This document is an informal draft of the PNG development group.
It is a proposal, and the format is subject to change.
Comments on this document can be sent to the PNG 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 presents the format of a MNG (Multiple-image Network Graphics) datastream. MNG is a multiple-image member of the PNG (Portable Network Graphics) format family, that can contain animations (slide shows) comprised of PNG single-image datastreams. It can also incorporate images in a highly compressible "Delta-PNG" format, defined herein, or a lossy JNG (JPEG Network Graphics) format, also defined herein.
The MNG format 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.
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 has been demonstrated to provide a much more compact way of representing subsequent images than using a complete PNG datastream for each.
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 valid PNG or JNG datastream is also a valid MNG datastream.
This document includes examples that demonstrate various capabilities of MNG including 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 the MNG (Multiple-image Network Graphics) and JNG (JPEG Network Graphics) datastreams.
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 home page,
http://www.cdrom.com/pub/png/
A MNG datastream describes a sequence of single frames, each of which can be composed of zero or more embedded images or directives to show previously defined images.
A typical MNG datastream consists of:
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" [or "M N G" in the local language pronunciation of Roman letters].
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 first eight bytes of a MNG datastream are
138 77 78 71 13 10 26 10
(decimal) 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 backwards-compatible manner. These issues are being discussed in the mpng-list@dworkin.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 in below (Chapter 7). 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 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.
An independent PNG or JNG datastream, with a PNG or JNG signature, is also a valid MNG datastream that must be recognized and decoded by MNG-compliant decoders. This kind of MNG datastream will contain only a single embedded image.
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.
The words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119, and the word "CAN" is equivalent to the word "MAY" as described therein. "NOT ALLOWED" and "NOT PERMITTED" describe conditions that "MUST NOT" occur. "ALLOWED" and "PERMITTED" describe conditions that "CAN" occur.
An image whose pixels have a hidden representation, and which does not necessarily carry PNG or JNG chunk data. An image delta cannot be applied to an abstract image. All abstract objects are viewable.
An image produced by applying an image delta to a parent object.
File gamma and chromaticity values, or an sRGB rendering intent, iCCP profile, or whatever is involved in mapping between RGB values and colors.
An image or object whose pixels have a publicly known representation, and which uses a publicly known color encoding. A concrete PNG or JNG image also carries data from other known PNG or JNG chunks that are present.
A concrete object or image that appears in-line in a MNG datastream.
A layout of a background and zero or more images, followed by a specified nonzero delay time or by the MEND chunk, that is to be displayed as a still frame or as part of an animation. An animation would ideally appear to a perfect observer (with an inhumanly fast visual system) as a sequence of still frames. A group of MNG images whose interframe delay time is zero makes up a "subframe" (see below) instead of a "frame". The final subframe in a segment completes a frame, regardless of its interframe delay.
The portion of the display surface whose pixels are inside the subframe clipping boundaries as defined by the FRAM chunk (see the FRAM chunk specification below).
The amount of time a frame should be visible when an animation is played. In reality, it takes a nonzero amount of time to display a frame. No matter which moment is picked as the "start" of the frame, the frame duration measures the time to the "start" of the next frame.
The upper left corner of the output device (frame buffer, screen, window, page, etc.) where the pixels are to be displayed. This is the {0,0} position for the purpose of defining frame clipping boundaries, image locations, and image clipping boundaries. Note that in a windowing system, the frame origin might be moved offscreen, but MOVE and CLIP values would still be measured from this offscreen origin.
An object whose object attributes set and whose object buffer are not allowed to be discarded, replaced, or modified.
An object that can be applied to a concrete image or object to produce another concrete image. For any two concrete images, there exists an image delta that will produce one from the other.
Shorthand for "the object with the object attribute set pointed to by
`object_id=N'".
A visible embedded image or a stored image that is displayed in response to a SHOW chunk directive, located with respect to the frame boundaries and clipped with respect to the subframe clipping boundaries and the image's own clipping boundaries, or the background image, clipped, located, and displayed against the background color at the beginning of a subframe. 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.
