of
color and alpha samples.
3: Magnified intervals with replication of color and
alpha samples from the closest pixel.
4: Magnified intervals with linear interpolation of
color samples and replication of alpha samples from
the closest pixel.
5: Magnified intervals with linear interpolation of
alpha samples and replication of color samples from
the closest pixel.
MX: 2 bytes. X magnification factor, range 1-65535. If
omitted, MX=1. Ignored if X_method is 0 and assumed to
be 1.
MY: 2 bytes. Y magnification factor. If omitted, MY=MX.
ML: 2 bytes. Left X magnification factor. If omitted, ML=MX.
MR: 2 bytes. Right X magnification factor. If omitted, MR=MX.
MT: 2 bytes. Top Y magnification factor. If omitted, MT=MY.
Ignored if Y_method is 0 and assumed to be 1.
MB: 2 bytes. Bottom Y magnification factor. If omitted,
MB=MY.
Y_method: 1 byte. If omitted, Y_method is the same as X_method.
The MAGN chunk causes the contents of the object buffers pointed to
by the specified range of objects to be immediately and irreversibly magnified.
The first_magnified_object_id can be zero. If so, any subsequent
embedded
objects whose object_id is 0 must be magnified immediately when they
appear in the datastream. Magnification factors and methods for object 0 are
updated by the appearance of a subsequent MAGN chunk whose
first_magnified_object_id is 0. Magnification of object 0 is turned
off by the appearance of an empty MAGN chunk or by a MAGN
chunk whose first_magnified_object_id is zero and
whose X_method
and Y_method are zero, explicitly or by omission.
The magnification factor for object 0 becomes undefined when a SEEK
chunk appears. Therefore, it is the encoder's responsibility either
to include a MAGN chunk that turns off magnification of object 0
prior to the end of any segment in which object 0 was magnified, or to
include a MAGN chunk for object 0 prior to the first embedded object 0
in every segment that contains an embedded object 0.
The last_magnified_object_id must be greater than or equal to the
first_magnified_object_id. It
is not an error to include a nonexistent object or an existing
"frozen"
object in the range; decoders must do nothing to any such objects. If an
object is potentially visible and viewable, it is displayed immediately
after it is magnified. If any object_id is nonzero, the result
of magnifying that object is stored in place of its original object buffer
for later use.
If the MAGN chunk is present, all existing objects in the specified range must conceptually be magnified immediately in accordance with the given magnification factors and methods. Decoders may wish to save the magnification factors and delay the magnification until display time, or until the object is used as the parent object of a Delta-PNG, to save memory. There is nothing preventing this, provided that the end effect is the same as if the magnification had been accomplished immediately. If object 0 is in the specified range, then any subsequent embedded objects with object_id=0 must be magnified immediately when they appear in the datastream.
When X_method is 0, all X magnification factors in
the MAGN chunk
are ignored and can be assumed to be 1.
When X_method is 1, X magnification is done by simple pixel
replication.
The leftmost pixel of each row is replicated ML-1 times.
If the original
width is greater than 1, the rightmost pixel is replicated MR-1 times.
If the original width is greater than 2, the original interior pixels are
replicated MX-1 times. The magnified width W is
W = ML; if (width > 1) W = W + MR; if (width > 2) W = W + (width-2)*MX;
When X_method is 2, X magnification is done by linear interpolation between
pixels. If the original width of the image is greater than 1, the interval
between the leftmost pixel and the second pixel of each row is subdivided
into ML equal intervals by inserting ML-1 pixels with
color and alpha values
that are obtained by linear interpolation. If the original width is 1, then
the pixel is simply magnified as if X method is 1. If the original width is
greater than 2, the rightmost interval is subdivided into MR equal intervals.
If the original width is greater than 3, each original interior interval is
subdivided into MX equal intervals. The magnified width W is
/* The orginal pixels: */
W = width;
/* Add the new pixels in the left interval: */
if (width > 1) W = W + ML-1;
/* Add the new pixels in the right interval: */
if (width > 2) W = W + MR-1;
/* Add the new interior pixels: */
if (width > 3) W = W + (width-3)*(MX-1);
When X_method is 3, intervals are subdivided as in X method 2,
and the
color and alpha values for the new pixels are obtained by replicating
the closest original pixel, with ties being broken by replicating the
pixel to the left. The magnified width is calculated in the same manner
as in X method 2.
When X_method is 4, the color samples are magnified as in
X method 2 and
the alpha samples are magnified as in X method 3.
When X_method is 5, the color samples are magnified as in
X method 3 and
the alpha samples are magnified as in X method 2.
When Y_method is 0, all Y magnification factors in
the MAGN chunk are ignored and can be assumed to be 1.
When Y_method is 1, Y magnification is done by simple pixel
replication.
The topmost pixel of each column is replicated MT-1 times. If the
original
height is greater than 1, the bottom pixel is replicated MB-1 times.
If the original height is greater than 2, the original interior pixels of each
column are replicated MY-1 times. The magnified height H is
H = MT; if (height > 1) H = H + MB; if (height > 2) H = H + (height-2)*MY;
When Y_method is 2, Y magnification is done by linear interpolation between
pixels. If the original height of the image is greater than 1, the interval
between the topmost pixel and the second pixel of each column is subdivided
into MT equal intervals by inserting MT-1 pixels with color and alpha
values
that are obtained by linear interpolation. If the original height is 1, then
the pixel is simply magnified as if Y method is 1. If the
original height is
greater than 2, the bottom interval is subdivided into MB equal intervals.
If the original height is greater than 3, each original interior interval is
subdivided into MY equal intervals. The magnified height H is
H = height; if (height > 1) H = H + MT-1; if (height > 2) H = H + MB-1; if (height > 3) H = H + (height-3)*(MY-1);
When Y_method is 3, intervals are subdivided as in Y method 2,
and the
color and alpha values for the new pixels are obtained by replicating
the closest original pixel, with ties being broken by replicating the
pixel above. The magnified width is calculated in the same manner
as in Y method 2.
When Y_method is 4, the color samples are magnified as in
Y method 2 and
the alpha samples are magnified as in Y method 3.
When Y_method is 5, the color samples are magnified as in
Y method 3 and
the alpha samples are magnified as in Y method 2.
When the image being magnified is a concrete object, it must not be a JNG or indexed-color PNG (the latter could be promoted to RGB or RGBA via a Delta-PNG PROM chunk first). The result of the magnification is also a concrete object. The Method 2 magnification is conceptually done first in the vertical (Y) direction, the results rounded to the sample depth, then in the horizontal (X) direction. Linear interpolation must be done on the raw pixels, prior to any color correction, using integer arithmetic, to ensure that the result is deterministic. For each channel, the m-1 interpolated samples s[i] are obtained from the two samples s0 and s1 by the following ISO C code or by any other method that obtains the identical results:
if(s1 == s0)
for (i=1; i < m; i++)
s[i] = s0;
else
for (i=1; i < m; i++)
s[i] = ((2*i*(s1-s0)+m)/(m*2) + s0;
Signed arithmetic in a precision large enough to hold the intermediate results must be used, and the final results must be modulo the sample depth.
When the image being magnified is an abstract object, which is always true of object 0, interpolation can be done by any means that achieves a visually similar but not necessarily identical result, such as rounding the results to the sample depth later, using video hardware that is capable of interpolation, or using floating point addition in the loop instead of integer multiplication and division as in:
float delta = ((float)(s1-s0)/(float)m);
float sf= (float)s0;
for (i=1; i < m; i++) {
sf = sf+delta;
s[i]=(int)(sf+0.5);
}
If the abstract object being magnified is being stored in an indexed representation, interpolation must be accomplished by a method that achieves a similar result to that obtained by interpolating between RGB or RGBA pixels.
Note that if an object and partial clones of it appear in the range of objects to be magnified, the object buffer will be magnified repeatedly.
Because the MAGN chunk was added late in the development of MNG-1.0, it is recommended that encoders place an empty MAGN chunk or a nEED MAGN chunk early in the datastream, so that pre-MNG-1.0 applications that do not recognize the MAGN chunk will encounter one quickly.
The DISC chunk can be used to inform the decoder that it can discard the object data associated with the associated object identifiers. Whether the decoder actually discards the data or not, it must not use it after encountering the DISC chunk.
The chunk contains a sequence of zero or more two-byte object identifiers. The number of objects to be discarded is the chunk's data length, divided by two.
Discard_id: 2 bytes (nonzero unsigned integer). ...etc...
If the DISC chunk is empty, all nonzero objects except those preceding the SAVE chunk (i.e., except for the "frozen" objects) can be discarded. If a SAVE chunk has not been encountered, all objects can be discarded. Note that each appearance of a SEEK chunk in the datastream implies an empty DISC chunk.
If the DISC chunk is not empty, the listed objects can be discarded.
When an object is discarded, any location, potential visibility, and clipping boundary data associated with it is also discarded.
It is not an error to include an object_id in the
discard_id list, when no such object has been stored, or when
the object has already been discarded.
It is an error to name explicitly any "frozen" object in the DISC list.
When the object is a partial clone or is the source of a partial clone that has not been discarded, only the set of object attributes (location, potential visibility, clipping boundaries, etc.) can be discarded. The data in the object buffer must be retained until the last remaining partial clone is discarded.
The TERM chunk suggests how the end of the MNG datastream should be handled, when a MEND chunk is found. It contains either a single byte or ten bytes:
Termination_action: 1 byte (unsigned integer)
0: Show the last frame indefinitely.
1: Cease displaying anything.
2: Show the first frame after the TERM chunk.
If processing the fPRI chunk, use a "cost"
of 255.
3: Repeat the sequence starting immediately
after the TERM chunk and ending with the
MEND chunk.
Action_after_iterations: 1 byte
0: Show the last frame indefinitely after
iteration_max iterations have been done.
1: Cease displaying anything.
2: Show the first frame after the TERM chunk.
If processing the fPRI chunk, use a "cost"
of 255.
This and the remaining fields must be present
if termination_action is 3, and must be omitted
otherwise.
Delay: 4 bytes (unsigned integer). Delay, in ticks,
before repeating the sequence.
Iteration_max: 4 bytes (unsigned integer). Maximum number of
times to execute the sequence. Infinity is
represented by 0x7fffffff.
The loop created by processing a TERM chunk must always be treated
by the decoder as if it were a cacheable <user-discretion> loop, with
iteration_min=1.
Applications must not depend on anything that has been drawn on the output buffer or device during the previous iteration. Its contents become undefined when the TERM loop restarts.
MNG editors that extract a series of PNG or JNG files from a MNG datastream are expected to execute the TERM loop only once, rather than emitting the files repeatedly.
The TERM chunk, if present, must appear either immediately after the MHDR chunk or immediately prior to a SEEK chunk. The TERM chunk is not considered to be a part of any segment for the purpose of determining the copy-safe status of any chunk. Only one TERM chunk is permitted in a MNG datastream.
Simple viewers and single-frame viewers can ignore the TERM chunk. It has been made critical only so MNG editors will not inadvertently relocate it.
The chunks in this section cause existing objects and embedded objects to be displayed on the output device, and control their location, clipping, and timing and the background against which they are displayed.
The BACK chunk suggests or mandates a background color, image, or both against which transparent, clipped, or less-than-full-frame images can be displayed. This information will be used whenever the application subsequently needs to insert a background layer, unless another BACK chunk provides new background information before that happens.
The BACK chunk contains 6, 7, 9, or 10 bytes. If any field is omitted, all remaining fields must also be omitted.
Red_background: 2 bytes (unsigned integer).
Green_background: 2 bytes (unsigned integer).
Blue_background: 2 bytes (unsigned integer).
Mandatory_background:
1 byte (unsigned integer).
0: Background color and background image are
advisory. Applications can use them if they
choose to.
1: Background color is mandatory. Applications
must use it. Background image is advisory.
2: Background image is mandatory. Applications
must use it. Background color is advisory.
3: Background color and background image are both
mandatory. Applications must use them.
This byte can be omitted if the remaining fields
are also omitted. If so, the background color is
advisory.
Background_image_id:
2 bytes (unsigned nonzero integer). Object_id of an
image that is to be used as the background layer or
part of it. If the image does not cover the area
defined by the layer clipping boundaries with opaque
pixels, the remainder of this area is filled with the
background color or application background and the
background image is composited against it. This
field can be omitted if the background_tiling byte is
also omitted; if so, no background image is defined,
and the background image_id from any previous BACK
chunk becomes undefined. This byte must be omitted
in MNG-LC and MNG-VLC datastreams, and when the
"stored object buffers" flag in the simplicity
profile is valid and is zero.
Background_tiling:
1 byte (unsigned integer).
0: Do not tile the background.
1: Tile the background with the background image.
This field can be omitted; if so, do not tile the
background. This byte must be omitted in MNG-LC and
MNG-VLC datastreams.
The first layer displayed by a viewer is always a background layer that fills the entire frame. The BACK chunk provides a background that the viewer can use for this purpose (or must use, if it is mandatory). If it is not "mandatory" the viewer can choose another background if it wishes. If the BACK chunk is not present, or if the background is not fully opaque or has been clipped to less than full frame, the viewer must provide or complete its own background layer for the first frame. Each layer after the first must be composited over the layers that precede it, until a FRAM chunk with framing mode 3 or 4 causes another background layer to be generated.
Viewers are expected, however, to composite every foreground layer against a fresh copy of the background, when the framing mode given in the FRAM chunk is 3, and to composite the first foreground layer of each subframe against a fresh copy of the background, when the framing mode is 4. Also, when the framing mode is 3 or 4 and no foreground layer appears between consecutive FRAM chunks, a background layer alone is displayed as a separate frame.
The images and the background are both clipped to the subframe boundaries given in the FRAM chunk. Anything outside these boundaries is inherited from the previous subframe. If the background layer is transparent and the subsequent foreground layers do not cover the transparent area with opaque pixels, the application's background becomes re-exposed in any uncovered pixels within the subframe boundaries. The background image (or tiled assembly) is also clipped to its own boundaries and located like any other image, and is only displayed if it is potentially visible. When the background image is used for tiling, the upper left tile is located according to the background image's location attributes and the entire assembly is clipped according to its clipping attributes. Viewers might actually follow some other procedure, but the final appearance of each frame must be the same as if they had filled the area within the subframe boundaries with the background color, then displayed the background image, and then displayed the foreground image (or images), without delay.
Note that any background layer, including the one that begins the first frame of the datastream, must be inserted at the latest possible moment, in case the background image is replaced or is modified by a Delta-PNG datastream or its location or clipping boundaries are changed by a MOVE or CLIP chunk, or in case a new BACK chunk appears, before that moment.
It is an error to specify a background_image_id when the
"stored object buffers" flag in the simplicity profile is
valid and zero.
It is not an error to specify a background_image_id
when such an image is not viewable and potentially visible or
does not yet exist or ceases to exist for some reason,
or to fail to specify one even when
the mandatory_background flag is 2 or 3.
Viewers must be prepared to fall back temporarily to using the background
color or application background in this event, and to resume using the
background image whenever a potentially visible viewable object with the
background_image_id becomes available.
They also must be prepared for the contents, viewability, location, potential
visibility, and clipping boundaries of the background image to change, just
like any other object, if it has not been "frozen". The
background image is allowed to have transparency, subject to any promises
made in the simplicity profile.
The three BACK components are always written as though for an RGBA PNG with 16-bit sample depth. For example, a mid-level gray background could be specified with the RGB color samples {0x9999, 0x9999, 0x9999}. The background color is interpreted in the current color space as defined by any top-level gAMA, cHRM, iCCP, sRGB chunks that have appeared prior to the BACK chunk in the MNG datastream. If no such chunks appear, the color space is unknown.
The color space of the background image, if one is used, is determined in the same manner as the color space of any other image.
The data from the BACK chunk takes effect the next time the decoder needs to insert a background layer, and remains in effect until another BACK chunk appears.
For the purpose of counting layers, when the background consists of both a background color and a background image, these are considered to generate a single layer and there is no delay between displaying the background color and the background image.
Multiple instances of the BACK chunk are permitted in a MNG datastream.
The BACK chunk can be omitted. If a background is needed and the BACK chunk is omitted, then the viewer must supply its own background. For the purpose of counting layers, such a viewer-supplied background layer is counted the same as a background supplied by the BACK chunk.
In practice, most applications that use MNG as part of a larger composition should ignore the BACK data if mandatory_background=0 and the application already has its own background definition. This will frequently be the case in World Wide Web pages, to achieve nonrectangular transparent animations displayed against the background of the page.
The FRAM chunk provides information that a decoder needs for generating frames and interframe delays. The FRAM parameters govern how the decoder is to behave when it encounters a FRAM chunk, an embedded image, or a SHOW chunk. The FRAM chunk also delimits subframes.
If bit 1 of the MHDR simplicity profile is 0 and bit 0 is 1, the FRAM chunk must not be present.
An empty FRAM chunk is just a subframe delimiter. A nonempty one is a subframe delimiter, and it also changes FRAM parameters, either for the upcoming subframe or until reset ("upcoming subframe" refers to the subframe immediately following the FRAM chunk). When the FRAM chunk is not empty, it contains a framing-mode byte, an optional name string, a zero-byte separator, plus four 1-byte fields plus a variable number of optional fields.
When the FRAM parameters are changed, the new parameters affect the subframe that is about to be defined, not the one that is being terminated by the FRAM chunk.
Framing_mode: 1 byte.
0: Do not change framing mode.
1: No background layer is generated, except for one
ahead of the very first foreground layer in the
datastream. The interframe delay is associated
with each foreground layer in the subframe.
2: No background layer is generated, except for one
ahead of the very first image in the datastream.
The interframe delay is associated only with the
final layer in the subframe. A zero interframe
delay is associated with the other layers in the
subframe.
3: A background layer is generated ahead of each
foreground layer. The interframe delay is
associated with each foreground layer, and a zero
delay is associated with each background layer.
4: The background layer is generated only ahead of
the first foreground layer in the subframe. The
interframe delay is associated only with the final
foreground layer in the subframe. A zero
interframe delay is associated with the background
layers, except when there is no foreground layer
in the subframe, in which case the interframe delay
is associated with the sole background layer.
Subframe_name: 0 or more bytes (Latin-1 Text). Can be omitted; if so,
the subframe is nameless.
Separator: 1 byte: (null). Must be omitted if all remaining fields
are also omitted.
Change_interframe_delay:
1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default.
This field and all remaining fields can be omitted as a
group if no frame parameters other than the framing mode
or the subframe name are changed.
Change_timeout_and_termination:
1 byte
0: No.
1: Deterministic, for the upcoming subframe only.
2: Deterministic, also reset default.
3: Decoder-discretion, for the upcoming subframe only.
4: Decoder-discretion, also reset default.
5: User-discretion, for the upcoming subframe only.
6: User-discretion, also reset default.
7: External-signal, for the upcoming subframe only.
8: External-signal, also reset default.
This field can be omitted only if the previous field is
also omitted.
Change_layer_clipping_boundaries:
1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default.
This field can be omitted only if the previous field is
also omitted.
Change_sync_id_list:
1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default list.
This field can be omitted only if the previous field is
also omitted.
Interframe_delay:
4 bytes (unsigned integer). This field must be omitted
if the change_interframe_delay field is zero or is
omitted. The range is [0..2^31-1] ticks.
Timeout: 4 bytes (unsigned integer). This field must be omitted
if the change_timeout_and_termination field is zero or
is omitted. The range is [0..2^31-1]. The value
2^31-1 (0x7fffffff) ticks represents an infinite
timeout period.
Layer_clipping_boundary_delta_type:
1 byte (unsigned integer).
0: Layer clipping boundary values are given directly.
1: Layer clipping boundaries are determined by adding
the FRAM data to the values from the previous
subframe.
This and the following four fields must be omitted if the
change_layer_clipping_boundaries field is zero or is
omitted.
Left_layer_cb or Delta_left_layer_cb:
4 bytes (signed integer).
Right_layer cb or Delta_right_layer_cb:
4 bytes (signed integer).
Top_layer_cb or Delta_top_layer_cb:
4 bytes (signed integer).
Bottom_layer_cb or Delta_bottom_layer_cb:
4 bytes (signed integer).
Sync_id: 4 bytes (unsigned integer). Must be omitted if
change_sync_id_list=0 and can be omitted if the new
list is empty; repeat until all sync_ids have been
listed. The range is [0..2^31-1].
Framing modes:
The framing_mode provides information to the decoder that it uses
whenever it is about to display an image, and when it is processing
the next FRAM chunk.
Any of these events generates a layer, even if no pixels are actually changed:
nominal_layer_count field of the MHDR
chunk.
When a decoder is ready to perform a display update, it must check the framing mode, to decide whether it should restore the background (framing modes 3 and 4) or not (framing modes 1 and 2), and whether it needs to wait for the interframe delay to elapse before continuing (framing modes 1 and 3) or not (framing modes 2 and 4).
When the interframe delay is zero, viewers are not required actually to update the display but can continue to process the remainder of the frame and composite the next image over the existing frame before displaying anything. The final result must appear the same as if each image had been displayed in turn with no delay.
Regardless of the framing mode, encoders must insert a background layer, with a zero delay, ahead of the first image layer in the datastream, even when the BACK chunk is not present or has been clipped to less than full-frame. This layer must be included in the layer count but not in the frame count.
Also, viewers that jump to a segment must insert a background layer, with a zero delay, ahead of the segment, even when the BACK chunk is not present in the prologue segment, if they jumped from the interior of a segment. Such layers are not included in either the layer count or the frame count.
framing_mode is 1, the decoder must wait until the
interframe delay for the previous frame has elapsed before displaying
each image. Each foreground layer is a separate subframe and frame.
In the usual case, the interframe delay is nonzero, and multiple layers are present, so each frame is a single subframe composed of several layers. When the interframe delay is zero, the subframe is combined with subsequent subframes until one with a nonzero interframe delay is encountered, to make up a single frame. The decoder must wait until the interframe delay for the previous frame has elapsed before displaying the frame. When framing_mode=2, viewers are expected to display all of the images in a frame at once, if possible, or as fast as can be managed, without clearing the display or restoring the background.
When the background layer is transparent or does not fill the clipping boundaries of the image layer, the application is responsible for supplying a background color or image against which the image layer is composited, and if the MNG is being displayed against a changing scene, the application should refresh the entire MNG frame against a new copy of the background layer whenever the application's background scene changes (see the "background transparency" bit of the simplicity profile).
framing_mode=4, the background layer is generated
and displayed immediately
before each frame, i.e., after each FRAM chunk, with no interframe
delay before each image. The decoder must wait until the
interframe delay for the previous frame has elapsed before displaying the
background layer.
Otherwise, framing mode 4 is identical to framing mode 2.
All of the foreground layers
between consecutive FRAM chunks, together with one background
layer, make up a single subframe.
A transparent or clipped background layer is handled as in framing mode 3.
The subframe name must conform to the same formatting rules as
those for a PNG tEXt keyword: It must consist only of printable
Latin-1 characters and must not have leading or trailing blanks, but
can have single embedded blanks. There must be at least one (unless
the subframe name is omitted) and no more than 79 characters in the
keyword. Keywords are case-sensitive. There is no null byte within
the keyword.
No specific use for the subframe name is specified in
this document, except that it can be included in the optional index
that can appear in the SAVE chunk.
Applications can use this
field for such purposes as constructing an external list of subframe
in the datastream. The subframe name only applies to the upcoming
subframe; subsequent subframes are unnamed unless they also have their
own frame_name field. It is recommended that the same name
not appear in any other FRAM chunk or in any SEEK
or eXPI
chunk. Subframe names should not begin with the
case-insensitive
strings "CLOCK(", "FRAME(", or "FRAMES(",
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier below).
The interframe delay value is the desired minimum time to elapse from the beginning of displaying one frame until the beginning of displaying the next frame. When the interframe delay is nonzero, which will probably be the usual case, layers are frames. When it is zero, a frame consists of any number of consecutive subframes, until a nonzero delay subframe is encountered and completed. Decoders are not obligated or encouraged to display such subframes individually; they can composite them offscreen and only display the complete frame.
There is no interframe delay before the first layer (the implicit background layer) in the datastream nor after the final frame, regardless of the framing mode.
The timeout field can be a number or <infinity>. Infinity can be represented by 0x7fffffff. Under certain termination conditions, the application can adjust the interframe delay, provided that it is not greater than the sum of the specified interframe delay and the timeout.
The termination condition given in the
change_timeout_and_termination field specifies whether and
over what range the normal interframe delay can be lengthened or
shortened. It can take the following values:
sync_id, but
must wait no less than the smaller of the timeout and the interframe
delay nor no longer than the greater of the timeout and the interframe
delay. If the decoder cannot receive signals, this condition degenerates
into "decoder-discretion".
The sync_id list can be omitted if the termination
condition is not "external-signal".
When the sync_id list is changed, the number of
sync_id entries is determined by the remaining length of the
chunk data, divided by four. This number can be zero, which either
inactivates the existing sync_id list for one frame or
deletes it.
The initial values of the FRAM parameters are:
Framing mode = 1
Subframe name = <empty string>
Interframe delay = 1
Left subframe boundary = 0
Right subframe boundary = frame_width
Top subframe boundary = 0
Bottom subframe boundary = frame_height
Termination = deterministic
Timeout = 0x7fffffff (infinite)
Sync id = <empty list>
The layer clipping boundaries from the FRAM chunk are only used for clipping, not for placement. The DEFI or MOVE chunk can be used to specify the placement of each image within the layer. The DEFI or CLIP chunk can be used to specify clipping boundaries for each image. Even when the left and top subframe boundaries are nonzero, the image locations are measured with respect to the {0,0} position in the display area. The left and top subframe boundaries are inclusive, while the right and bottom boundaries are exclusive.
If the layers do not cover the entire area defined by the layer clipping boundaries with opaque pixels, they are composited against whatever already occupies the area, when the framing mode is 1 or 2. When the framing mode is 3 or 4, they are composited against the background defined by the BACK chunk, or against an application-defined background, if the BACK chunk is not present or does not define a mandatory background. The images, as well as the background, are clipped to the layer clipping boundaries for the subframe. Any pixels outside the layer clipping boundaries remain unchanged from the previous layer.
The interframe_delay field gives the duration of
display, which is the minimum time that must elapse from the
beginning of displaying one layer until the beginning of displaying
the next (unless the termination condition and timeout permit this
time to be shortened). It
is measured in "ticks" using the tick length determined from
ticks_per_second defined in the MHDR chunk.
When the interframe delay is zero, it indicates that
the layer is to be combined with the subsequent layer or layers into a
single frame, until a nonzero interframe delay is specified or
the MEND chunk is reached.
A viewer does not actually have to follow the procedure of erasing the screen, redisplaying the background, and recompositing the images against it, but what is displayed when the frame is complete must be the same as if it had. It is sufficient to redraw the parts of the display that change from one frame to the next.
The sync_id list provides a point at which the processor
must wait for all pending processes to reach the synchronization
point having the same sync_id before resuming, perhaps
because of a need to synchronize a sound datastream (not defined
in this specification) with the display, to synchronize stereo
images, and the like. When the period defined by the sum of the
interframe_delay and the timeout fields
elapses, processing can resume even though the processor has not
received an indication that other processes have reached the
synchronization point.
Note that the synchronization point does not occur immediately, but at the end of the first frame that follows the FRAM chunk.
The identifier sync_id=0 is reserved to represent
synchronization with a user input from a keyboard or pointing device.
The sync_id values 1-255 are reserved to represent the
corresponding ASCII letter, received from the keyboard (or a simulated
keyboard), and values 256-1023 are reserved for future definition
by this specification. If multiple channels (not defined in this
specification) are not present, viewers can ignore other values
appearing in the sync_id list.
Note that the rules for omitting the interframe delay, timeout, clipping boundary, and sync id fields of the FRAM chunk are different from the general rule stated in MNG Chunks, above (Chapter 4). These fields are either present in the chunk data or omitted from it according to the contents of the corresponding "change" byte.
The MOVE chunk gives a new location of an existing object or objects (replacing or incrementing the location given in the DEFI chunk).
The position is measured downward and to the right of the frame origin, in pixels, where the named object or group of objects is to be located.
The chunk's contents are:
First_object: 2 bytes (unsigned integer).
Last_object: 2 bytes (unsigned integer).
Location_delta_type:1 byte (unsigned integer).
0: MOVE data gives X_location and Y_location
directly.
1: New locations are determined by adding the MOVE
data to the location of the parent object.
X_location or delta_X_location:
4 bytes (signed integer).
Y_location or delta_Y_location:
4 bytes (signed integer).
The new location applies to a single object, if
first_object=last_object, or to a group of consecutive
object_ids, if they are different. Last_object
must not be less than first_object. Negative values are
permitted for the X and Y location. The positive directions are
downward and rightward from the frame origin. The MOVE chunk
can specify an image placement that is partially or wholly outside the
display boundaries. In such cases, the resulting image must be clipped
to fit within its clipping boundaries, or not displayed at all if it
falls entirely outside its clipping boundaries. The clipping boundaries
are determined as described in the specification for the CLIP
chunk below (Paragraph 4.3.4).
The left and top boundaries are inclusive, while the right and bottom
boundaries are exclusive.
It is not an error for the MOVE chunk to name an object that has not previously been defined. In such cases, nothing is done to the nonexistent object. It is permitted to move "frozen" objects provided that the encoder includes chunks to move them back to their original positions prior to then end of the segment.
When an object is discarded, its set of object attributes, which includes the MOVE data, is also discarded.
This chunk gives the new boundaries (replacing or incrementing those from the DEFI chunk) to which an existing object or group of objects must be clipped for display. It contains the following 21 bytes:
First_object: 2 bytes (unsigned integer).
Last_object: 2 bytes (unsigned integer).
Clip_delta_type: 1 byte (unsigned integer).
0: CLIP data gives boundary values
directly.
1: CLIP boundaries are determined by
adding the CLIP data to their
previous values for this object.
Left_cb or delta_left_cb: 4 bytes (signed integer).
Right_cb or delta_right_cb: 4 bytes (signed integer).
Top_cb or delta_top_cb: 4 bytes (signed integer).
Bottom_cb or delta_bottom_cb: 4 bytes (signed integer).
The new clipping boundaries apply to a single object, if
first_object=last_object, or to a group of consecutive
objects, if they are different. the last_object must not be less
than first_object.
The clipping boundaries are expressed in pixels, measured rightward and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and bottom clipping boundaries are exclusive, i.e., the pixel located at {x,y} is only displayed if the pixel falls within the physical limits of the display hardware and all of the following are true:
0 <= x < frame_width (from the MHDR chunk) 0 <= y < frame_height Left_lcb <= x < right_lcb (from the FRAM chunk) Top_lcb <= y < bottom_lcb Left_cb <= x < right_cb (from the CLIP chunk) Top_cb <= y < bottom_cb
It is not an error for the CLIP chunk to name an object that has not previously been defined. In such cases, nothing is done to the nonexistent object. It is permitted to clip "frozen" objects provided that another CLIP chunk resets them to their original values prior to the end of the segment.
When an object is discarded, its set of object attributes, which includes the CLIP data, is also discarded.
The SHOW chunk is used to change the potential visibility of one or more previously-defined objects and to direct that they be displayed. It contains 2, 4, or 5 bytes, or it can be empty. When any field is omitted, all remaining fields must also be omitted.
First_image: 2 bytes (nonzero unsigned integer).
Last_image: 2 bytes (nonzero unsigned integer). This field can be
omitted if the show_mode byte is also omitted. If so,
decoders must assume the default values, show_mode=0 and
last_image=first_image.
Show_mode: 1 byte (unsigned integer).
0: Make the images potentially visible and display them
(set do_not_show=0).
1: Make the images invisible (set do_not_show=1).
2: Do not change do_not_show flag; display those that are
potentially visible.
3: Mark images "potentially visible" (do_not_show=0), but
do not display them.
4: Toggle do_not_show flag; display any that are
potentially visible after toggling.
5: Toggle do_not_show flag, but do not display even if
potentially visible after toggling.
6: Step through the images in the given range, making the
next image potentially visible (set do_not_show=0) and
display it. Set do_not_show=1 for all other images in
the range. Jump to the beginning of the range when
reaching the end of the range. Perform one step for
each SHOW chunk (in reverse order
if last_image < first_image).
7. Make the next image in the range (cycle) potentially
visible (do_not_show=0), but do not display it. Set
do_not_show=1 for the rest of the images in the range.
This field can be omitted. If so, decoders must assume the
default, show_mode=0.
The decoder processes the objects (or images) named in the
SHOW chunk in the order first_image through
last_image, and resets the do_not_show flag for
each of the objects. If show_mode is even-valued, it also
displays the images if they are potentially visible and are viewable
images.
When the SHOW chunk is empty, the decoder displays
all existing potentially visible images, without changing their
do_not_show status. The empty SHOW chunk is
equivalent to
SHOW 1 65535 2
If last_image < first_image the images are processed in
reverse order.
When show_mode is odd-valued, nothing is displayed
unless a subsequent SHOW chunk with an even-valued
show_mode appears.
show_mode is even-valued, each visible image that
is displayed generates a separate layer, even
if it is offscreen and no pixels are actually displayed. In such cases,
the layer is totally transparent. When show_mode is odd, or
when show_mode is 2 or 4 or is empty and no image is visible, no
layer is generated.
When show_mode is 1, 4, 5, 6, or 7, images can
be made invisible. This is not permitted when the framing mode is 2 or 4
in the FRAM chunk and the images have already appeared in the
frame, because simple viewers will have already drawn them and have no
way to make them invisible again without redrawing the entire frame.
When show_mode is 6 or 7, a single layer is generated.
The decoder must make the
next image in the "cycle" visible. To do this, it must examine
the do_not_show flag for each image in the range
first_image through last_image, and make the next
one (the one with the next higher value of image_id that
exists and is "viewable") after the first visible one it finds visible
and the rest invisible. When first_image > last_image,
the cycle is reversed, and the "next" image is the one with the next
lower value of image_id. In either case, if the first
visible one found was last_id, or none were visible, it must make
first_image visible. These modes are useful for manipulating
a group of sequential images that represent different views of an
animated icon. See Example 8, below (Chapter 18).
If no "viewable" object is in the specified range, an empty layer
must be generated.
When show_mode is 0, 2, 4, or 6, separate layers will be
generated, each containing an instance of one visible image at the
location specified by the DEFI, CLON, or MOVE
chunk and clipped according to the boundaries specified by the
CLIP and FRAM chunks. When the MOVE or
CLON chunk is used in the delta form, which will frequently be
the case, each image must be displaced from its previous position by the
values given in the MOVE or CLON chunk.
Assuming a nonzero interframe delay, any of the following sequences
would cause the image identified by object_id=6 in a
composite frame to blink:
LOOP 0 0 10 FRAM 4 # Show background SHOW 1 10 # Show images 1 thru 10. FRAM # Show background SHOW 1 5 # Show images 1 thru 5. SHOW 7 10 # Show images 7 thru 10. ENDL FRAM 4 # Show background LOOP 0 0 10 SHOW 1 5 # Show images 1 thru 5. SHOW 6 6 4 # Toggle potential visibility of image 6 SHOW 7 10 # and show it; show images 7 thru 10. FRAM ENDL FRAM 4 # Show background LOOP 0 0 10 SHOW 6 6 5 # Toggle potential visibility of image 6. SHOW 1 10 2 # Show potentially visible images in 1 FRAM # through 10. ENDL
It is not necessary to follow an IHDR-IEND,
JHDR-IEND, BASI-IEND, or DHDR-IEND
sequence or PAST chunk with a SHOW chunk to
display the resulting image, if it was already caused to appear by
do_not_show=0 in the DEFI chunk that introduced the
image. Similarly, the CLON chunk need not be followed by a
SHOW chunk, if do_not_show=0 in the CLON
chunk.