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.
An image or a nonviewable basis object. The object_id is
an unsigned sixteen-bit number that serves as the identifier of a set of
object attributes.
Properties of an object such as its existence, potential visibility, location, clipping boundaries, and a pointer to an object buffer. See Object attributes below.
A 2D array of pixels or pixel deltas, each of which has color and transparency information. More than one object can point to a given object buffer. See Object buffers below.
An object to which a delta is applied.
do_not_show flag to zero.
do_not_show flag to zero.
The first segment, when there is more than one segment.
Any segment other than the first (also the first segment, when there is only one).
A part of a MNG datastream starting with the MHDR chunk or with a SEEK chunk and extending to just before the next SEEK chunk (or the MEND chunk if there is no next SEEK chunk). The MHDR, MEND, SAVE, SEEK, and TERM chunks are not considered to be a part of any segment.
An entity with a number that can arrive asynchronously at the decoder. More detailed semantics, like whether multiple signals of the same number (or even different numbers) can be queued, are beyond the scope of this specification.
A group of one or more layers, with nonzero delay time between them and following them, that are to be displayed as a part of a still frame or as part of an animation. Subframes are defined in MNG for convenience in applying frame parameters only to a subset of the layers making up a complete frame.
A stored object that is capable of being made visible. An image is viewable, while some objects resulting from decoding a BASI datastream are not viewable.
Actually drawn on a display. If an object is visible, a person looking at the display can see it.
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.
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:
object_id appears.
object_id replaces it
without an intervening DEFI chunk (in this case, the new object
inherits the object attribute set from the previous object with the same
nonzero object_id; when object_id = 0, it receives
the default DEFI attribute set).
object_id = 0 and its IEND chunk appears
(in this case, the object attribute set can be immediately discarded).
Every object (except for object 0) has an object buffer. Multiple objects can point to the same object buffer. The representation of a pointer is decided by the application; pointers never appear explicitly in a MNG datastream. Decoders can also create an object buffer for object 0, if that is more convenient, but the information in that buffer cannot be depended upon to exist after the image has been displayed, nor can that buffer become "frozen".
All objects are initially "not frozen". Any objects in existence (except for object 0) when the SAVE chunk is encountered become "frozen", along with the object buffers that they point to.
The "potential visibility" of an object is determined by the
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" object be displayed.
The X and Y location of an object is determined by the DEFI chunk that introduced it, and can be changed by the MOVE chunk.
The clipping boundaries of an object are determined by the DEFI chunk that introduced it, and can be changed by means of the CLIP chunk.
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:
Any object that points to a viewable object buffer can be made potentially visible, but one that points to a nonviewable one cannot. Any attempt to do so must be ignored.
The data format can be:
A concrete object must be stored by the decoder in a form that retains the complete object description, sufficient to regenerate the original object description or its equivalent without loss. Its pixels have a publicly known representation, and it uses a publicly known color encoding.
An abstract image can be stored by the decoder in any form that is convenient, such as an X Window System "pixmap", even though that form might not have sufficient resolution for exact, lossless conversion. In the case of a PNG image, the pixels could be stored after the gamma and chromaticity corrections have been made, and the sample depth could be the same as the display hardware, even though it is smaller than the original sample depth. Similarly, a JNG image could be stored in the same form, after the pixels have been decoded, converted to RGB form, and gamma-corrected. It is always safe, however, to store an abstract image as though it were concrete, if decoders do not wish to take advantage of the distinction between abstract and concrete objects.
All object buffers are initially "not frozen". Any object buffers in existence when the SAVE chunk is encountered become "frozen". Decoders do not actually have to store this flag except as a sanity check, because they can depend on the fact that a "frozen" object buffer will always have at least one "frozen" object whose "buffer pointer" points to it.
When an object buffer is first created, its reference count is set to 1.