It is not an error for the SHOW chunk to name a nonviewable object or an object that has not previously been defined. In such cases, nothing is done to the nonexistent object. It is permitted to change the potential visibility of "frozen" objects provided that another SHOW chunk resets them to their original values prior to the end of the segment.
The SAVE chunk marks a point in the datastream at which objects are "frozen" and other chunk information is "saved". The SEEK chunk marks positions in the MNG datastream where a restart is possible, and where the decoder must restore the "saved" information, if they have jumped or skipped to a SEEK point from the interior of a segment. They only need to restore information that they will use, e.g., a viewer that processes gAMA and global PLTE and tRNS, but ignores iCCP and sPLT, need only restore the value of gamma and the global PLTE and tRNS data from the prologue segment but not the values of the iCCP and sPLT data.
Simple decoders that only read MNG datastreams sequentially can safely ignore the SAVE and SEEK chunks, although it is recommended that, for efficient use of memory, they at least mark existing objects as "frozen" when the SAVE chunk is processed and discard all "unfrozen" objects whenever the SEEK or empty DISC chunk is processed.
The SAVE chunk marks a point in the datastream at which objects are "frozen" and other chunk information is "saved"; a decoder skipping or jumping to a SEEK chunk from the interior of a segment must restore the "saved" chunk information if it has been redefined or discarded. In addition, the SAVE chunk can contain an optional index to the MNG datastream.
The SAVE chunk can be empty, or it can contain an index consisting of the following:
Offset_size: 1 byte (unsigned integer).
4: Offsets and nominal start times are expressed as
32-bit integers.
8: Offsets and nominal start times are expressed as
64-bit integers.
plus zero or more of the following index entries:
Entry_type: 1 byte (unsigned integer).
0: Segment with nominal start time, nominal layer number,
and nominal frame number.
1: Segment.
2: Subframe.
3: Exported image.
Offset: 4 or 8 bytes (unsigned integer). Must be omitted if
entry_type > 1, set equal to zero if the offset is
unknown.
Nominal_start_time:
4 or 8 bytes: (unsigned integer). Start time of the segment,
measured in ticks from the beginning of the sequence,
assuming that all prior segments were played as intended on
an ideal player, ignoring any fPRI chunks. Must be omitted
if entry_type > 0.
Nominal_layer_number:
4 bytes (unsigned integer). Sequence number of the first
layer in the segment, assuming that all prior segments were
played as intended on an ideal player, ignoring any fPRI
chunks; the first layer of the first segment being layer 0.
Must be omitted if entry_type > 0.
Nominal_frame_number:
4 bytes (unsigned integer). Sequence number of the first
frame in the segment, assuming that all prior segments were
played as intended on an ideal player, ignoring any fPRI
chunks; the first frame of the first segment being frame 0.
Must be omitted if entry_type > 0.
Name: 1-79 bytes (Latin-1 text). Must be omitted for unnamed
segments. The contents of this field must be the same as
the name field in the corresponding SEEK, FRAM, or eXPI
chunk.
Separator: 1 byte (null) (must be omitted after the final entry).
The SAVE chunk must be present when the SEEK chunk is present. It appears after the set of chunks that define information that must be retained for the remainder of the datastream. These chunks, collectively referred to as the prologue segment, are no different from chunks in other segments. They can be chunks that define objects, or they can be chunks that define other information such as gAMA, cHRM, and sPLT. If any chunks appear between the SAVE chunk and the first SEEK chunk, these chunks also form a part of the prologue segment, but their contents become undefined when the SEEK chunk appears.
Only one instance of the SAVE chunk is permitted in a MNG datastream. It is not allowed anywhere after the first SEEK chunk.
It is not permitted, at any point beyond the SAVE chunk, to modify or discard any object that was defined ahead of the SAVE chunk.
An object appearing ahead of the SAVE chunk can be the subject of a CLON chunk. If the clone is a partial clone, modifying it is not permitted, because this would also modify the object buffer that the original object points to.
A chunk like gAMA that overwrites a single current value is permitted after the SAVE chunk, even if the chunk has appeared ahead of the SAVE chunk. Decoders are responsible for saving a copy of the chunk data (in any convenient form) when the SAVE chunk is encountered and restoring it when skipping or jumping to a SEEK chunk from the interior of a segment. If no instance of the chunk appeared ahead of the SAVE chunk, the decoder must restore the chunk data to its original "unknown" condition when it skips or jumps to a SEEK chunk from the interior of a segment.
It is the encoder's responsibility, if it changes or discards any "saved" data, to restore it to its "saved" condition (or to nullify it, if it was unknown) prior to the end of the segment. This makes it safe for simple decoders to ignore the SAVE/SEEK mechanism.
Known chunks in this category include DEFI, FRAM, BACK, PLTE, cHRM, tRNS, fPRI, gAMA, iCCP, bKGD, sBIT, pHYg, pHYs, and sRGB. In addition, it is the responsibility of the encoder to include chunks that restore the potential visibility, location, and clipping boundaries of any "frozen" objects to their "saved" condition.
In the case of chunks like sPLT that can occur multiple times, with different "purpose" fields, additional instances of the chunk are permitted after the SAVE chunk, but not with the same keyword as any instances that occurred ahead of the SAVE chunk. The decoder is required to forget such additional instances when it skips or jumps to a SEEK chunk from the interior of a segment, but it must retain those instances that were defined prior to the SAVE chunk. Encoders are required to nullify such additional instances prior to the end of the segment. Known chunks in this category include only sPLT.
If an entry for a segment (entry type 0 or 1) appears in the optional index, there must also be an entry for every segment, whether named or not, except for the prologue segment, that precedes it. All entries must appear in the index in the same order that they appear in the MNG datastream. There must never be a segment entry (type 0 or 1) for the prologue segment, but there can be entries for named images or subframes in the prologue, placed ahead of the first segment entry. Only named images or subframes are permitted, and it is not an error to omit any or all named images or subframes. Nor is it an error to omit a contiguous set of segments at the end of the datastream from the index.
Offsets are calculated from the first byte of the MNG 8-byte signature, which has offset=0. This is true even if the MNG datastream happens to be embedded in some other file and the signature bytes are not actually present.
Applications with direct access to the datastream can use the index to find segments, subframes, and exported images quickly. After processing the prologue segment, they can jump directly to any segment and then process the remaining datastream until the desired subframe, image, or time is found. Applications that have only streaming access to the datastream can still use the index to decide whether to decode the chunks in a segment or to skip over them.
Only one instance of the SAVE chunk is permitted in a MNG datastream. If the SEEK chunk is present, the SAVE chunk must be present, prior to the first SEEK chunk. The only chunks not allowed ahead of the SAVE chunk are the SEEK chunk and the MEND chunk. The SAVE chunk must not appear inside a LOOP-ENDL pair.
The SEEK chunk marks positions ("seek points") in the MNG datastream where a restart is possible, and where the decoder must restore certain information to the condition that existed when the SAVE chunk was processed, if it has skipped or jumped to the SEEK chunk from the interior of a segment.
The SEEK chunk can be empty, or it can contain a segment name.
Segment_name: 1-79 bytes (Latin-1 string).
The segment name is optional. It must follow the format of a tEXt keyword: It must consist only of printable Latin-1 characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the keyword. There is no null byte terminator within the segment name, nor is there a separate null byte terminator. Segment names are case-sensitive. Use caution when printing or displaying keywords (Refer to Security considerations, below, Chapter 17). No specific use for the segment name is specified in this document, but applications can use the segment name for such purposes as constructing a menu of seek points for a slide-show viewer. It can be included in the optional index that can appear in the SAVE chunk. It is recommended that the same name not appear in any other SEEK chunk or in any FRAM or eXPI chunk. Segment names should not begin with the case-insensitive strings "CLOCK(", "FRAME(", or "FRAMES(", which are reserved for use in URI queries and fragments (see Uniform Resource Identifier below).
Applications must not use any information preceding the SEEK chunk, except for:
When the SEEK chunk is encountered, the decoder can discard any objects appearing after the SAVE chunk, as though an empty DISC chunk were present.
In addition to providing a mechanism for skipping frames or backspacing over frames, the SEEK chunk provides a means of dealing with a corrupted datastream. The viewer would abandon processing and simply look for the next SEEK chunk before resuming. Note that looking for a PNG IHDR chunk would not be sufficient because the PNG datastream might be inside a loop or a Delta-PNG datastream, or it might need data from preceding MOVE or CLIP chunks.
When a decoder jumps to a seek point from the interior of a segment, it must restore the information that it saved when it processed the SAVE chunk, and it must reset the object attributes and magnification factors for object 0 to their default values. When it encounters a SEEK chunk during normal sequential processing of a MNG datastream, it need not restore anything, because the encoder will have written chunks that restore all saved information.
Multiple instances of the SEEK chunk are permitted. The SEEK chunk must not appear prior to the SAVE chunk. The SAVE chunk must also be present if the SEEK chunk is present. The SEEK chunk must not appear between a LOOP chunk and its ENDL chunk.
This section describes ancillary MNG chunks. MNG-compliant decoders are not required to recognize and process them.
The eXPI chunk takes a snapshot of a viewable object (either concrete or abstract), associates the name with that snapshot, and makes the name available to the "outside world" (like a scripting language).
The chunk contains an object identifier (snapshot id) and a name:
Snapshot_id: 2 bytes (unsigned integer). Must be zero in
MNG-LC and MNG-VLC datastreams.
Snapshot_name: 1-79 bytes (Latin-1 text).
When the snapshot_id is zero, the snapshot is the first instance of an embedded image with object_id=0 following the eXPI chunk. When the snapshot_id is nonzero, the snapshot is an already-defined object with that object_id as it already exists when the eXPI chunk is encountered.
Note that the snapshot_name is associated with the
snapshot, not with the snapshot_id nor its subsequent contents;
changing the image identified by snapshot_id will not
affect the snapshot.
The snapshot_name means nothing inside the scope of the
MNG specification, except that it can be included in
the optional index that can appear in the SAVE chunk.
If two eXPI chunks use the same name, it is the outside world's
problem (and the outside world's prerogative to regard it as an error).
It is recommended, however, that the snapshot_name not be
the same as that appearing in any other eXPI
chunk or in any FRAM
or SEEK
chunk. A decoder that knows of no
"outside world" can simply ignore the eXPI chunk. This
chunk could be used in MNG datastreams that define libraries of related
images, rather than animations, to allow applications to extract
images by their snapshot_id.
Names beginning with the word "thumbnail" are reserved for snapshot images that are intended to make good icons for the MNG. Thumbnail images are regular PNG or Delta-PNG images, but they would normally have smaller dimensions and fewer colors than the MNG frames. They can be defined with the potential visibility field set to "invisible" if they are not intended to be shown as a part of the regular display.
The snapshot_name string must follow the format of a
tEXt keyword: It must consist only of printable Latin-1
characters and must not have leading or trailing blanks, but can have
single embedded blanks. There must be at least one and no more than
79 characters in the keyword. Keywords are case-sensitive. There is
no null byte terminator within the snapshot_name string,
nor is there a separate null byte terminator. Snapshot names should
not begin with the case-insensitive strings
"CLOCK(", "FRAME(", or "FRAMES("
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier below).
Multiple instances of the eXPI chunk are permitted
in a MNG datastream, and they need not have different values of
snapshot_id.
The fPRI chunk allows authors to assign a priority to a portion of the MNG datastream. Decoders can decide whether or not to decode and process that part of the datastream based on its "priority" compared to some measure of "cost".
The fPRI chunk contains two bytes:
fPRI_delta_type: 1 byte (unsigned integer).
0: Priority is given directly.
1: Priority is determined by adding the fPRI
data to the previous value, modulo 256.
Priority or delta_priority:
1 byte (signed integer). Value to be assigned to
subsequent chunks until another fPRI chunk is
reached.
While 256 distinct values of priority are possible, it
is recommended that only the values 0 (low priority), 128 (medium
priority), and 255 (high priority) be used. Viewers that can only
display a single image can look for one with priority=255 and
stop after displaying it. If the datastream contains a large number
of frames and includes periodic "initial" frames that do not contain
Delta-PNG datastreams, each "initial" frame could be preceded by a
fPRI with priority=128 and followed by one with
priority=0, and the best representative initial frame could
be preceded by a fPRI chunk with priority=255. Then
single-image viewers would just display the representative frame, slow
viewers would display just the "initial" frames, and fast viewers
would display everything.
If a viewer has established a nonzero "cost", it must skip any portion of the datastream whose priority is less than that "cost". The "cost" must be established prior to processing the proloque segment. If the decoder changes its "cost" it must process again according to the new "cost", unless it knows that there were no fPRI chunks in the prologue segment.
The SAVE, SEEK, and MEND chunks always
have priority=255; decoders must look for these chunks in
addition to the fPRI chunk while skipping a low-priority
portion of the datastream.
It is not permissible for a portion of the datastream to depend on any portion of the datastream having a lower value, because a decoder might have skipped the lower value portion. Use of the fPRI chunk is illustrated in Example 5 and Example 9.
Viewers that care about the priority must assume
priority=255 for any portion of the MNG datastream that is
processed prior to the first fPRI chunk.
Multiple instances of the fPRI chunk are permitted.
The nEED chunk can be used to specify needed resources, to provide a quick exit path for viewers that are not capable of displaying the MNG datastream.
The nEED chunk contains a list of keywords that the decoder must recognize. Keywords are typically private critical chunk names.
Keyword: 1-79 bytes. Separator: 1 byte (null). ...etc...
The nEED chunk should be placed early in the MNG datastream, preferably very shortly after the MHDR chunk.
The keywords are typically 4-character private critical chunk names, but they could be any string that a decoder is required to recognize. No critical chunks defined in this specification or in the PNG specification should be named in a nEED chunk, because MNG-compliant decoders are required to recognize all of them, whether they appear in a nEED chunk or not. The purpose of the nEED chunk is only to identify requirements that are above and beyond the requirements of this document and of the PNG specification.
Each keyword string must follow the format of a tEXt keyword: It must consist only of printable Latin-1 characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the keyword. Keywords are case-sensitive. There is no null byte terminator within the keyword. A null separator byte must appear after each keyword in the nEED chunk except for the last one.
Decoders that do not recognize a chunk name or keyword in the list should abandon the MNG datastream or request user intervention. The normal security precautions should be taken when displaying the keywords.
The MNG pHYg chunk is identical in syntax to the PNG pHYs chunk. It applies to complete full-frame MNG layers and not to the individual images within them.
Conceptually, a MNG viewer that processes the pHYg chunk will first composite each image into a full-frame layer, then apply the pHYg scaling to the layer, and finally composite the scaled layer against the frame. MNG datastreams can include both the PNG pHYs chunk (either at the MNG top level or within the PNG and JNG datastreams) and the MNG pHYg chunk (only at the MNG top level), to ensure that the images are properly displayed either when displayed by a MNG viewer or when extracted into a series of individual PNG or JNG datastreams and then displayed by a PNG or JNG application. The pHYs and pHYg chunks would normally contain the same values, but this is not necessary.
The MNG top-level pHYg chunk can be nullified by a subsequent empty pHYg chunk appearing in the MNG top level.
The namespace for MNG chunk names is separate from that of PNG. Only those PNG chunks named in this paragraph are also defined at the MNG top level. They have exactly the same syntax and semantics as when they appear in a PNG datastream:
A MNG editor that writes PNG datastreams should not include the top-level iTXt, tEXt, tIME, and zTXt chunks in the generated PNG datastreams.
These PNG chunks are also defined at the MNG top level. They provide default values to be used in case they are not provided in subsequent PNG datastreams. Any of these chunks can be nullified by the appearance of a subsequent empty chunk with the same chunk name. Such empty chunks are not legal PNG or JNG chunks and must only appear in the MNG top level.
In the MNG top level, all of these chunks are written as though for 16-bit RGBA PNG datastreams. Decoders are responsible for reformatting the chunk data to suit the actual bit depth and color type of the datastream that inherits them.
A MNG editor that writes PNG or JNG datastreams is expected to include the top-level cHRM, gAMA, iCCP, and sRGB chunks in the generated PNG or JNG datastreams, if the embedded image does not contain its own chunks that define the color space. When it writes the sRGB chunk, it should write the gAMA chunk (and perhaps the cHRM chunk), in accordance with the PNG specification, even though no gAMA or cHRM chunk is present in the MNG datastream. It is also expected to write the pHYs chunk and the reformatted top-level bKGD chunk in the generated PNG or JNG datastreams, and the reformatted sBIT chunk only in generated PNG datastreams, when the datastream does not have its own bKGD, pHYs, or sBIT chunks.
The top-level sRGB chunk nullifies the preceding top-level gAMA and cHRM chunks, if any, and either the top-level gAMA or the top-level cHRM chunk nullifies the preceding top-level sRGB chunk, if any.
This PNG chunk is also defined at the MNG top
level. It
provides a value that takes precedence over those that might be provided
in subsequent PNG or JNG datastreams and provides a value to be used
when it is not provided in subsequent PNG or JNG datastreams.
It also takes precedence over the
PLTE chunk in a subsequent PNG datastream when the
PLTE and hIST chunks are being used as a suggested
palette (i.e., color_type != 3). This chunk can appear for
any color type. There can be multiple sPLT chunks in a MNG
datastream. If a palette_name is repeated, the previous
palette having the same palette_name is replaced. It is
not permitted, at the MNG top level, to redefine a palette after the
SAVE chunk with the same palette_name as one that
appears ahead of the SAVE chunk. It is permitted, however, to
define and redefine other palettes with other palette_name
fields. A single empty sPLT chunk can be used to nullify all
sPLT chunks that have been previously defined in the MNG top
level, except for those that appeared ahead of the SAVE chunk,
when the SAVE chunk has been read.
When a decoder needs to choose between a suggested palette defined at the MNG level and a suggested palette defined in the PNG datastream (either with the sPLT chunk, or with the PLTE/hIST chunks for grayscale or truecolor images), it should give precedence to the palette from the MNG level, to avoid spurious layer-to-layer color changes.
MNG editors that write PNG datastreams should ignore the sPLT data from the MNG level and simply copy any sPLT chunks appearing within the embedded PNG datastreams.
JNG (JPEG Network Graphics) is the lossy sub-format for MNG objects.
MNG-LC and MNG-VLC applications can choose to support JNG or not. Those that do not can check bit 4 (JNG is present/absent) of the MHDR simplicity profile to decide whether they can process the datastream.
Note: This specification depends on the PNG Portable Network Graphics specification [PNG]. The PNG specification is available at the PNG home page,
http://www.libpng.org/pub/png/
A JNG datastream consists of a header chunk (JHDR), JDAT chunks that contain a complete JPEG datastream, optional IDAT chunks that contain a PNG-encoded grayscale image that is to be used as an alpha mask, and an IEND chunk. The alpha mask, if present, must have the same dimensions as the image itself. The JDAT and IDAT chunks can be interleaved. Some of the PNG ancillary chunks are also recognized in JNG datastreams.
While JNG is primarily intended for use as a sub-format within MNG, a single-image JNG datastream can be written in a standalone file. If so, the JNG datastream begins with an 8-byte signature containing
139 74 78 71 13 10 26 10 (decimal)
91 4a 4e 47 0d 0a 1a 0a (hexadecimal)
\213 J N G \r \n \032 \n (ASCII C notation)
which is similar to the PNG signature with "\213 J N G" instead of "\211 P N G" in bytes 0-3.
We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
image/x-jng can be used. It is recommended that the
file extension ".jng" (lower case preferred) be used.
JNG is pronounced "Jing."
This section specifies the critical chunks that are defined in the JNG format.
The format of the JHDR chunk introduces a JNG datastream. It contains:
Width: 4 bytes (unsigned integer, range 0..65535).
Height: 4 bytes (unsigned integer, range 0..65535).
Color_type: 1 byte
8: Gray (Y).
10: Color (YCbCr).
12: Gray-alpha (Y-alpha).
14: Color-alpha (YCbCr-alpha).
Image_sample_depth:
1 byte
8: 8-bit samples and quantization tables.
12: 12-bit samples and quantization tables.
20: 8-bit image followed by a 12-bit image.
Image_compression_method:
1 byte
8: ISO-10918-1 Huffman-coded baseline JPEG.
Image_interlace_method:
1 byte.
0: Sequential JPEG, single scan.
8: Progressive JPEG.
Alpha_sample_depth:
1 byte.
0, 1, 2, 4, 8, or 16, if the Alpha compression method is 0
(PNG)
8, if the Alpha compression method is 8 (JNG).
Alpha_compression_method:
1 byte.
0: PNG grayscale IDAT format.
8: JNG 8-bit grayscale JDAA format.
Alpha_filter_method:
1 byte.
0: Adaptive PNG (see PNG spec) or not applicable (JPEG).
Alpha_interlace_method:
1 byte.
0: Noninterlaced PNG or sequential single-scan JPEG.
The width, height, image_sample_depth, image_compression_method, and image_interlace_method fields are redundant because equivalent information is also embedded in the JDAT datastream. They appear in the JHDR chunk for convenience. Their values must be identical to their equivalents embedded in the JDAT chunk. We use four bytes in the width and height fields for similarity to MNG and PNG, and to leave room for future expansion, even though two bytes would have been sufficient.
When the color_type is 8 or 10 (no alpha channel), the
last four bytes, which describe the IDAT or JDAA data,
must be set to zero. The alpha_sample_depth must be nonzero
when the alpha channel is present.
A JNG datastream must contain one or more JDAT chunks, whose data, when concatenated, forms a complete JNG JPEG datastream. JNG decoders are required to read all baseline JNG JPEG and eight-bit progressive JNG JPEG datastreams. Twelve-bit capability is not required.
JDAT chunks are like PNG IDAT chunks in that there may be multiple JDAT chunks, the data from which are concatenated to form a single datastream that can be sent to the decompressor. No chunks are permitted among the sequence of JDAT chunks, except for interleaved IDAT chunks. The ordering requirements of other ancillary chunks are the same with respect to JDAT as they are in PNG with respect to the IDAT chunk.
A JNG JPEG is a baseline, extended-sequential, or progressive JPEG as defined by JPEG Part 1 [ISO/IEC-10918-1]. JNG uses only JFIF-compatible [JFIF] component interpretations, and imposes a few additional restrictions that reflect limitations of many existing JPEG implementations. In particular, only Huffman entropy coding is permitted.
Actually, a JNG may contain two separate JNG JPEG datastreams (one eight-bit and one twelve-bit), each contained in a series of JDAT chunks, and separated by a JSEP chunk (see the JSEP chunk specification below, Paragraph 5.1.6). Decoders that are unable to (or do not wish to) handle twelve-bit datastreams are allowed to display the eight-bit datastream instead, if one is present.
The core of the JNG JPEG definition is baseline JNG JPEG, which is JPEG Part 1's definition of baseline JPEG further restricted by JFIF restrictions and JNG-specific restrictions. JNG JPEG also includes progressive JPEG, which is also defined in JPEG Part 1 and has JNG-specific restrictions.
A baseline JPEG according to JPEG Part 1 is DCT-based (lossy) sequential JPEG, using 8-bit sample precision and Huffman entropy coding, with the following further restrictions:
The SOF marker type for baseline JPEG is SOF0.
JDAT datastreams must always follow "interchange JPEG" rules: all
necessary quantization and Huffman tables must be included in the
datastream; no tables can be omitted.
The image data is always stored left-to-right, top-to-bottom.
The encoded data shall have one of the two color space interpretations allowed by the JFIF specification:
Y = Luma_red*R + Luma_green*G + Luma_blue*B Cb = (B - Y) / (2 - 2*Luma_blue) + Half_scale Cr = (R - Y) / (2 - 2*Luma_red) + Half_scale
By convention, the luminance coefficients are always those defined by CCIR Recommendation 601-1:
Luma_red = 0.299 Luma_green = 0.587 Luma_blue = 0.114
The constant Half_scale is 128 when dealing with eight-bit data, 2048 for twelve-bit data. With these equations, Y, Cb, and Cr all have the same range as R, G, and B: 0 to 255 for eight-bit data, 0 to 4095 for twelve-bit data.
The JFIF convention for YCbCr differs from typical digital television practice in that no headroom/footroom is reserved: the coefficient values range over the full available 8 or 12 bits.
Intercomponent sample alignment shall be such that the first (upper leftmost) samples of each component share a common upper left corner position. This again differs from common digital TV practice, in which the first samples share a common center position. The JFIF convention is simpler to visualize: subsampled chroma samples always cover an integral number of luminance sample positions, whereas with co-centered alignment, chroma samples only partially overlap some luminance samples.
JNG imposes three additional restrictions not found in the text of either JPEG Part 1 or the JFIF specification:
In other words, the chroma components may be downsampled 2:1 or 1:2 horizontally or vertically relative to luminance, or they may be left full size. These four sampling ratios are the only ones supported by a wide spectrum of implementations (1x2 is relatively uncommon, and is usually the result of a lossless rotation of a 2x1 sampling).
For grayscale images, the sampling factors are irrelevant according to a strict reading of JPEG Part 1. Hence decoder authors should accept any sampling factors for grayscale. However, we recommend that encoders always emit sampling factors 1h1v for grayscale, since some decoders have been observed to malfunction when presented with other sampling factors.
For JNG progressive JPEG datastreams, the JPEG process is progressive Huffman coding (SOF marker type SOF2) rather than baseline (SOF0). All JNG-compliant decoders must support full progression, including both spectral-selection and successive-approximation modes, with any sequence of scan progression parameters allowed by the JPEG Part 1 standard.
Otherwise, all the restrictions listed above apply, except these:
We require full progression support since relatively little code savings can be achieved by subsetting the JPEG progression features. In particular, successive approximation offers significant gains in the visual quality of early scans. Omitting successive-approximation support from a decoder does not save nearly enough code to justify restricting JNG progressive encoders to spectral selection only.
No particular progressive scan sequence is specified or recommended by this specification. Not enough experience has been gained with progressive JPEG to warrant making such a recommendation. To allow for future experimentation with scan sequences, decoders are expected to handle any JPEG-legal sequence. Again, the code savings that might be had by making restrictive assumptions are too small to justify a limitation.
When the JSEP chunk is present, both images must be progressive if one of them is progressive.
JNG JPEGs may optionally use 12-bit sample precision as defined in JPEG Part 1.
For a sequential image, the SOF marker type must be SOF1 (extended sequential) not SOF0, and the baseline restriction of two Huffman tables is removed. Also, the encoder may use either 8-bit or 16-bit quantization tables. All other JNG baseline restrictions still apply. It is recommended that JNG encoders not use extended-sequential mode except to encode 12-bit data.
For a progressive image, the only difference between 8-bit and 12-bit modes is that the sample precision is 12 bits and the encoder may use either 8-bit or 16-bit quantization tables. All other JNG restrictions still apply.
This chunk is exactly like the IDAT chunk in a PNG grayscale
image, except that it is interpreted as an alpha mask to be applied to
the image data from the JDAT chunks,
when alpha_compression_method=0. The alpha channel, if
present, can have sample depths 1, 2, 4, 8, or 16.
The filter method can be any filter method that is defined for PNG
datastreams that are embedded in MNG datastreams.
The IDAT
chunks can be interleaved with the JDAT chunks
(see Recommendations for Encoders:
JNG interleaving below).
No other chunk
type can appear among the sequence of IDAT and JDAT
chunks. No other chunk type can appear between the sequences of
IDAT and JDAT chunks when they are not interleaved.
The samples in the IDAT must be presented in noninterlaced order, left
to right, top to bottom. As in PNG, zero means fully transparent and
2^alpha_sample_depth-1 means fully opaque.
The IDAT chunks must precede the JSEP chunk, if the JSEP chunk is present. Minimal viewers that ignore the twelve-bit JDAT chunks must read the IDAT chunks and apply the alpha samples to the eight-bit image that is contained in the JDAT chunks that precede the JSEP chunk. Viewers that skip the eight-bit JDAT chunks must decode the IDAT chunks that precede the JSEP chunk and apply the alpha samples to the twelve-bit image that is contained in the JDAT chunks that follow the JSEP chunk.
This chunk is exactly like the JDAT chunk in a non-progressive
JNG 8-bit grayscale image, except that it is interpreted as an alpha mask to
be applied to the image data from the JDAT chunks,
when alpha_compression_method=8. The alpha channel, if
present, can have only sample depth 8. The JDAA
chunks can be interleaved with the JDAT chunks
(see Recommendations for Encoders:
JNG interleaving below).
Like IDAT chunks, the JDAA chunks must precede the JSEP chunk, if the JSEP chunk is present, and are handled similarly.
The JNG IEND chunk is identical to its counterpart in PNG. Its data length is zero, and it serves to mark the end of the JNG datastream.
JNG permits storage of both an 8-bit and a 12-bit JPEG datastream in a single JNG file. This feature allows an 8-bit image to be provided for non-12-bit-capable decoders. The JSEP chunk is used to separate the two datastreams.
The JSEP chunk is empty.
A JSEP chunk must appear between the JDAT
chunks of an eight-bit datastream and those of a twelve-bit datastream, when
image_sample_depth=20 in the JHDR chunk. When
image_sample_depth != 20, the JSEP chunk must not
be present. The eight-bit
datastream must appear first. Both images must have the same width,
height, color type, compression method, and interlace method. Viewers can
choose to display one or the other image, but not both.
Some PNG ancillary chunks can also appear in JNG datastreams, and are used for the same purposes as described in the PNG specification [PNG] and the Extensions to the PNG Specification document [PNG-EXT].
If the bKGD chunk is present, it must be written as if it were written for a PNG datastream with sample_depth=8. It has one 2-byte entry for grayscale JNGs and three 2-byte entries for color JNGs. The first (most significant) byte of each entry must be 0.
The following chunks have exactly the same meaning and have the same syntax as given in the PNG specification: cHRM, gAMA, iCCP, sRGB, pHYs, oFFs, sCAL. If they are present, they must appear prior to the first JDAT chunk. The following chunks also have the same meaning and syntax as in PNG: iTXt, tEXt, tIME, and zTXt. They can appear prior to the first or after the last JDAT chunk.
The PNG PLTE, hIST, pCAL, sBIT, sPLT, tRNS, fRAc, and gIF* chunks are not defined in JNG.
When cHRM, gAMA, iCCP, or sRGB are present, they provide information about the color space of the decoded JDAT image, and they have no effect on the decoded alpha samples from the IDAT or JDAA chunks. Any viewer that processes the gAMA chunk must also recognize and process the sRGB chunk. It can treat it as if it were a gAMA chunk containing the value .45455 and it can ignore its "intent" field.
The chunk copying and ordering rules for JNG are the same as those in PNG, except for the fact that the JDAT chunks and IDAT or JDAA chunks can be interleaved.
A Delta-PNG datastream describes a single image, by giving the changes from a previous PNG (Portable Network Graphics) image or nonviewable PNG-like object, a JNG (JPEG Network Graphics) image, or another Delta-PNG image.
No provision is made in this specification for storing a Delta-PNG datastream as a standalone file. A Delta-PNG datastream will normally be found as a component of a MNG datastream. Applications that need to store a Delta-PNG datastream separately should use a different file signature and filename extension, or they can wrap it in a MNG datastream consisting of the MNG signature, the MHDR chunk, a BASI chunk with the appropriate dimensions and an IEND chunk, the Delta-PNG datastream, and the MEND chunk.
The decoder must have available a parent (decoded) object that has an object buffer from which the original chunk data is known. The parent object can be the result of decoding a PNG, another Delta-PNG datastream, or it could have been generated by a PNG-like datastream introduced by a BASI chunk.
The child image is always of the same basic type (at present only PNG and JNG are defined) as the parent object. The child is always a viewable image even if the parent is not.
The decoder must not have modified the pixel data in the parent object by applying output transformations such as gAMA or cHRM, or by compositing the image against a background. Instead, the decoder must make available to the Delta-PNG decoder the unmodified pixel data along with the values for the gAMA, cHRM, and any other recognized chunks from the parent object datastream.
A Delta-PNG datastream consists of a DHDR and IEND
enclosing other optional chunks (if there are no other chunks, the
decoder simply copies the parent image, and displays it if its
do_not_show=0).
Chunk structure (length, name, CRC) and the chunk-naming system
are identical to those defined in the PNG specification. Definitions
of compression_method and interlace_method are
also the same as defined in the PNG specification. The definition
of filter_method is the same as for PNG datastreams that
are embedded in MNG datastreams
(see the IHDR chunk specification, above, Paragraph 4.2.3).
This section describes critical Delta-PNG chunks. MNG-compliant decoders must recognize and process them.
The DHDR chunk introduces a Delta-PNG datastream. Subsequent chunks, through the next IEND chunk, are interpreted according to the Delta-PNG format.
The DHDR chunk can contain 4, 12, or 20 bytes:
Object_id: 2 bytes (nonzero unsigned integer). Identifies the parent
object from which changes will be made. This is also the
object_id of the child image, which can be used as the
parent image for a subsequent Delta-PNG.
Image_type: 1 byte.
0: Image type is unspecified. An IHDR, JHDR, IPNG, or
IJNG chunk must be present. If JHDR or IJNG is
present, delta_type must not be 1, 3, 4, or 6.
1: Image type is PNG. IHDR and IPNG can be omitted under
certain conditions.
2: Image type is JNG. JHDR and IJNG can be omitted under
certain conditions. Delta_type must not be 1, 3, 4,
or 6.
Delta_type: 1 byte.
0: Entire image replacement.
1: Block pixel addition, by samples, modulo 2^sample_depth.
2: Block alpha addition, by samples, modulo 2^sample_depth.
Regardless of the color type of the parent image, the
IDAT data are written as a grayscale image (color type
0), but the decoded samples are used as deltas to the
alpha samples in the parent image. The parent image
must have (or be promoted to via the PROM chunk) a
color type that has an alpha channel.
3: Block color addition. Similar to delta type 1 except
that only the color channels are updated even when the
parent has an alpha channel.
4: Block pixel replacement.
5: Block alpha replacement.
6: Block color replacement.
7: No change to pixel data.
Block_width: 4 bytes (unsigned integer). This field must be omitted
when delta_type=7.
Block_height: 4 bytes (unsigned integer). This field must be omitted
when delta_type=7.
Block_X_location:
4 bytes (unsigned integer), measured in pixels from the
left edge of the parent object. This field must be
omitted when delta_type=0 or when delta_type=7.
Block_Y_location:
4 bytes (unsigned integer), measured in pixels from the
top edge of the parent object. This field must be
omitted when delta_type=0 or when delta_type=7.
The object_id must identify an existing object, and the
object must be a "concrete" object, i.e., it must have the property
concrete_flag=1.
The image_type, whether given explicitly as 1 or 2
or implied by the presence of an IHDR, IPNG,
JHDR, or IJNG chunk, must be the same as that of the
parent object.
When delta_type=0, the width and height of the child image
are given by the block_width and block_height
fields.
For all other values of delta_type, the width and height
of the child image are inherited from the parent object.
When delta_type=1-6, the block_width
and block_height fields give the size of the block of
pixels to be modified or replaced, and block_X_location and
block_Y_location give its location with respect to the top
left corner of the parent object. The block must fall entirely within
the parent object.
delta_type=0 in the DHDR chunk, the
pixel data in the IDAT chunks represent a completely new
image, with dimensions given by the block_width and
block_height fields of the DHDR chunk. Data from
chunks other than IDAT or JDAT can be inherited from
the parent object. If the IHDR or JHDR chunk is present,
all of its fields except width and height
(which must be ignored by decoders) provide
new values that are inherited by subsequent objects. The "pixel
sample depth" and "alpha sample depth" are also reset
equal to the
IHDR sample_depth value (in the case of a JNG object,
the new "alpha sample depth" is taken from the JHDR
alpha_sample_depth field). If the IHDR or JHDR
chunk is not present,
the IDAT chunks are decoded according to the parent object's
sample depth, and not according to the "pixel
sample depth" or "alpha sample depth" which are used for
decoding the IDAT chunks in subsequent Delta-PNG datastreams
when delta_type is nonzero.
delta_type=1 in the DHDR chunk, the pixel
data in the IDAT chunks represent deltas from the pixel data in
a parent object known to the decoder, including the alpha channel, if the
parent object has an alpha channel.