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, the reference count of the object buffer that it points to is decremented, and the object buffer is also discarded if the resulting reference count is zero.
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 there.
This section describes critical MNG control chunks. MNG-compliant decoders must recognize and process them.
The MHDR chunk is always first in all MNG datastreams except for those that consist of a PNG datastream with a PNG signature.
-[This chunk contained only the first three fields (12 bytes) prior to draft 51, and 21 bytes in draft 51]
The MHDR chunk contains exactly 28 bytes:
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).
0: Presence or absence of any features
is unspecified.
all other even numbers: Invalid.
1: JNG, transparency, and complex MNG
features are absent.
3: Complex MNG features are present.
Transparency and JNG are absent.
5: Transparency is present. JNG and
complex MNG features are absent.
7: JNG is absent. Transparency and
complex MNG features are present.
9: JNG is present. Transparency and
complex MNG features are absent.
11: JNG and complex MNG features are
present. Transparency is absent.
13: JNG and transparency are present.
Complex MNG features are absent.
15: JNG, transparency, and complex MNG
features are present.
17-65535: Reserved for public expansion.
65536-((2^31)-1): Available for private
or experimental expansion.
(2^31) and up: Invalid.
Decoders can ignore the "informative" frame-count, layer-count, 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 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 frame duration and sync timeout.
It must be nonzero if the datastream contains an animation. 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 sync_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 frame duration, viewers should display
animations 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 and
TERM chunks. If the frame count is greater than (2^31)-1),
encoders should write (2^31)-1, representing an infinite frame count.
If the layer-count field contains a zero, the layer
count is unspecified. If it is nonzero, it contains the number of
layers in the datastream, ignoring the fPRI and TERM
chunks. If the layer count is greater than (2^31)-1), encoders should
write (2^31)-1, representing an infinite layer count.
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 and
TERM chunks. 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 4-bit integer. If the frame count is greater
than (2^31)-1) ticks, encoders should write (2^31)-1,
representing an infinite nominal play time.
When the simplicity profile is zero, the simplicity (or complexity) of the MNG datastream is unspecified.
If the simplicity profile is nonzero, it can be regarded as a 31-bit profile, with the ones bit being a "profile-validity" flag, the twos bit being a "simple MNG" flag, the fours bit being a "transparency" flag, and the eights bit being a "JNG" flag. a profile-validity flag. The upper 15 bits (except for the most significant bit, which must be zero) are available for private test or experimental versions, and the remaining bits are reserved for future MNG versions, and must be zero in this version. If a bit is zero, the corresponding feature is guaranteed to be absent, and if a bit is one, the corresponding feature may be present in the MNG datastream.
A "simple MNG" (simplicity profile = 1, 5, 9, or 13) does not contain the LOOP/ENDL, DEFI, BASI/IEND, CLON, DHDR/IEND, PAST, DISC, FRAM, MOVE, CLIP, and SHOW chunks. If the BACK chunk is present, it does not define a background image. A MNG with a "complex MNG feature" (simplicity profile = 3, 7, 11, or 15) may contain at least one of them.
"Transparency is not present" means that the MNG or PNG tRNS chunk is not present and no PNG or JNG image has an alpha channel.
Encoders that write a nonzero simplicity profile should endeavor to be accurate, so that decoders that process it will not unnecessarily reject datastreams. For example, the simplicity profile 15 indicates that transparency, JNG, and at least one "complex" MNG feature are all present. If the simplicity profile promises that a 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. Its contents are the first two, five, six, or more of the following fields:
Nest level: 1 byte (unsigned integer).
Repeat count: 4 bytes (unsigned integer),
range [0..2^31-1].
Termination
condition: 1 byte (unsigned integer). Omit if
termination_condition=<deterministic>.
1: Decoder discretion.
2: User discretion.
3: External signal.
Iteration min: 4 bytes(unsigned integer). Omit
if termination_condition is also
omitted.
Iteration max: 4 bytes (unsigned integer). Omit
if termination_condition is also
omitted.
Signal number: 4 bytes (unsigned integer). Omit
if termination_condition != 3.
Additional
signal number: 4 bytes. Omit if
termination_condition != 3.
...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
repeat_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:
This is the default behavior, when the termination condition field is omitted. 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.
The number of iterations can be chosen by the decoder, and
must not be less than 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.
The number of iterations should be chosen by the user (e.g. by
pressing the <escape> key), but the decoder must enforce the
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.
The number of iterations must not be less than
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.
The iteration_min and iteration_max
can be omitted only if the condition is <deterministic>.
The repeat_count, iteration_min, and
iteration_max can be any non-negative integers or
<infinity>, but they must satisfy iteration_min <=
repeat_count <= iteration_max. Infinity can be represented by
0x7fffffff.
If repeat_count is zero, the loop is done zero times.
Upon encountering a LOOP chunk with repeat_count=0,
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 ASCII character,
received from a keyboard or simulated keyboard, and values 256-1023 are
reserved for future definition by this specification.
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
repeat_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.
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.
The DEFI chunk sets the default object attribute
set (object_id, potential_visibility,
concrete_flag, location, and clipping boundaries) for
any subsequent images that are defined with IHDR-IEND,
JHDR-IEND, or BASI-IEND datastreams. The
DEFI chunk contains 2, 3, 4, 12, or 28 bytes:
Object id: 2 bytes (unsigned integer) identifier
to be given to the objects that
follow the DEFI chunk.
Do_not_show
flag: 1 byte (unsigned integer)
0: Make the objects potentially
visible.
1: Do not make the objects potentially
visible. This field can be omitted
if the concrete_flag, location, and
clipping boundary fields are also
omitted. When it is omitted, the
image is potentially visible
(do_not_show=0).
Concrete flag: 1 byte (unsigned integer)
0: Make the objects "abstract" (image
can not be the source for a
Delta-PNG)
1: Make the objects "concrete" (object
can be the source for a Delta-PNG).
This field can be omitted if the
location and clipping boundary fields
are also omitted. When it is omitted,
the object is made "abstract"
(concrete_flag=0).
X_location: 4 bytes (signed integer).
The X_location and Y_location fields can
be omitted if the clipping boundaries
are also omitted. If so, decoders must
assume default values {X_location=0,
Y_location=0}.
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. If so, decoders
must assume default values
{0, frame_width, 0, frame_height}.
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 positive directions are downward and rightward from the frame origin.
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. When object_id=0, the DEFI
chunk values are discarded after the object's IEND chunk is
processed. 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 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
If object_id is an identifier that already exists when
a DEFI chunk appears, the object attribute set (except for
the pointer to the object buffer) is immediately replaced, but the
contents of the object buffer do not change until an IHDR,
JHDR, or BASI chunk is encountered; then the contents
of the object buffer previously associated with the identifier are
replaced with the new embedded object data. Note that if the object has
partial clones, the clones will also be 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.
A PNG (Portable Network Graphics) datastream.
See the PNG specification [PNG] and the PNG Special Purpose Chunks document [PNG-EXT] for the format of the PNG chunks.
Any chunks between IHDR and IEND are written and decoded according to the PNG specification.
If a global PLTE chunk appears in the top-level MNG datastream, the PNG datastream can have an empty PLTE chunk, to direct that the global PLTE data be used. If an empty PLTE chunk is not present, the data is not inherited. MNG applications that recreate PNG files must write the global PLTE chunk rather than the empty one in the output PNG file. It is an error for the PNG datastream to contain an empty PLTE chunk when the global PLTE chunk is not present or has been nullified.