The IDAT chunk data contains a filtered and perhaps
interlaced set of delta pixel samples. The delta samples are presented
in the order specified by interlace_method, filtered
according to the filter_method and compressed according to
the compression_method given in the IHDR chunk.
The pixel data includes alpha
samples, if the parent object has an alpha channel.
An encoder calculates the delta sample values from the samples
in the
parent object and those in the child image by subtracting the parent
object samples from the child image samples, modulo 2^sample_depth.
When decoding the IDAT chunk, the child image bytes are
obtained by adding the delta bytes to the parent object bytes, modulo
2^sample_depth. This is similar in operation to the PNG SUB filter,
except that it works by samples instead of working by bytes.
Only the pixels in the block defined by the block location and dimensions given in the DHDR chunk are changed. The size of the IDAT data must correspond exactly to this rectangle.
When the parent object has color_type=3, the
deltas are differences between index values, not between color samples.
The color type must match that of the parent,
except that
when the parent has PNG color_type=3, the delta can have
color_type=0, and vice versa, since the contents of
the IDAT chunks of either color type are indistinguishable.
If the
pixel_sample_depth
does not match the
object_sample_depth, the delta must be scaled to the
object_sample_depth using the zero-fill or right-shift method
described in the PNG specification, before performing the pixel addition.
When the IHDR chunk is present, the compression method, filter method, and interlace method need not be the same as those of the parent object. The new values are used in decoding the IDAT data, and the new values are inherited by the child object.
Whenever the sample depth differs from that of the parent object, the resulting object inherits the original value from the parent. The value from the IHDR chunk is only used for decoding the IDAT data in this and subsequent Delta-PNGs. Implicit in this is the requirement for decoders to remember in the object buffer not only the sample depth of the object but (separately) the "pixel sample depth" for use in decoding the IDAT chunks of subsequent Delta-PNG datastreams that do not contain their own IHDR chunk. The parent object cannot have alpha samples that were carried in JPEG-encoded JDAA chunks.
delta_type=2 in the DHDR chunk, the pixel
data in the IDAT chunks represent deltas from the alpha
data in a parent object known to the decoder. The color samples
are not changed, and the updated alpha samples are calculated in
the same manner as the updated pixel samples are calculated when
delta_type=1.
The color_type is 0 (grayscale), regardless of the
color_type of the parent object. The parent object must have
an alpha channel or must have been promoted to a type that has an alpha
channel. The compression method, filter method, and interlace method
need not be the same. If they are different, the child object inherits
the new values, and the new values will be used in decoding the data in
any subsequent IDAT chunks. Neither the parent object nor the
delta object can have alpha samples that were carried in JPEG-encoded
JDAA chunks.
The sample_depth value from the IHDR chunk is
interpreted as a new value of alpha_sample_depth and is
only used for decoding the IDAT data in this and subsequent
Delta-PNGs. Implicit in this is the requirement for decoders to remember
in the object buffer not only the sample depth of the object
but (separately) the alpha_sample_depth
for use in decoding the IDAT chunks in any subsequent Delta-PNG
datastreams.
If the
alpha_sample_depth
does not match the
object_sample_depth, the delta must be scaled to the
object_sample_depth,
using the zero-fill or right-shift method described in the PNG specification,
before performing the pixel addition.
delta_type=3 is similar to delta_type=1
except that the alpha channel is not included in the IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
delta_type=4 in the DHDR chunk, the pixel
data in the IDAT chunks represent replacement values for the
pixel samples in the rectangle given by the block location and dimension
fields in the DHDR chunk, including the alpha channel, if the
parent object has an alpha channel.
If the pixel_sample_depth does not match
the object_sample_depth,
the pixel data must be scaled to the object_sample_depth
before making the replacements,
using the left bit replication method described in the PNG specification,
or by the right shift method in the unlikely event that the
pixel_sample_depth is larger than
the object_sample_depth.
The color type must match that of the parent, except for the cases mentioned for delta type 1, above.
delta_type=5 in the DHDR chunk, the pixel
data in the IDAT chunks represent replacement values of the
alpha samples in the rectangle given by the block location and dimension
fields in the DHDR chunk. The sample depth of the
data (i.e. the "alpha sample depth") need not match the sample depth
of the parent object, and color_type is
0 (grayscale), regardless of the color_type of the parent
object.
If the sample depths differ, the samples must be scaled
to the object_sample_depth, using
the left bit replication method or right shift method described
in the PNG specification, depending on whether
the alpha_sample_depth is larger or smaller than
the object_sample_depth.
The parent object must have an alpha channel or must have been
promoted to a type that has an alpha channel. The compression method,
filter method, and interlace method need not be the same. If they
differ, the child object inherits the new values.
It is permitted to use JPEG-encoded JDAA chunks to convey the new alpha data. If this is done, then the alpha channel of the object can no longer be used as the parent for block-pixel-addition or block-alpha-addition.
delta_type=6 is similar to delta_type=4
except that the alpha channel is not included in the IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
delta_type=7 in the DHDR chunk, there is
no change to the pixel data, and it is an error for
IDAT, JDAT, or JDAA to appear. If the IHDR
or JHDR chunk appears, the width, height, and color_type fields are
ignored, the PNG sample depth
(or JNG alpha_sample_depth) is used to update the pixel_sample_depth
and alpha_sample_depth, and the data in the remaining fields
are inherited by the child object.
delta_type is 0. The decoder needs to remember the
pixel_sample_depth and alpha_sample_depth to use with each
object. They are initialized to the sample_depth value
from the IHDR chunk that appears when the object is first
created but can be changed by the appearance of the IHDR chunk in
a Delta-PNG datastream that has a nonzero delta_type.
If the object is a JNG image, they are initialized from the value
of alpha_sample_depth from
the original JHDR chunk, and can be changed by the appearance
of the JHDR chunk in a Delta-PNG datastream that has
delta_type != 0.
This chunk is used to "promote" a parent object to a higher bit depth or to add an alpha channel, before making changes to it.
New color type: 1 byte.
New sample depth: 1 byte.
Fill method: 1 byte.
0: Left-bit-replication
1: Zero fill
When a decoder encounters the PROM chunk, it must promote the pixel data. The cases are:
color_type 0 -> 4)
color_type 2 -> 6)
color_type 0 -> 2)
color_type 4 -> 6)
color_type 0 -> 6)
color_type 3 -> 2)
color_type 3 -> 6)
JNG color_type 8 -> 10)
JNG color_type 8 -> 12)
JNG color_type 8 -> 14)
JNG color_type 10 -> 14)
JNG color_type 12 -> 14)
color_type
If the sample depth has been changed, the sample values must be
widened. The decoder must use left-bit-replication or zero-fill
according to the specified fill_method to fill the additional
bits of each sample. If cheap transparency information is present in a
grayscale or truecolor object, its sample values must also be widened in
the same manner. If the image type is JNG, then the new sample depth
refers to the alpha_sample_depth and only the alpha channel
is affected, if one is present. If the color_type has been
promoted from indexed-color, the original bit depth is always considered to be
8. See the PNG specification [PNG]
for
further information on these filling methods. Any alpha channel added
in this manner is eligible to be updated by block-alpha-addition in this
or a subsequent Delta-PNG.
If the basis object contains data from the PNG bKGD chunk,
this data must be promoted as well. If a grayscale object is being
promoted to a truecolor object, the background RGB samples are set equal
to the grayscale background sample. If the bit depth has been changed,
the background samples are widened in accordance with the specified
fill_method. If the basis object is a JNG, the bKGD
chunk is not affected.
If the basis object contains data from the PNG sBIT chunk, this data must also be promoted. If a grayscale object is being promoted to a truecolor object, the new RGB bytes are set equal to the grayscale byte. When an alpha channel is added, the alpha byte is set equal to the sample depth of the basis image. If the sample depth has been changed, the sBIT bytes do not change.
The PROM chunk is not permitted to "demote" a parent object to an object with a lesser bit depth or from one with an alpha channel to one without an alpha channel.
The PROM chunk must appear ahead of the IHDR chunk, if IHDR is present, and ahead of any chunks that would have followed IHDR, if IHDR is omitted.
Inside a Delta-PNG datastream, the IHDR chunk introduces an incomplete PNG (Portable Network Graphics) datastream. The parent object must be a PNG or PNG-based Delta-PNG. The datastream can be introduced by a complete PNG IHDR chunk or by an IPNG chunk, which is empty.
If the IHDR chunk is present, its width
and height
fields are ignored. The values for these parameters are inherited from
the parent object or from the DHDR chunk.
The sample_depth, color_type,
compression_method, interlace_method, and
filter_method fields, if different from those of the parent
object, are used in decoding any subsequent IDAT chunks, and
the new values will be inherited by any subsequent image that uses this
object as its parent. These do not change the sample_depth
and color_type of the object itself; those can only be
changed by using the PROM chunk or by using delta_type=0.
See the PNG specification
and the Extensions to the PNG Specification document [PNG-EXT]
for the format of the PNG chunks.
The filter_method can be any filter_method that is
allowed in PNG datastreams that are embedded in a MNG datastream.
The
PNG datastream must contain at least IHDR and IEND
(whether actually present in the datastream or omitted and included
by implication, as described below), but can inherit other chunk data
from the parent object. Except for IDAT and PPLT,
any chunks appearing between IHDR and IEND are always
treated as replacements or additions and not as deltas.
The IPNG chunk is empty.
The IPNG chunk can be used instead of the IHDR
chunk if the IHDR chunk is not needed for resetting the
value of compression_method, filter_method, or
interlace_method. The purpose of this chunk is to identify
the beginning of the PNG datastream, so decoders can start interpreting
PNG chunks instead of Delta-PNG chunks. The decoder must treat this
datastream as though the IHDR chunk were present in the
location occupied by the IPNG chunk.
The IHDR chunk can also be omitted when
image_type=1 and the PNG datastream begins with a
PLTE chunk, a PPLT chunk, or an IDAT chunk.
In this case, no IPNG chunk is required, either. The decoder
must treat this datastream as though the IHDR chunk were
present, immediately preceding the first PNG chunk. If the first PNG
chunk is neither a PLTE chunk, a PPLT chunk, nor an
IDAT chunk, then either the IPNG or IHDR chunk
must be present to introduce the PNG datastream.
If the PLTE chunk is present, it need not have the same
length as that inherited from the parent object, but it must contain
the complete palette needed in the child image. If it is shorter than
the palette of the parent object, decoders can discard the remaining
entries and the child image must not refer to them. Decoders can also
truncate any tRNS data inherited from an indexed-color parent
object. If the new palette is longer than the parent palette, and
a new tRNS chunk is not present in an indexed-color image,
the tRNS data must be extended with opaque entries. The new
palette must not be longer than the object's sample_depth
would allow, and must not have more than 256 entries.
When processing the tRNS chunk, if color_type=3
and PLTE is not supplied, then the number of allowable entries
is determined from the number of PLTE entries in the parent
object. A tRNS chunk appearing in a Delta-PNG datastream is
always treated as a complete replacement for the tRNS chunk
data in the parent object. All entries beyond those actually supplied
are overwritten with the "opaque" value (255).
If it is desired only to overwrite or add palette entries, the PPLT chunk can be used. This might be useful for palette-animation applications. This chunk can also be used to overwrite or add entries to the transparency (alpha) data from the parent's tRNS chunk.
The PPLT chunk contains a delta_type byte and one
or more groups of palette entries:
PPLT_delta_type: 1 byte.
0: Values are replacement RGB samples.
1: Values are delta RGB samples.
2: Values are replacement alpha samples.
3: Values are delta alpha samples.
4: Values are replacement RGBA samples.
5: Values are delta RGBA samples.
First_index,
first group: 1 byte.
Last_index,
first group: 1 byte.
First set of
samples: 1, 3, or 4 bytes.
...etc...
Last set of
samples: 1, 3, or 4 bytes.
First index,
second group: 1 byte.
...etc...
The last_index must be equal to or greater than
first_index. The groups are not required to appear in
ascending order. If any index of any group is beyond the end of the
parent object's palette, the palette and tRNS data must be
extended just as if a longer complete PLTE chunk had appeared.
If there are gaps in the resulting extended palette, the colors must be
filled with {0,0,0} and the alphas filled with 255. If alpha samples
are supplied (PPLT_delta_type > 1) and no tRNS
data is present in the parent object, a tRNS chunk must be
created in the child object as though a complete tRNS chunk
had appeared. The new palette must not be longer than the object's
sample_depth would allow.
When PPLT_delta_type=0, the values are replacements for
the existing samples in the palette.
When PPLT_delta_type=1, the values are added to the
existing samples (modulo 256) to obtain the new samples.
If the new entry is beyond the range of the original palette,
the values are simply appended, regardless of the contents of
PPLT_delta_type.
Inside a Delta-PNG datastream, the JHDR chunk introduces an incomplete JNG (JPEG Network Graphics) datastream. The parent object must be a JNG or JNG-based Delta-PNG. The datastream is introduced by a complete JHDR chunk.
If the JHDR chunk is present, its width,
height, image_sample_depth,
image_color_type, image_filter_method, and
image_interlace_method fields are ignored. The values for these
parameters are inherited from the parent object.
The alpha_compression_method,
alpha_interlace_method, and alpha_filter_method fields,
if different from those of the parent object, are used in decoding any
subsequent IDAT chunks, and the new values will be inherited by
any subsequent image that uses this object as its parent. If the
alpha_sample_depth differs, it will be used in decoding the
IDAT chunk data of the Delta-PNG and subsequent Delta-PNG
datastreams; but the child object itself will retain the original sample depth,
and must also retain the "alpha sample depth" for use in decoding
subsequent Delta-PNG datastreams. The decoded alpha samples must be
scaled to the object's sample depth before the replacements or delta
calculations are done.
See the JNG specification above for the format of the JNG chunks. The PNG datastream must contain at least JHDR and IEND, but can inherit other chunk data from the parent object. Except for IDAT, any chunks appearing between JHDR and IEND are always treated as replacements or additions and not as deltas.
The IJNG chunk is empty.
The IJNG chunk can be used instead of the JHDR chunk if the JHDR chunk is not needed for resetting the value of any of the JHDR fields. The purpose of this chunk is to identify the beginning of the JNG datastream, so decoders can start interpreting JNG chunks instead of Delta-PNG chunks. The decoder must treat this datastream as though the JHDR chunk were present in the location occupied by the IJNG chunk.
The JHDR chunk can also be omitted when
image_type=2 and the JNG datastream begins with a
JDAT or JDAA chunk.
In this case, no IJNG
chunk is required, either. The decoder must treat this datastream as
though the JHDR chunk were present, immediately preceding
the first JDAT chunk. If the first JNG chunk is not a
JDAT or JDAA chunk, then either the IJNG
or JHDR chunk must be present to introduce the JNG datastream.
All chunks in the parent object with the specified name are inhibited from being copied into the child image.
The DROP chunk contains a one or more 4-byte chunk names:
Chunk_name: 4 bytes (ASCII text). etc.
Multiple DROP chunks are permitted in a Delta-PNG datastream.
If multiple names appear in the DROP chunk, it is shorthand for multiple DROP chunks.
The DBYK chunk contains one or more sequences, each containing a chunk name, polarity byte, and a keyword:
Chunk_name: 4 bytes (ASCII text).
Polarity: 1 byte (unsigned integer).
0: Only.
1: All-but.
Keywords (null-separated Latin-1 text strings).
etc.
The chunk name must be the name of a chunk whose data begins with a null-terminated text string. Some parent object chunks with the specified chunk name are inhibited from being copied into the child image. If polarity is <only>, then any parent chunk whose keyword appears in the keywords list is inhibited. If polarity is <all-but>, then any parent object chunk whose keyword does not appear in the keywords list is inhibited.
The format of the keyword is the same as that specified for the parent chunk. Comparisons of keywords in the parent chunk and the DBYK chunk are case sensitive.
Use caution when printing or displaying keywords (Refer to Security considerations, below, Chapter 17).
The ORDR chunk informs the applier of the Delta-PNG of the ordering restrictions for ancillary chunks. It contains one or more 5-byte sequences:
Chunk_name: 4 bytes (ASCII text).
Order_type: 1 byte.
0: Anywhere.
1: After IDAT and/or JDAT or JDAA.
2: Before IDAT and/or JDAT or JDAA.
3: Before IDAT but not before PLTE.
4: Before IDAT but not after PLTE.
etc.
Critical chunk names must not appear in the ORDR chunk. The applier needs to know everything about them anyway.
If a chunk name appears in the ORDR chunk, it is a promise that any chunk of that name appearing in the parent object which is not inhibited by DROP/DBYK will not be broken by this Delta-PNG, and therefore the applier must copy it into the child image at a location compatible with its ordering restrictions.
If any ancillary chunk appears in the parent object, and it is not inhibited, and its name does not appear in the ORDR chunk, then the applier should copy it into the child only if it knows the chunk well enough to be sure that it is consistent with the changes made by the Delta-PNG, and knows where it can be placed in the child. Those conditions are always true of safe-to-copy chunks.
If any critical chunk defined in neither this specification nor the PNG specification appears in the parent object or in the Delta-PNG, it is a fatal error unless the applier knows how to handle it. The specification of the critical chunk can include provisions for this scenario.
This section describes ancillary Delta-PNG chunks. MNG-compliant decoders should recognize and process them, but are not required to.
A gAMA, cHRM, iCCP, sRGB or similar chunk existing in the parent object would not affect the pixel data in a concrete object inherited by this Delta-PNG datastream because they are not used in decoding the pixel data. Applications are responsible for ensuring that the pixel values that are inherited from the parent object are the raw pixel data that existed prior to any transformations that were applied while displaying the parent image. These color transformations are applied to the resulting pixel data for display purposes.
MNG viewers must ignore oFFs and pHYs chunks that appear inside a PNG or JNG datastream or are inherited from the MNG top level. MNG editors are expected to treat them as if they were unknown copy-safe chunks.