If the PNG sRGB, gAMA, iCCP, or cHRM chunks appear in the top-level MNG datastream (and have not been nullified), but none of them appear in the PNG datastream, then the values are inherited from the top level as though the chunks had actually appeared in the PNG datastream. Data from such chunks appearing in the PNG datastream take preference over the inherited values. If any one of these chunks, or any future chunk that defines the color space, appears in the PNG datastream, none of them is inherited. MNG applications that recreate PNG files must write these chunks, if they are inherited, in the output PNG files. If the sRGB chunk is present, it need not be accompanied by gAMA and cHRM chunks, as recommended in the PNG specification. 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. If the sRGB chunk is present, editors that write PNG files should add the gAMA and cHRM chunks, even though they are not present in the MNG datastream.
If the PNG sPLT chunk appears in the top-level MNG datastream, it takes preference over any sPLT chunk appearing in the PNG datastream. MNG applications that recreate PNG files should not copy top-level sPLT chunks to the output PNG files, because a suggested palette for rendering a group of images is not necessarily the best palette for rendering a single image.
When the framing mode is 2, 3, or 5 in the MNG FRAM chunk, the PNG oFFs and pHYs chunks and any future chunks that attempt to set the pixel dimensions or the drawing location must be ignored by MNG viewers and simply copied (according to the copying rules) by MNG editors. When the framing mode is 1 or 4 (i.e., when each PNG image is an individual frame), these chunks must be treated as described in the PNG specification.
The PNG gIFg, gIFt, and gIFx chunks must be ignored by viewers and must be copied according to the copying rules by MNG editors.
If do_not_show=0 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=0, there is no need to store the pixel data
after displaying it.
If concrete_flag=1 and object_id != 0,
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=0, the decoder can store the pixel data in any
form that it chooses.
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.
Any chunks between JHDR and IEND are written and decoded according to the JNG specification.
The remaining discussion in the previous paragraph about PNG datastreams also applies to JNG datastreams.
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. An optional 8 additional bytes 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 be present.
Width: 4 bytes (unsigned integer).
Height: 4 bytes (unsigned integer).
Sample_depth: 1 byte (unsigned integer).
Color_type: 1 byte (unsigned integer).
Compression_method: 1 byte (unsigned integer).
Filter_type: 1 byte (unsigned integer).
Interlace_type: 1 byte (unsigned integer).
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 (unsigned integer).
0: Basis object is not viewable.
1: Basis object is viewable.
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. The decoder is responsible for converting the color and
alpha samples to the appropriate format and sample depth for the
specified color_type. When color_type=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. If the viewable field is omitted, the object is not
viewable.
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=3, the least significant byte
of each color sample is used and the upper byte must be zero. When
color_type=0 or color_type=4, the green and blue
samples must be present but must be ignored by decoders.
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 are:
color_type=3. If so, the subsequent Delta-PNG that uses this
as the parent object can supply a complete PLTE chunk, if the
single-entry palette that is generated is not desired.
The BASI chunk can be used to introduce such things as a library of faLT 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 into which other images will be pasted by means of the PAST chunk.
A BASI chunk appearing in a MNG datastream must be
preceded by a DEFI chunk that gives the object_id,
location, and potential visibility for the basis object. 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 do_not_show=0 or viewable=1, the
resulting image, after the pixel samples are filled in, must be a legal
PNG image. If do_not_show=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, 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, or 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.
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 object attributes
set and the object buffer.
1: Partial clone; only object attributes
set (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").
This field can be omitted if the
do_not_show field is also omitted.
If it is omitted, the clone_type defaults
to zero (full clone).
Do_not_show: 1 byte (unsigned integer).
0: Make the clone potentially visible.
1: Do not make the clone potentially
visible.
This field can be omitted if the concrete
flag and location fields are also omitted.
When it 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).
This field can be omitted if the location
fields are also omitted. When it 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.
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, or they can modify, show, or discard the parent object or modify its object attribute set without affecting the clone.
The concrete_flag byte must be zero 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.
A Delta-PNG (PNG-Delta) 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=0. The parent
object must be concrete (i.e., it must have concrete_flag=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
immediately preceding 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, to fill
the clipping boundaries. The
upper left corner of the assembly
is positioned according to the
prescribed offsets.
Offset
delta_type: 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.
Xoffset or
delta xoffset: 4 bytes (signed integer).
Yoffset or
delta yoffset: 4 bytes (signed integer).
Boundary
delta_type: 1 byte (unsigned integer).
0: Boundaries are measured from
the {0,0} pixel in the
destination image.
1: Boundaries are measured from the
{target_x,target_y} pixel in the
destination image.
Left_pb or delta
left_pb: 4 bytes (signed integer).
Right_pb or delta
right_pb: 4 bytes (signed integer).
Top_pb or delta
top_pb: 4 bytes (signed integer).
Bottom_pb or delta
bottom_pb: 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
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. Note that 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 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 assembly of tiles.
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_id's 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 MOVE or CLIP information associated with the
destination_id and the source_id's is not used in
the PAST operation (but if a decoder is simultaneously updating
and displaying the destination_id, the MOVE and
CLIP for the destination_id is used in the display
operation).
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 objects except those preceding the SAVE chunk (i.e., 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 object attribute set (location, potential visibility, and clipping boundaries) can be discarded. The data in the object buffer must be retained until the last remaining partial clone is discarded.
The BACK chunk suggests a background color against which transparent, clipped, or less-than-full-frame images can be displayed.
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
background_image_id is 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
displayed as the background of each
frame. If it is not full-frame, the
remainder of the frame is filled
with the background color. This
field can be omitted; if so, no
background image is defined, and the
background image from a previous
BACK chunk becomes undefined.
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.
Viewers are expected to composite every subframe in the MNG datastream against a fresh copy of the background, if the framing mode given in the FRAM chunk is 3, 4, or 5. The images and the background are both clipped to the frame boundaries given in the FRAM chunk. The background image (or tiled assembly) is also clipped to its own boundaries and located like any other image. 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 frame boundaries with the background color, then displayed the background image, and then displayed the foreground image (or images), without delay.
It is not an error to specify a background_image_id
when such an image does not exist or ceases to exist for some reason.
Viewers must be prepared to fall back to using the background color in
this event. They also must be prepared for the contents, location, and
clipping boundaries of the background image to change, just like any
other object, if it has not been "frozen".
The three BACK components are always 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 colorspace of the background image, if one is used, is determined in the same manner as the colorspace of any other image.
When the BACK chunk appears between FRAM chunks, it applies to the upcoming frame, not to the current one. When the framing_mode is 4, it takes effect immediately prior to the next IHDR, JHDR, DHDR, PAST, or SHOW chunk in the datastream.
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. When the framing mode is 3, 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 required and the BACK chunk is omitted, then the viewer must supply its own background.
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 page's background.
The FRAM chunk provides information that a decoder needs for generating layers, subframes, and frames. The FRAM parameters govern how the decoder is to behave when it encounters a FRAM chunk, a SHOW chunk, or an embedded image. The FRAM chunk also delimits subframes.
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. 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.
Framing mode: 1 byte.
0: Don't change framing mode.
1: Each embedded image or SHOW'ed
image generates a subframe
consisting of a single image layer.
There is no background layer.
Use this mode to avoid unnecessary
clearing of the display when the
first image covers the entire
frame area, and subsequent frames can
be displayed properly by simply
overlaying them on the prior frame.
This is the default framing mode.
2: The group of embedded images or
SHOW'ed images appearing prior
to the next FRAM chunk form a
composite subframe consisting of
zero or more image layers. The
level, or stacking order, of each
layer is given by its order of
appearance in the datastream.
No interframe delay occurs between
the layers.
3: The group of embedded images or
SHOW'ed images appearing prior to
the next FRAM chunk define individual
subframes, each consisting of a
single layer. A background layer
is also generated as a separate
subframe preceding the group. An
interframe delay occurs after
each layer (including the background
layer) is displayed.