Any other ancillary PNG chunks that the decoder recognizes when processing a PNG datastream should also be recognized and handled when processing a delta-PNG datastream. Any chunks that it does not recognize should be processed as instructed by the ORDR, DROP, and DBYK chunks. MNG viewers are free to ignore any ancillary chunks, while MNG editors should handle them in accordance with the copying rules.
End of Delta-PNG datastream. An IEND chunk must be present for each DHDR chunk in a MNG datastream. A single IEND terminates both the Delta-PNG datastream and any embedded PNG or JNG datastream within it.
The IEND chunk is empty.
The PNG specification places ordering requirements on many chunks with respect to the PLTE and IDAT chunks. If neither of these two chunks is present, and the ORDR chunk is not present, known chunks (always including all standard chunks described in the PNG specification) are considered to have appeared in their proper order with respect to the critical chunks. Unknown chunks are ordered as described above (Paragraph 6.1.12).
New public chunk types, and additional options in existing public chunks, can be proposed for inclusion in this specification by contacting the PNG/MNG specification maintainers at png-info@uunet.uu.net, png-group@w3.org, or at mng-list@ccrc.wustl.edu.
New public chunks and options will be registered only if they are of use to others and do not violate the design philosophy of PNG and MNG. Chunk registration is not automatic, although it is the intent of the authors that it be straightforward when a new chunk of potentially wide application is needed. Note that the creation of new critical chunk types is discouraged unless absolutely necessary.
Applications can also use private chunk types to carry data that is not of interest to other applications.
Decoders must be prepared to encounter unrecognized public or private chunk type codes. If the unrecognized chunk is critical, then decoders should abandon the segment, and if it is ancillary they should simply ignore the chunk. Editors must handle them as described in the following section, Chunk Copying Rules.
The chunk copying rules for MNG are the same as those in PNG, except that a MNG editor is not permitted to move unknown chunks across any of the following chunks, or across any critical chunk in a future version of this specification that creates or displays an image:
The copy-safe status of an unknown chunk is determined from the chunk name, just as in PNG. If bit 5 of the first byte of the name is 0 (Normally corresponding to an uppercase ASCII letter), the unknown chunk is critical and cannot be processed or copied. If it is 1 (usually corresponding to a lowercase ASCII letter), the unknown chunk is ancillary and its copy-safe status is determined by bit 5 of the fourth byte of the name, 0 meaning copy-unsafe and 1 meaning copy-safe.
If an editor makes changes to the MNG datastream that render unknown chunks unsafe-to-copy, this does not affect the copy-safe status of any chunks beyond the next SEEK chunk or prior to the previous one. However, if it makes such changes prior the SAVE chunk, this affects the copy-safe status of all top-level unknown chunks in the entire MNG datastream.
Changes to the MHDR chunk do not affect the copy-safe status of any other chunk.
The SAVE, SEEK, and TERM chunks are not considered to be a part of any segment. Changes to the data in the SAVE or SEEK chunks do not affect the copy-safe status of any other chunks. Adding or removing a SEEK chunk affects the copy-safe status of unknown chunks in the newly-merged or newly-separated segments. Adding, removing, or changing the TERM chunk has no effect on the copy-safe status of any chunk.
As in PNG, unsafe-to-copy ancillary chunks in the top-level MNG datastream can have ordering rules only with respect to critical chunks. Safe-to-copy ancillary chunks in the top-level MNG datastream can have ordering rules only with respect to the SAVE, SEEK, SHOW, and PAST chunks, DHDR-IEND, BASI-IEND, IHDR-IEND, JHDR-IEND sequences, or with respect to any other critical "header-end" sequence that might be defined in the future that could contain IDAT or similar chunks.
The copying rules for unknown chunks inside IHDR-IEND, BASI-IEND, DHDR-IEND, and JHDR-IEND sequences are governed by the PNG and JNG specifications, and any changes inside such sequences have no effect on the copy-safe status of any top-level MNG chunks.
The copy-safe status of chunks inside a DHDR-IEND sequence depends on the copy-safe status of the chunks in its parent object.
This section specifies the minimum level of support that is expected of MNG, MNG-LC, or MNG-VLC-compliant decoders, and provides recomendations for viewers that will support slightly more than the minimum requirements. All critical chunks must be recognized, but some of them can be ignored after they have been read and recognized. Ancillary chunks can be ignored, and do not even have to be recognized.
Anything less than this level of support requires subsetting. Applications that provide less than minimal MNG support should check the MHDR "simplicity profile" for the presence of features that they are unable to support or do not wish to support. A specific subset, in which "complex MNG features" and JNG are absent, is called "MNG-LC". In MNG-LC datastreams, bit 0 of the simplicity profile must be 1 and bits 2 and 4 must be 0. Another subset is called "MNG-VLC". In MNG-VLC datastreams, "simple MNG features" are also absent, and bit 1 must therefore also be 0.
Subsets are useable when the set of MNG datastreams to be processed is known to be (or is very likely to be) limited to the feature set in MNG-LC or MNG-VLC. Limiting the feature set in a widely-deployed WWW browser to anything less than MNG with 8-bit JNG support would be highly inappropriate.
Some subsets of MNG support are listed in the following table, more or less in increasing order of complexity.
MHDR Profile bits Profile Level of support
31-10 9 8 7 6 5 4 3 2 1 0 value
0 0 0 0 1 0 0 0 0 0 1 65 MNG-VLC without transparency
0 0 1 1 1 0 0 1 0 0 1 457 MNG-VLC
0 0 1 1 1 0 1 1 0 0 1 473 MNG-VLC with JNG
0 0 1 1 1 0 0 1 0 1 1 459 MNG-LC
0 0 1 1 1 0 1 1 0 1 1 475 MNG-LC with JNG
0 0 1 1 1 0 1 1 1 1 1 479 MNG without stored object buffers
0 1 1 1 1 0 0 1 1 1 1 975 MNG without JNG or Delta-PNG
0 1 1 1 1 0 1 1 1 1 1 991 MNG without Delta-PNG
0 1 1 1 1 1 0 1 1 1 1 1007 MNG without JNG
0 1 1 1 1 1 1 1 1 1 1 1023 or 0 MNG
0 1 1 1 1 1 1 1 1 1 1 1023 or 0 MNG with 12-bit JNG support
| | | | | | | | | |
| | | | | | | | | +- Validity
| | | | | | | | +--- Simple MNG features
| | | | | | | +----- Complex MNG features
| | | | | | +------- Transparency
| | | | | +--------- JNG
| | | | +----------- Delta-PNG
| | | +------------- Validity of bits 7,8, and 9
| | +--------------- Semitransparency
| +----------------- Background transparency
+------------------- Stored objects
One reasonable path for an application developer to follow might be to develop and test the application at each of the following levels of support in turn:
An equally reasonable development path might be
On the other hand, a developer working on an application for storing multi-page fax documents might have no need for more than "MNG-VLC without transparency".
We are allowing conformant decoders to skip twelve-bit JNGs because those are likely to be rarely encountered and used only for special purposes. There is no profile flag to indicate the presence or absence of 12-bit JNGs.
ticks_per_second must be supported by animation viewers.
The simplicity profile, frame count, layer count, and nominal play time
can be ignored. Decoders that provide less than minimal support can use
the simplicity profile to identify datastreams that they are incapable
of processing.
iteration_count must be supported. The
nest_level should be used as a sanity check but is not
required. When iteration_min=1 either explicitly or
when it is omitted and the termination_condition is not 0 or 4, the
LOOP chunk
and its ENDL chunk can be ignored (bit 2 of the simplicity profile
can be used to promise that this is true for all loops).
framing_mode and clipping parameters must be
supported. The interframe_delay must be supported
except by single-frame viewers. The sync_id and
timeout data can be ignored. Bit 1 of the simplicity profile
can be used to promise that the FRAM chunk is not present.
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported. Interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded. The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported. Alpha is not present, or can be ignored because it has no
effect on the appearance of any frame, if bit 3 of the simplicity profile is 0.
Bit 1 of the simplicity profile can be used to promise that only filter methods
defined in the PNG specification are present.
Bit 4 of the simplicity profile can be used to promise that JNG chunks are not present. Viewers that choose not to support JNG can check this bit before deciding to proceed. MNG-compliant decoders must support JNG, but MNG-LC and MNG-VLC decoders are not required to support JNG.
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported. Interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded. The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported.
image_sample_depth=8 must be supported. The JSEP
chunk must be recognized and must be used by minimal decoders to select
the eight-bit version of the image, when both eight-bit and twelve-bit versions
are present, as indicated by image_sample_depth=20 in the
JHDR chunk. When image_sample_depth=12, minimal
decoders are not obligated to display anything. Such decoders can
choose to display nothing or an empty rectangle of the width and height
specified in the JHDR chunk.
This can be done by processing the JNG as though a
viewable transparent BASI object had appeared:
BASI width height 1 4 0 0 0 0 00 00 00 00 1 IEND
MNG-compliant decoders are required to support Delta-PNG, but MNG-LC and MNG-VLC decoders are not. Bit 2 or 5 of the simplicity profile can be used to promise that Delta-PNG datastreams are not present.
The following recommendations do not form a part of the specification.
It is a good idea to use a single color space for all of the layers in an animation, where speed and fluidity are more important than exact color rendition. This is best accomplished by defining a single color space at the top level of MNG, using either an sRGB chunk or the gAMA and cHRM chunks and perhaps the iCCP chunk, and removing any color space chunks from the individual images after converting them to the common color space.
When the encoder converts all images to a single color space before putting them in the MNG datastream, decoders can improve the speed and consistency of the display.
For single-frame MNG datastreams, however, decoding speed is less important and exact color rendition might be more important. Therefore, it is best to leave the images in their original color space, as recommended in the PNG specification, retaining the individual color space chunks if the images have different color spaces. This will avoid any loss of data due to conversion.
Always use framing mode 1 or 2 when all of the images are opaque. This avoids unnecessary screen clearing, which can cause flickering.
If it
is necessary to establish a synchronization point immediately, this can
be done by using two consecutive FRAM chunks, the first setting
a temporary interframe_delay=0, timeout, and
sync_id, and the second establishing the synchronization
point:
FRAM 2 0 1 1 0 1 0000 timeout sync_id FRAM 0 name
Embedded images should not be enclosed in loops unless absolutely necessary. It is better to store them ahead of time and then use SHOW chunks inside the loops. Otherwise, decoders will be forced to repeatedly decode them. See Examples 2, 8, 11, and 12, below (Chapter 18).
Authors of MNG files that are intended for transmission over a network should consider whether it is more economical for the client to rebuild the index from scratch than it is to transmit it. Web pages that are likely to be downloaded over slow lines, and whose clients are unlikely to use the index anyway, generally should have empty SAVE chunks. No information is lost by deleting the index, because the MNG datastream contains all of the information needed to build the index. If an application does build an index, and the file is going to be kept as a local file, the application should replace the empty SAVE chunk with one containing the index. See above (Paragraph 4.4.1).
When a JNG datastream contains an alpha channel, and the file is intended for transmission over a network, it is useful to interleave the IDAT or JDAA and the JDAT chunks. In the case of sequential JPEG, the interleaving should be arranged so that the alpha data arrives more or less in sync with the color data for the scanlines. In the case of progressive JPEG, the alpha data should be interleaved with the first JPEG pass, so that all of the alpha data has arrived before the beginning of the second JPEG pass.
It is recommended that the JDAA chunk be used only to convey smoothly varying alpha channels and not to convey binary transparency which is more precisely and efficiently conveyed in IDAT chunks.
The simplicity profile in the MHDR chunk can be ignored or it can be used for
If a decoder reads an ENDL chunk for which the matching
LOOP chunk is missing, or has been skipped for some reason, any
active loops with a higher nest_level should be terminated,
and processing can resume after the next SEEK chunk. Simple
viewers that do not process the SAVE chunk should abandon the
MNG datastream. See above.
The PNG specification gives a good explanation of how to composite a partially transparent image over an opaque image, but things get more complicated when both images are partially transparent.
Pixels in PNG and JNG images are represented using gamma-encoded RGB (or gray) samples along with a linear alpha value. Alpha processing can only be performed on linear samples. This chapter assumes that R, G, B, and A values have all been converted to real numbers in the range [0..1], and that any gamma encoding has been undone.
For a top pixel {Rt,Gt,Bt,At} and a bottom pixel {Rb,Gb,Bb,Ab}, the composite pixel {Rc,Gc,Bc,Ac} is given by:
Ac = 1 - (1 - At)(1 - Ab)
if (Ac != 0) then
s = At / Ac
t = (1 - At) Ab / Ac
else
s = 0.0
t = 1.0
endif
Rc = s Rt + t Rb
Gc = s Gt + t Gb
Bc = s Bt + t Bb
When the bottom pixel is fully opaque (Ab = 1.0), the function reduces to:
Ac = 1 Rc = At Rt + (1 - At) Rb Gc = At Gt + (1 - At) Gb Bc = At Bt + (1 - At) Bb
When the bottom pixel is not fully opaque, the function is much simpler if premultiplied alpha is used. A pixel that uses non-premultiplied alpha can be converted to premultiplied alpha by multiplying R, G, and B by A.
For a premultiplied top pixel {Rt,Gt,Bt,At} and a premultiplied bottom pixel {Rb,Gb,Bb,Ab}, the premultiplied composite pixel {Rc,Gc,Bc,Ac} is given by:
Ac = 1 - (1 - At)(1 - Ab) Rc = Rt + (1 - At) Rb Gc = Gt + (1 - At) Gb Bc = Bt + (1 - At) Bb
As mentioned in the PNG specification, the equations become much simpler when no pixel has an alpha value other than 0.0 or 1.0, and the RGB samples need not be linear in that case.
The decoder must retain information about each object (except for
objects with object_id=0) for possible redisplay with the
SHOW chunk or for possible use as the parent object for a
subsequent Delta-PNG datastream.
The following information must be retained, for each nonzero object that is defined and not subsequently discarded:
target_x and
target_y, if the object was the destination of a
PAST chunk.
When the encoder knows that data in the object buffer will not be
needed later, it help decoders operate more efficiently by
using object_id=0 or by using the DISC or the
SEEK chunk. Abstract images rather than concrete objects
should be used if the encoder knows that the data will not later be used
as the parent object for a Delta-PNG. If no object buffer in the entire
datastream will be needed later, the "stored object buffers" flag
can be set appropriately in the simplicity profile field of the MHDR
chunk.
When a fatal error is encountered, such as a bad CRC or an unknown critical MNG chunk, minimal viewers that do not implement the SAVE/SEEK mechanism should simply abandon the MNG datastream. More capable MNG viewers should attempt to recover gracefully by abandoning processing of the segment and searching for a SEEK chunk. If such errors occur before the SAVE chunk is reached, the viewer should abandon the MNG datastream.
When an error occurs within a image datastream, such as an unknown critical PNG chunk or a missing parent object where one was required, only that image should be abandoned and the associated object should be discarded. If a bad CRC is found, indicating a corrupted datastream, the entire segment should be abandoned, as above.
MNG editors, on the other hand, should be more strict and reject any datastream with errors unless the user intervenes.
Decoders are required to be able to interpret datastreams that contain interlaced PNG images, but are only required to display the completed frames; they are not required to display the images as they evolve. Viewers that are decoding datastreams coming in over a slow communication link might want to do that, but MNG authors should not assume that the frames will be displayed in other than their final form.
When a PLTE chunk is received, it only affects the display of the PNG datastream that includes or inherits it. Decoders must take care that it does not retroactively affect anything that has already been decoded.
If PLTE or PPLT is present in a Delta-PNG datastream, the new palette is used in displaying the image defined by the Delta-PNG; if no IDAT chunk is present and the image type is PNG indexed-color, then the resulting image is displayed using the old pixel samples as indices into the new palette, which provides a "palette animation" capability.
If a frame contains two or more images, the PLTE chunk in
one image does not affect the display of the
other, unless one image
is a subsequent Delta-PNG that has no PLTE chunk and has been
declared by the DHDR object_id field to depend on the
other.
A composite frame consisting only of indexed-color images should not be assumed to contain 256 or fewer colors, since the individual palettes do not necessarily contain the same set of colors. Encoders can supply a top-level sPLT chunk with a suggested reduced global palette, to help decoders build an appropriate palette when necessary.
Viewers that can only display a single frame must display the first frame that they encounter. It is strongly recommended that such viewers understand the fPRI chunk above (Paragraph 4.5.2), which will enable them to find and display the best representative frame, if the encoder has identified one.
MNG provides four types of clipping, in addition to any clipping that might be required due to the physical limitations of the display device.
frame_width and frame_height are defined in
the MHDR chunk and cannot be changed by any other MNG chunk.
This is the only type of clipping available in MNG-VLC datastreams.
Decoders can use these parameters to establish the size of
a window in which to display the MNG frames. When the frame_width
or frame_height exceeds the physical dimensions of the
display hardware, the contents of the area outside those dimensions is
undefined. If a viewer chooses, it can create "scroll bars" or the
like, to enable persons to pan and scroll to the offscreen portion
of the frame. If this is done, then the viewer is responsible for
maintaining and updating the offscreen portion of the frame.
In the case of a MNG datastream that consists of a PNG or JNG
datastream, with the PNG or JNG signature, the frame_width
and frame_height are defined by the width and
height fields of the IHDR (or JHDR) chunk.
frame_width and frame_height)
will remain on the display from frame to frame without being explicitly
redisplayed.
See Example 8,
which displays a
large background image once, and then, in each frame, only redisplays
the portion of the background surrounding the moving sprite.
The clipping boundaries are expressed in pixels, measured rightward and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and bottom clipping boundaries are exclusive, i.e., the pixel located at {x,y} is only displayed if the pixel falls within the physical limits of the display hardware and all of the following are true:
0 <= x < frame_width (from the MHDR chunk) 0 <= y < frame_height Left_lcb <= x < right_lcb (from the FRAM chunk) Top_lcb <= y < bottom_lcb Left_cb <= x < right_cb (from the DEFI or CLIP chunk) Top_cb <= y < bottom_cb
Editors must recreate or delete the optional SAVE chunk index whenever they make any change that affects the offsets of chunks following the portion of the datastream that is changed. If the changes do not involve the addition, deletion, or relocation of segments, frames, and images, then it is sufficient to zero out the offsets.