4: This is the same as framing_mode=1,
except that a background layer is
generated. Each embedded image or
SHOW'ed image generates a subframe
consisting of a background layer
followed by an image layer. No
interframe delay occurs between the
background layer and the image layer.
5: This is the same as framing_mode=2,
except that a background layer is
generated prior to the image layers.
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 next subframe only.
2: Yes, also reset default.
This field and the next three must be
omitted as a group if no frame parameters
other than the framing mode are changed.
Change sync timeout and
termination: 1 byte
0: No.
1: Deterministic, for the next subframe
only.
2: Deterministic, also reset default.
3: Decoder-discretion, for the next
subframe only.
4: Decoder-discretion, also reset
default.
5: User-discretion, for the next subframe
only.
6: User-discretion, also reset default.
7: External-signal, for the next subframe
only.
8: External-signal, also reset default.
Change subframe clipping
boundaries: 1 byte.
0: No.
1: Yes, for the next subframe only.
2: Yes, also reset default.
Change sync id
list: 1 byte.
0: No.
1: Yes, for this subframe only.
2: Yes, also reset default list.
Interframe
delay: 4 bytes (unsigned integer). Must
be omitted if change_interframe_delay=0.
The range is [0..2^31-1] ticks.
Sync timeout: 4 bytes (unsigned integer). Omit if
change_sync_timeout=0. The range is
[0..2^31-1]. The value 2^31-1
(0x7fffffff) ticks represents an
infinite timeout period.
Subframe boundary
delta type: 1 byte (unsigned integer).
0: Subframe clipping boundary values are
given directly.
1: Subframe clipping boundaries
are determined by adding the FRAM
data to their previous values.
This and the following four
fields must be omitted if
change_frame_clipping_boundaries=0.
Left_fb or delta
left_fb: 4 bytes (signed integer).
Right_fb or delta
right_fb: 4 bytes (signed integer).
Top_fb or delta
top_fb: 4 bytes (signed integer).
Bottom_fb or delta
bottom_fb: 4 bytes (signed integer).
Sync id: 4 bytes (unsigned integer). Omit if
change_sync_id_list=0 or if the new
list is empty; repeat until all
sync_id's have been listed. The
range is [0..2^31-1].
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 modes:
When framing_mode=1, each image generates a separate
subframe consisting of a single image layer. In the usual case, the
interframe delay is nonzero, so each subframe is a frame. FRAM
chunks need not appear to separate them.
The following events generate a subframe:
Any of these events generates a subframe, even if the visible image is outside the clipping boundaries and no pixels are actually changed.
For example (assuming that objects 1 through 5 are all viewable objects and a nonzero interframe delay time), the sequence
FRAM 1 SHOW 1 5
will generate five frames, each containing a single subframe consisting of an image layer.
If the BACK chunk is present, eecoders should insert a background layer, with a zero delay, ahead of the first image layer in a segment, even when the framing_mode is 1. Such layers must be included in the layer count but not in the frame count.
Framing mode 2 is the same as framing mode 1, except that the
interframe delay is occurs between subframes rather than between images.
In the usual case, the interframe delay is nonzero, so the subframes are
composite frames. When framing_mode=2, viewers are expected
to display all of the images at once, if possible, or as fast as can be
managed, without clearing the display or restoring the background. The
next FRAM chunk delimits the subframe. A subframe boundary
also occurs when a SEEK chunk or the MEND chunk
appears.
For example, the sequence
FRAM 2 SHOW 1 5 (shows images 1, 2, 3, 4, and 5) FRAM
will result in a single subframe containing six layers: a background layer and five images, each displayed according to its location and CLIP data.
When images in a subframe overlap, viewers are expected to composite the later images against the partially completed subframe that includes all earlier images.
This framing_mode is fundamentally declarative; it
describes the elements that go into an individual subframe. It is up
to the decoder to work out an efficient way of making the screen match
the desired composition. 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.
If the BACK chunk is present, eecoders should insert a background layer, with a zero delay, ahead of the first image layer in a segment, even when the framing_mode is 2. Such layers must be included in the layer count but not in the frame count.