The SAVE chunk is not considered to be in any MNG segment, so changing it has no effect on the copy-safe status of unknown chunks in any other part of the MNG datastream.
When the SAVE chunk is expanded to include an index, all chunks that follow will have their offsets changed by an amount equal to the change in the length of the data segment of the SAVE chunk, so the offset table will have to be adjusted accordingly. If a SAVE chunk is already present with zero offsets, the correct offsets can be written without adjustment.
Editors that create a series of PNG or JNG datastreams from a MNG datastream
should check the termination condition of any LOOP chunks and execute
loops only iteration_min times. The loop created by the TERM
chunk should be executed only once.
On systems where file names customarily include an extension
signifying file type, the extension .mng is recommended for
MNG (including MNG-LC and MNG-VLC)
files. Lowercase .mng is
preferred if file names are case-sensitive. The extension .jng is
recommended for JNG files.
When and if the MNG format becomes finalized, the
MNG authors intend to register video/mng
as the Internet Media Type for
MNG [RFC-2045],
[RFC-2048].
At the date of this document, the media
type registration process had not been started. It is recommended
that implementations also recognize the interim media type
video/x-mng.
Although we define a standalone JNG format, we recommend that such
files be used only temporarily while compiling or disassembling MNG
datastreams. We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
image/x-jng can be used.
An Extended Backus-Naur Form (EBNF) description of the chunk ordering in MNG, PNG, and JNG is being developed. The current draft, together with some supporting software, is available at ftp://swrinde.nde.swri.edu/pub/mng/documents/ebnf/.
Segments, subframes, and objects are externally accessible via named SEEK, eXPI, and FRAM chunk names. They can be referred to by URI, as in
SRC=file.mng#segment_name SRC=file.mng#subframe_name SRC=file.mng#snapshot_name SRC=file.mng?segment_name#segment_name SRC=file.mng?snapshot_name#snapshot_name
When the URI specializer ("#" or "?") is "#", and the fragment identifier (the string following the specializer) is the name of a segment, i.e., a named SEEK chunk, the viewer should display the sequence from the beginning of the named segment up to the next segment. When it refers to a subframe or an image, i.e., a named FRAM or eXPI chunk, it should display the single frame (as it exists when the next FRAM chunk is encountered) or image that is identified by the fragment identifier. The client can find the needed segment quickly if the SAVE chunk is present and contains the optional index.
When the URI specializer is "?" (server side query), the "query component" is the string following the "?" specializer and up to but not including the "#" if the "#" specializer is also present. The server should find the segment that is named in the query component or the segment that contains the subframe or image named in the query component, and it should return a datastream consisting of:
If no SAVE chunk is present, the server must simply return the entire MNG datastream. Servers that are unwilling to parse the MNG datastream and are unconcerned about bandwidth can return the entire MNG datastream even when the SAVE chunk is present. Authors should defend against this behavior by including both a query and a fragment in the URI even when a segment is being requested.
The client can process this as a complete MNG datastream, either displaying the entire segment, if no fragment identifier is present, or extracting the segment, frame or image that is named in a fragment identifier and displaying it, if a fragment identifier is present (a fragment identifier must be present if a frame or image is being requested). To "extract a frame" means to decode the returned datastream through the end of the frame that contains the named subframe and to display the result as a single still image. If the layers of the named subframe do not cover the entire frame, pixels from the background and from earlier subframes must be included in the resulting composition.
A part of the MNG datastream can also be requested by timecode, as in
SRC=file.mng#clock(10s-20s) SRC=file.mng#clock(0:00-0:15) SRC=file.mng?clock(0:00-0:15)#clock(0:00-0:15)
or by frame number, as in
SRC=file.mng#frame(10) SRC=file.mng#frames(30-60) SRC=file.mng?frames(30-60)#frames(30-60)
The timecode must consist of starting and ending clock values, as defined in the W3C SMIL recommendation, separated by a hyphen (ASCII code 45).
When the URI specializer is "#", the viewer should play that part of the sequence beginning and ending at the requested times, measuring from zero time at the beginning of the MNG datastream, or beginning and ending with the specified frame numbers. To do this it must start with the segment containing the requested time and decode any part of the segment up to that time, composing but not displaying the frames; this will provide the background against which the desired frames are displayed.
When the URI specializer is "?", the server can send the entire MNG datastream, or, preferably, it should construct a complete MNG file containing:
If the server does not send the entire MNG datastream, and the first segment after the SAVE chunk is not sent but a later segment is sent, the optional index must be written even if it does not exist in the source file. The index must contain at least one "type 0" entry that gives the nominal start time and frame number for the first segment that is sent after the SAVE chunk. The offset field can be set to zero and the segment name can be omitted.
The query component should always be repeated as a fragment identifier, so clients can find the requested item in case the server sends more than what was requested.
MNG datastreams should not contain segment, subframe, or image names that begin with the case-insensitive strings "CLOCK(", "FRAME(", or "FRAMES(", which are reserved for use in URI queries and fragments (see Uniform Resource Identifier below).
See [RFC-2396] and the W3C SMIL recommendation at http://www.w3.org/TR/.
This (incomplete as of version 0.999) section does not form a part of the specification. It provides the rationale behind some of the design decisions in MNG.
Explain why the interframe delay has to be provided before the subframes of layers are defined, instead of having a simpler DELA chunk that occurs in the stream where the delay is wanted.
Some delta types are not allowed when the parent object is a JNG image. Explain why types 4 and 6 (pixel replacement and color channel replacement) are not allowed under these circumstances.
Filter method 64 could have been implemented as a new critical chunk in embedded PNG datastreams.
FILT
method (1 byte)
64: intrapixel differencing
data (variable, depends on method)
method 64 requires no data
The FILT chunk would turn on this type of filtering.
The choice of using a new filter method instead of a new critical chunk was made based on simplicity of implementation and possible eventual inclusion of this method in PNG. Also, using the filter-method byte helps implementors avoid confusion about whether this is a color transform (which could affect the implementation of tRNS and other color-related chunks) or part of the filtering mechanism (which would not conceivably affect color-related chunks).
We considered using an ancillary chunk (e.g., fILt or fILT) to turn on the new filtering method. This would have the advantage that existing applications could manipulate the files, but viewers that ignore the chunk would display the image in unacceptably wrong colors, and editors could mistakenly discard the chunk.
Q. Why not just use a BASI chunk to encode solid-color rectangles?
A. The MAGN chunk also allows encoding of gradient-filled rectangles.
Q. Why not just use PNG to encode gradient-filled rectangles?
A. While PNG can encode vertical and horizontal gradients fairly efficiently, it cannot do diagonal ones efficiently, and none are as efficient as a 30-byte MAGN chunk plus a 4-pixel PNG.
Q. Why not use full-scale low-quality JPEG/JNG?
A. Low-quality JPEG with reduced dimensions can be much smaller than even the lowest-quality full-sized JPEG. Such images can then be magnified to full scale with the MAGN chunk, for use as preview ("LOWSRC") images. this has been demonstrated to be about 40 to 50 times as efficient as using Adam7 interlacing of typical natural images,
It appears that in general, usable preview images of truecolor photographic images can be made at compression ratios from M*800:1 to M*2500:1, where M is the number of megapixels in the original image, by reducing the original image spatially to width and height in the range 64 to 200 pixels and then compressing the result to a medium-quality JNG.
Q. Why not use the pHYg chunk?
A. It is not mandatory for decoders to process the pHYg chunk and it does not apply to individual images; it is used to scale the entire MNG frame. The pHYs chunk cannot be used either because MNG decoders are required to ignore it.
Q. Why not 4-byte magnification factors instead of 2-byte ones?
A. Encoders can start with a larger object or, except for object 0, magnify it twice.
Q. Why not 1-byte magnification factors, then?
A. With typical screen widths currently 1280 or 1600 pixels and film and printer pages currently about 3000 pixels wide, magnifying a 1x1 image to a width of more than 255 pixels would not be uncommon.
Q. I want to magnify a "frozen" object.
A. You can make a full clone and magnify that.
Q. Why define Methods 4 and 5?
A. Method 4 is useful for magnifying an alpha-encoded image while maintaining binary transparency. Method 5 is useful for making an alpha-gradient while preserving sharp edges in the main image.
It has been suggested that a new global MNG chunk, JTAB, be defined to hold global JPEG quantization and Huffman tables that could be inherited by JNG datastreams from which these have been omitted. This has not been tested, and we are reluctant to add new critical chunks to the MNG specification now.
Released 27 January 2001
Released 10 December 2000
Released 01 October 2000
image_sample_depth != 20
Released 28 February 2000.
Released 18 July 1999.
The changes that are not simple editorial changes were approved by votes of the PNG Development group that closed 16 July 1999 (pHYg and change to treatment of the pHYs chunk), 14 July 1999 (global bKGD and sBIT) and 25 June 1999 (change to LOOP chunk and treatment of the DEFI chunk and nonviewable objects).
termination_condition byte of the LOOP chunk was
extended to include a "cacheable" bit.
JDAT_sample_depth" to
"image_sample_depth" and
"IDAT_sample_depth" to
"alpha_sample_depth", etc.
See also Pennebaker, William B., and Joan L. Mitchell, "JPEG : Still Image Data Compression Standard" Van Nostrand Reinhold, ISBN:0442012721, September 1992
See also the PNG-1.2 specification:
Randers-Pehrson, G., et. al., "PNG (Portable Network Graphics
Format) Version 1.2", which is available at
ftp://swrinde.nde.swri.edu/pub/png/documents/.
Security considerations are addressed in the PNG specification.
An infinite or just overly long loop could give the appearance
of having locked up the machine, as could an unreasonably long
interframe delay or a misplaced sync_id with a long
timeout value. Therefore a decoder should always
provide a simple method for users to escape out of a loop or delay,
either by abandoning the MNG entirely or just proceeding to the next
SEEK chunk. Decoders should check for user input after
each loop iteration (not just after each frame) in case of infinite loops
that are empty or that generate layers with zero interframe delay.
The SEEK chunk makes it safe for a
viewer to resume processing after it encounters a corrupted portion of a
MNG datastream or jumps out of the interior of a segment for any reason.
Some people may experience epileptic seizures when they are exposed to certain kinds of flashing lights or patterns that are common in everyday life. This can happen even if the person has never had any epileptic seizures. All graphics software and file formats that support animation and/or color cycling make it possible to encode effects that may induce an epileptic seizure in these individuals. It is the responsibility of authors and software publishers to issue appropriate warnings to the public in general and to animation creators in particular.
No known additional security concerns are raised by this format.
We use the "#" character to denote commentary in these examples; such comments are not present in actual MNG datastreams.
The simplest MNG datastream is a single-image PNG datastream. The simplest way to create a MNG from a PNG is:
copy file.png file.mng
The resulting MNG file looks like:
\211 P N G \r \n ^z \n # PNG signature. IHDR 720 468 8 0 0 0 0 # Width and Height, etc. sRGB 2 gAMA 45455 IDAT ... IEND
If file.png contains an sRGB chunk and also gAMA and cHRM chunks that are recommended in the PNG specification for "fallback" purposes, you can remove those gAMA and cHRM chunks from file.mng because any MNG viewer that processes the gAMA chunk is also required to recognize and process the sRGB chunk, so those chunks will always be ignored. Any MNG editor that converts the MNG file back to a PNG file is supposed to insert the recommended gAMA and cHRM chunks.
This example demonstrates a very simple movie, such as might result from directly converting an animated GIF that contains a simple series of full-frame images:
\212 M N G \r \n ^z \n # MNG signature.
MHDR 256 300 # Width and height.
1 # 1 tick per second.
5 4 4 # Layers, frames, play time
7 # Simplicity profile
DEFI 1 0 0 IHDR ... IDAT ... IEND # Four PNG datastreams
DEFI 2 0 0 IHDR ... IDAT ... IEND # are read and stored
DEFI 3 0 0 IHDR ... IDAT ... IEND # and are displayed as
DEFI 4 0 0 IHDR ... IDAT ... IEND # they are read.
SAVE # This is needed so we can place TERM before SEEK.
TERM 3 0 120 10 # When done, repeat from TERM 10 times.
SEEK
SHOW
MEND
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 1 # Width and height, 1 tick per second.
6 5 5 # Layers, frames, play time.
1 # Simplicity profile (MNG-VLC)
FRAM 1 0 2 2 0 2 1 600 0 # Set interframe_delay to 1,
# timeout to 600 sec, and sync_id list to {0}.
SAVE
SEEK "Briefing to the Workforce"
IHDR ... IDAT ... IEND # DEFI 0, visible, abstract
SEEK "Outline" # is implied.
IHDR ... IDAT ... IEND
SEEK "Our Vision" IHDR ... IDAT ... IEND
SEEK "Our Mission" IHDR ... IDAT ... IEND
SEEK "Downsizing Plans" IHDR ... IDAT ... IEND
MEND
This slideshow gives exactly the same output as Example 3, but the storage in the datastream is more efficient (the IDAT chunks will be smaller) while the memory requirements in the decoder are larger. Image ID 1 is used to store the ornate logos and frame design that appear on every slide. The DHDR-IEND datastreams only contain deltas due to the text and other information that is unique to each slide.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
1 6 5 5 15 # 1 tick per second, complex, no JNG.
DEFI 1 1 1 # Define image 1, invisible, concrete.
IHDR ... IDAT ... IEND
FRAM 1 0 2 2 0 2 1 600 0 # set interframe_delay to 1,
# timeout to 600 sec and sync_id list to {0}.
SAVE
SEEK "Briefing to the Workforce"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Outline"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Vision"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Mission"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Downsizing Plans"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
MEND
This movie is still fairly simple, but it capitalizes on frame-to-frame similarities by use of Delta-PNG datastreams, and also demonstrates the use of the fPRI chunk.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
30 6 5 15 # 30 ticks per second.
47 # Delta-PNG, transparent, complex
tEXtTitle\0Sample Movie
fPRI 0 128 # Default frame priority is "medium".
FRAM 1 0 2 0 0 0 3 # Set interframe_delay to 1/10 sec.
DEFI 1 0 1 # Set default image to 1 (concrete).
SAVE
SEEK "start"
IHDR 720 468 8 2 0 0 0 # DEFI 1 is implied.
IDAT ...
IEND
DHDR 1 1 1 20 30 100 220 # A PNG-delta frame.
IDAT ... # The IDAT gives the 20x30 block
IEND # of deltas.
DHDR 1 1 1 20 30 102 222 # Another PNG-delta frame.
IDAT ... # This time the deltas are in a 20 x 30
IEND # block at a slightly different location.
SEEK "frame 3" # OK to restart here because a
# complete PNG frame follows.
fPRI 0 255 # This is the representative frame that
IHDR 720 468 ...# will be displayed by single-frame
IDAT ... # viewers.
IEND
fPRI 0 128 # Return to medium frame priority.
DHDR 1 1 1 720 468 0 0 # Another PNG-delta frame.
IDAT ... # The entire 720x468 rectangle changes
IEND # this time.
SEEK "end"
MEND # End of MNG datastream.
Here is an example single-composite-frame MNG, with thumbnails, which takes a grayscale image and draws it side-by-side with a false-color version of the same image:
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 512 0 # Width, height, ticks per second
4 1 0 47 # Layers, frames, time, simplicity
BACK 16448 16448 52800 1 # Must use sky blue background.
PLTE ... # Define global PLTE
gAMA 50000 # Define global gAMA
DEFI 1 1 # Define invisible abstract thumbnail image.
IHDR 64 64 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 1"
DEFI 1 1 # Also define a larger thumbnail.
IHDR 96 96 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 2"
DISC # Discard the thumbnail image.
FRAM 4 "Two views of the data"
DEFI 1 0 1 6 6 # Define first (bottom) image.
IHDR 500 500 16 0 .. # A 16-bit graylevel image.
IDAT ...
IEND # End of image.
CLON 1 2 0 1 0 0 518 6 # Make full invisible concrete clone.
SHOW 2 2 3 # Mark it for immediate display during
# the upcoming delta-PNG operation.
DHDR 2 1 7 # Modify it (no change to pixels).
ORDR faLT 2 # Establish chunk placement.
gAMA 100000 # Local gamma value is 100000 (gamma=1.0).
tEXtComment\0The faLT chunk is described in ftp://swrinde...
faLT ... # Apply pseudocolor to parent image.
IEND # End of image.
DEFI 3 0 0 900 400 # Overlay near lower right-hand corner.
IHDR 101 101 2 3 ...
PLTE ... # Use a local PLTE and global gAMA.
tRNS ... # It is transparent (maybe a logo).
IDAT ... # Note that the color type can differ
IDAT ... # from that of the other images.
IEND # End of image.
MEND # End of MNG datastream.
Here is another movie, illustrating the use of Delta-PNG datastreams as sprites:
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 47 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame
# sets interframe_delay=3 ticks.
DEFI 1 # Define image 1 (abstract, LOCA 0 0).
IHDR 512 512 ... # It is a full-display PNG image.
etc # Chunks according to PNG spec.
IEND # SHOW 1 is implied by DEFI 1.
DEFI 2 0 1 300 200 # Define image 2, concrete.
IHDR 32 32 ... # It is a small PNG.
gAMA 50000
IDAT ...
IEND
FRAM 0 "frame 2" # Start new frame.
# New location for image 1 is still 0,0.
SHOW 1 # Display image 1 from previous frame.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
SHOW 2 # Retrieve image 2 from previous frame,
CLON 2 3 0 1 0 # make a full clone of it as image 3.
0 400 500 # Location for image 3.
DHDR 3 1 7 0 0 0 0 # Modify image 3 (no change to pixels).
tRNS ... # Make it semitransparent.
IEND # SHOW 3 is implied by CLON visibility.
FRAM 0 "frame 3" # Next frame (repeat this FRAM-SHOW 1 3
# sequence with different locations to
# move the images around).
# New location for image 1 is still 0,0.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
MOVE 3 3 1 5 -2 # New location for image 3.
SHOW 1 3 # Show images 1 through 3.
FRAM 0 "frame 4" # Another frame.
etc.
FRAM 0 "frame 99"
etc. # More frames.
MEND # End of MNG datastream.
This movie illustrates the use of several abstract images with Show_mode=6 to describe an animated sprite, and the PAST chunk to turn it around. The sprite runs back and forth across the background ten times. The FRAM clipping boundaries restrict the screen updates to the small region that changes, with a little "wiggle room" to make sure the disturbed part of the background gets updated.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 15 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame.
DEFI 1 IHDR 512 512 ... # Background PNG image.
etc ... IEND # Chunks according to PNG spec.
DEFI 10 1 0 x0 y0 # Static part of sprite.
IHDR 64 64 ... IDAT ... IEND
DEFI 11 1 0 x0 y1 # View 1 of animated part.
IHDR 64 32 ... IDAT ... IEND # (y1=y0+64)
DEFI 12 1 0 x0 y1 # View 2 of animated part.
IHDR 64 32 ... IDAT ... IEND
DEFI 13 1 0 x0 y1 # View 3 of animated part.
IHDR 64 32 ... IDAT ... IEND
FRAM 0 0 0 0 2 0 0 x0-dx x0+64+dx y0-dy y1+32+dy
LOOP 0 0 10
LOOP 1 0 150
FRAM 0 "left-to-right" 0 0 2 0 1 dx dx dy dy
MOVE 10 13 1 dx dy # Move animated icon {dx, dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
FRAM SHOW 1
PAST 10 0 0 0 10 1 4 0 0 0 0 0 64 64
PAST 11 0 0 0 11 1 4 0 0 0 0 0 64 32
PAST 12 0 0 0 12 1 4 0 0 0 0 0 64 32
PAST 13 0 0 0 13 1 4 0 0 0 0 0 64 32
LOOP 1 0 150
FRAM 0 "right-to-left" 0 0 2 0 1 -dx -dx -dy -dy
MOVE 10 13 1 -dx -dy # Move animated icon {-dx, -dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
ENDL 0 FRAM
MEND
The opaque parts of this image will "fade in" gradually. This example also illustrates the use of the PPLT and fPRI chunks.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 64 64 30 0 0 0 47 # Width, height, ticklength, ....
BACK 52800 52800 52800 # "Browser gray" default background.
FRAM 3 0 2 0 0 0 3 # Set interframe_delay=3 ticks. Use
# framing mode 3 so background gets restored.
DEFI 1 1 1 # Invisible and "concrete".
IHDR ... # PNG header.
PLTE ...
tRNS 0 # Entries are zero for the transparent (0)
# color and 255 for the nontransparent ones.
IDAT ...
IEND
fPRI 0 0 # Give the fade-in sequence a low priority.
CLON 1 2 # Make a working concrete copy of the image
# that will be modified during the low-priority
# part of the datastream. It is a full clone.
DHDR 2 1 7 # No change to pixel data.
tRNS 0 0 0 0 0 0 ... # Make all pixels fully transparent.
IEND
SHOW 2 2 3 # Make it visible but do not show it now.
LOOP 0 0 15
DHDR 2 1 7 # A Delta-PNG.
# Delta-type 7 means no change to pixels.
PPLT 1 10 3 16 16 16 16 ... # Increment all alphas except
IEND # for entry 0 by 16.
SHOW 2
ENDL 0 # Nontransparent pixel alpha=15, 31, ... 240.
DISC 2 # Discard the working copy.
fPRI 0 255 # Give the final frame the highest value
FRAM 0 0 1 0 0 0 60 # Hold the last frame for at least
# 60 ticks (2 sec). Applications might show it longer.
SHOW 1 # This copy still has alpha=255 for the
# opaque pixels and alpha=0 for the others.
MEND # End of MNG.
In this example, we store a series of twenty-four 150 x 150 x 150 blocks of eight-bit voxels. Each block is stored as a composite frame with the first image being a PNG whose pixels represent the top layer of voxels, which is followed by 149 Delta-PNG images representing the rest of the layers of voxels. Only one image is defined, through which the parent image is passed along from PNG to Delta-PNG to Delta-PNG. This example also illustrates the use of unregistered ancillary chunks that describe the x, y, and z scales and pixel calibration.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 150 150 1 # Width, height, ticklength.
0 0 0 47 # Layers, frames, time, simplicity.
tEXtTitle\0Weather modeling results
tEXtComment\0The xxSC, yySC, zzSC, and ttSC chunks
in this file are written according to the Proposed
Chunk Specifications version 19970203 and Sci-Vis
Chunks Specification version 19970203 available at
ftp://swrinde.nde.swri.edu/pub/png-group/documents/
xxSC kch\0 [sig\0] kilometers\0 0\0 150
yySC kch\0 [sig\0] kilometers\0 0\0 150
zzSC kch\0 [sig\0] Height (kilometers)\0 0\0 15
ttSC kch\0 [sig\0] Time (hours)\0 0\0 24
pCAL kch\0 0 255 0 2 Degrees Celsius\0 0\0 45
DEFI 1 0 1 # All images will have image = 1
SAVE # and be visible and "concrete".
SEEK
FRAM 2 # Initial composite image.
IHDR 150 150 16 # Width, height, bit depth for top layer.
0 0 0 0 # Color, comp, filter, interlace.
IDAT ...
IEND # No DEFI chunk, so it is image 0.
DHDR 1 1 0 # Source=0, PNG, pixel addition,
150 150 0 0 # Block is entire image.
IDAT ... # IHDR is omitted; everything matches top.
IEND # IEND is also omitted.
etc. # Repeat DHDR through IEND 148 more times.
SEEK
FRAM # End of first block.
etc. # Repeat FRAM through SEEK 19 more times.
SEEK
MEND # End of MNG.
Here is another composite frame, illustrating the use of the LOOP syntax to tile a large (1024 by 768) image area with a small (128 by 64) image.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
98 1 0 15 # Layers, frames, time, simplicity.
FRAM 2
DEFI 1 1 0 0 -64 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
LOOP 0 0 12 # Y loop -- make 12 rows of tiles.
MOVE 1 1 1 0 64 # Move the first copy down 64 rows.
SHOW 1 # Display it.
CLON 1 2 1 # Create a partial clone of the tile.
LOOP 1 0 7 # X loop - 7 additional columns.
MOVE 2 2 1 0 128 # Move it to the right 128 columns.
SHOW 2 # Use the second copy.
ENDL 1
ENDL 0
MEND
Here is a better approach, which creates a reusable tiled image by means of the PAST chunk.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
3 1 0 15 # Layers, frames, time, simplicity.
DEFI 1 1 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
DEFI 2 # The abstract, visible, viewable image to
BASI 1024 768 8 2 0 0 0 0 0 0 0 1 # be tiled. Initially
IEND # all pixels are zero.
PAST 2 0 0 0 # Destination and target location.
# src mod orient offset clipping
1 0 8 0 0 512 0 0 1024 0 768
# End of PAST chunk data.
MEND
Here is an example of scrolling a 3000-line-high image (perhaps an image of some text, but could be anything) through a 256-line-high window with an alpha-blended border.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 256 30 # Width, height, ticks per second
6513 3257 3257 15 # Layers, frames, time, simplicity.
BACK 50000 50000 50000 0 # advisory gray background
DEFI 1 1 0 0 256 # Define image 1 but do not display now.
# Initially it is offscreen, just
# below the 512 by 256 window.
IHDR 512 3000 1 0 ... # A PNG datastream containing the
PLTE ... # text (or whatever) to be scrolled.
IDAT ...
IEND
DEFI 2
IHDR 512 256 8 6 ... # A PNG datastream containing some kind
PLTE ... # of alpha-blended border that is
tRNS ... # transparent in the center.
IDAT ...
IEND
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack image 1 up one scanline, 3256 times.
# It ends up just above the 512 by 256 window.
# The border does not move.
FRAM 1 0 2 0 0 0 0 # Interframe_delay = 0 ticks.
# We use Framing_mode=1 to avoid unnecessary
# screen clearing between frames.
SHOW 1 # Show first image and continue without delay.
FRAM 1 0 2 0 0 0 1 # Interframe_delay = 1 tick.
SHOW 2 # Composite second image over first, wait 1 tick.
ENDL 0
MEND
Alternatively, we can declare the scrolling object to be the background and use framing-mode 3:
(Same as above down to the LOOP chunk.)
BACK 50000 50000 50000 2 1 # Advisory gray background.
# Mandatory image background.
FRAM 3 0 2 0 0 0 1 # Interframe_delay = 1 tick.
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack background up one scanline, 3256 times.
SHOW 2 # Composite the second image over it, wait 1 tick.
ENDL 0
MEND
This demonstrates the use of the SHOW chunk with
show_mode=6 to create animations that cycle through
a series of ten objects.
This will cycle through the ten objects in the forward direction, 100 times, unless terminated sooner by the user or the decoder.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 30 # Width, height, ticks per second
11 1001 1001 7 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
LOOP 0 100 6 # 100 iterations, user-discretion, cacheable
SHOW 1 10 6
ENDL 0
MEND
This will cycle through the ten objects, back and forth, 50 times, unless terminated sooner by the user or the decoder.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 6 # Width, height, ticks per second
11 901 901 7 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
CLON 11 9 1 # Make partial clones of objects 2-9
etc. # in reverse order, as objects 11-18.
CLON 18 2 1
LOOP 0 50 6 # 50 iterations, user-discretion, cacheable
SHOW 1 18 6
ENDL 0
MEND
Outline of a program to convert GIF animations to MNG format:
begin
write "MHDR" chunk
saved_images := 0; Interframe_delay := 0
First_frame := TRUE
if(loops>1) "write TERM 3 0 0 loops" chunk
write "BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 4 0 2 2 0 2 0 interframe_delay 0" chunk
First_frame := FALSE
else if(First_frame == TRUE)then
write "FRAM 4" chunk
First_frame := FALSE
else
write "FRAM" chunk
endif
if(X_loc == 0 AND Y_loc == 0) then
write "DEFI saved_images 1 1" chunk
else
write "DEFI saved_images 1 1 X_loc Y_loc" chunk
write "<image>"
write "SHOW 0 saved_images" chunk
if (gif_disposal_method == 0
OR gif_disposal_method == 2) then
/* (undefined or restore background) */
write "DISC" chunk
saved_images := 0
else if (gif_disposal_method == 1) then
/* (keep) */
saved_images := saved_images + 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
write "DISC saved_images" chunk
endif
endfor
write "FRAM" and "MEND" chunks
end
Where "<image>" represents a PNG or Delta-PNG containing a GIF frame converted to PNG format.
Caution: if you write such a program, you might have to pay royalties in order to convey it to anyone else.
Outline of a program to convert simple GIF animations that do not use the "restore-to-previous" disposal method to "simple" MNG (or "MNG-LC") format:
begin
write "MHDR" chunk
Interframe_delay := 0; Previous_mode := 1
Framing_mode := 1
if(loops>1) "write TERM 3 0 0 loops"
write "mandatory BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 0 0 2 2 0 2 0 interframe_delay 0"
endif
if(X_loc != 0 OR Y_loc != 0) then
write "DEFI 0 0 0 X_loc Y_loc" chunk
endif
write "<image>"
if (gif_disposal_method < 1) then
/* (none or keep) */
Framing_mode := 1
else if (gif_disposal_method == 2) then
/* (restore background) */
write "FRAM 4 0 1 0 1 0 0 L R T B"
Previous_mode := 4; Framing_mode := 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
error ("can't do gif_disposal method = previous.")
endif
if(Framing_mode != Previous_mode) then
write "FRAM Framing_mode" chunk
Previous_mode := Framing_mode
endif
end
write "MEND" chunk
end
Where "<image>" represents a PNG datastream containing a GIF frame that has been converted to PNG format.
Caution: if you write such a program, you might have to pay royalties in order to convey it to anyone else.
This demonstrates the determination of the layer count and frame count that should be written in the MHDR chunk. For framing_modes 1 and 2, the FRAM chunks themselves do not generate layers. For framing_modes 3 and 4, they do generate layers ("B" for background), and also generate frames if there is no embedded image with which to combine the background layer. Note that every framing_mode creates a "B" layer at the beginning.
Given the following chunk stream:
MHDR sRGB Fn F I I I F F I I I F F I I I MEND
in which
Fn represents a FRAM chunk with framing_mode n F represents an empty FRAM chunk; I represents an embedded image
This table shows the layer count and frame count for each of the four possible values of framing-mode:
Framing Layer count Frame count
mode
1 B,I,I,I, I,I,I, I,I,I = 10 BI,I,I, I,I,I, I,I,I = 9
2 B,I,I,I, I,I,I, I,I,I = 10 BIII,III,III = 3
3 3*(B, B,I, B,I, B,I) = 21 3*(B,BI,BI,BI) = 12
4 3*(B,B,I,I,I) = 15 B,BIII,B,BIII,B,BIII = 6
Here is an example of storing a library of icons in a MNG-LC datastream. All of the icons use the same palette, transparency, and colorspace, so these are put in global chunks at the beginning. Empty PLTE chunks in the embedded images are used to import the global palette and transparency data.
MHDR 96 96 1 6 5 5 11 # Profile 11 is MNG-LC sRGB 2 # Global sRGB PLTE ... # Global PLTE tRNS 0 # Global tRNS eXPI 0 "thumbnail" IHDR 32 32 ... PLTE IDAT ... IEND eXPI 0 "left arrow" IHDR 96 96 ... PLTE IDAT ... IEND eXPI 0 "right arrow" IHDR 96 96 ... PLTE IDAT ... IEND eXPI 0 "up arrow" IHDR 96 96 ... PLTE IDAT ... IEND eXPI 0 "down arrow" IHDR 96 96 ... PLTE IDAT ... IEND MEND
This is similar, but it uses Delta PNG datastreams to create modified versions by replacing the palette. This can be more storage-efficient, but requires a full MNG decoder because of the presence of Delta PNG datastreams.
MHDR 96 96 1 6 5 5 47 # Profile 47 is MNG without JNG sRGB 2 # Global sRGB PLTE ... # Global PLTE tRNS 0 # Global tRNS eXPI 0 "thumbnail" IHDR 32 32 ... PLTE IDAT ... IEND SAVE <index> SEEK "left arrows" DEFI 1 IHDR 96 96 ... PLTE IDAT ... IEND eXPI 1 "red left arrow" DHDR 1 1 7 PPLT ... IEND # Change some palette entries. eXPI 1 "blue left arrow" SEEK "right arrows" IHDR 96 96 ... PLTE IDAT ... IEND eXPI 1 "red right arrow" DHDR 1 1 7 PPLT ... IEND eXPI 1 "blue right arrow" MEND
This demonstrates the methods used in the MAGN chunk.
Original 3x2 object or embedded image:
1 9 1 9 17 9
Magnification method 1, XM = 5, YM = 3. Replicates each pixel 4 additional times in the X direction and 2 additional times in the Y direction; new size is 15x6:
1 1 1 1 1 9 9 9 9 9 1 1 1 1 1 1 1 1 1 1 9 9 9 9 9 1 1 1 1 1 1 1 1 1 1 9 9 9 9 9 1 1 1 1 1 9 9 9 9 9 17 17 17 17 17 9 9 9 9 9 9 9 9 9 9 17 17 17 17 17 9 9 9 9 9 9 9 9 9 9 17 17 17 17 17 9 9 9 9 9Magnification method 2, XM = 8, YM = 4. Fills the X intervals with 7 new pixels and the Y interval with 3 new pixels and interpolates to get pixel values; new size is 17x5:
1 2 3 4 5 6 7 8 9 8 7 6 5 4 3 2 1 3 4 5 6 7 8 9 10 11 10 9 8 7 6 5 4 3 5 6 7 8 9 10 11 12 13 12 11 10 9 8 7 6 5 7 8 9 10 11 12 13 14 15 14 13 12 11 10 9 8 7 9 10 11 12 13 14 15 16 17 16 15 14 13 12 11 10 9
Magnification method 3, XM = 8, YM = 4 Fills the X intervals with 7 new pixels and the Y interval with 3 new pixels replicating the closest pixel to get pixel values; new size is 17x5:
1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1 1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1 1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1 9 9 9 9 9 17 17 17 17 17 17 17 17 9 9 9 9 9 9 9 9 9 17 17 17 17 17 17 17 17 9 9 9 9
This example demonstrates the use of MNG to display a region of interest (ROI) at a higher quality than the rest of the frame, and the MAGN chunk to convey a highly-compressed but very lossy image, a drop shadow, and a diagonal gradient background.
MHDR 600 600 0 5 1 0 19 # Gradient background MAGN 00 00 2 599 sRGB 1 IHDR IDAT IEND <dblue2x2.png> # 93 bytes # Drop shadow DEFI 0 0 0 52 52 BASI 512 512 1 4 0 0 0 51 51 51 153 1 IEND # Grey-Alpha object, 46 bytes # Main image, with most of the region of interest # replaced with a solid rectangle, and reduced to # 128x128 dimensions, low quality JPEG compression. DEFI 0 0 0 36 36 MAGN 00 00 2 04 04 06 05 06 05 JHDR 128 128 10 8 8 0 0 0 0 0 JDAT <lena_q25_fourth.jpg> # 2514 bytes IEND # Region of interest, full scale, cropped to # dimensions 200x313 at location 192,200, # high quality JPEG compression. MAGN # Turn off magnification of all subsequent object 0 DEFI 0 0 0 228 236 JHDR 200 312 10 8 8 0 0 0 0 0 JDAT <lena_face_q65.jpg> # 8001 bytes IEND MEND
For this image, the resulting 600x600 frame occupies about 2.6
times the file size when written as a simple JNG and about 26 times the
file size when written as a simple PNG. The particular image used in
this example was the 512x512 color Lena from
http://links.uwaterloo.ca/bragzone.base.html.
Contributors' names are presented in alphabetical order:
This document was built from the file mng-master-20010127 on 27 January 2001.
Copyright (C) 1998-2001, by Glenn Randers-Pehrson
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