When framing_mode=3, a subframe boundary occurs after
the background and after each image appears. A background layer is
generated after the FRAM chunk is encountered. Subframes
are triggered by the same events that trigger a subframe when
framing_mode=1. A subframe boundary also occurs when a
SEEK chunk or the MEND chunk appears.
When multiple images are caused to appear with a single SHOW chunk, a subframe boundary occurs after each one. For example, the sequence
FRAM 3 SHOW 1 5 (shows images 1, 2, 3, 4, and 5) FRAM
would result in six layers being generated, namely the background and the five images. What appears on the screen is exactly equivalent to what would have been accomplished (but less efficiently) with
FRAM 2 SHOW 1 FRAM SHOW 1 2 (shows background, images 1 and 2) FRAM SHOW 1 3 (shows background, images 1, 2, and 3) FRAM SHOW 1 4 (shows background, images 1, 2, 3, and 4) FRAM SHOW 1 5 (shows background, images 1, 2, 3, 4, and 5) FRAM
When framing_mode=4, a subframe consisting of the
background layer and an image layer is generated for each image.
A subframe boundary occurs after each image appears. Otherwise,
framing_mode=4 is identical to framing_mode=1.
Subframes are triggered by the same events that trigger a subframe
when framing_mode=1. A subframe is also triggered by the
appearance of the next FRAM chunk. Decoders should, however,
clear the frame area when the very first FRAM chunk in the
datastream is found, or the very first FRAM chunk after a
SEEK chunk is found, even when framing_mode=4.
When framing_mode=5, a subframe boundary and interframe
delay occurs only when the next FRAM chunk appears. No
background layer is generated. Otherwise, framing_mode=5 is
identical to framing_mode=2. A subframe boundary also occurs
when a SEEK chunk or the MEND chunk appears.
This table summarizes the behavior of a viewer under the various framing modes.
+--------------+--------------------+-------------------+ | Framing mode | Restore background | Interframe delay | +--------------+--------------------+-------------------+ | 1 | Before first image*| Before each FRAM | | | after first FRAM | after the first | | | in the datastream | in the datastream | | | (or segment, if | | | | jumped to segment) | | +--------------+--------------------+-------------------+ | 2 | Before first image | Before each image | | | after first FRAM | after the first | | | in the datastream | in the datastream | | | (or segment, if | | | | jumped to segment) | | +--------------+--------------------+-------------------+ | 3 | Before first image | Before each image | | | following each | after the first | | | FRAM chunk | in the datastream | +--------------+--------------------+-------------------+ | 4 | Before each image | Before each image | | | | after the first | | | | in the datastream | +--------------+--------------------+-------------------+ | 5 | Before first image | Before each FRAM | | | following each | after the first | | | FRAM chunk | in the datastream | +--------------+--------------------+-------------------+ | *"Each image" means each image that is displayed in | | response to decoding an embedded object or a SHOW | | directive, even if no pixels are actally drawn due | | to the image being outside the clipping boundaries. | +-------------------------------------------------------+
The framing_mode also affects the way decoders handle the
pHYs chunk (see below).
The subframe name must conform to the same formatting rules as
those for a SEEK 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 subframes
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 strings
"clock(", "frame(", or "frames(" (all case-insensitive), which are
reserved for use in URI queries and fragments.
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, subframes are frames. When it is zero, a frame consists of any number of consecutive subframes, until a nonzero delay subframe is encountered. Decoders are not obligated to display such subframes individually; they can composite them offscreen and only display the final complete frame.
The sync timeout field can be a number or <infinity>. Infinity can be represented by 0x7fffffff.
The termination condition given in the
change_sync_timeout_and_termination field specifies how much
longer, after the normal interframe delay has elapsed, the frame will
endure. It can take the following values:
The frame endures no longer than the normal interframe delay. Even though this is the default, a streaming encoder talking to a real-time decoder might write a FRAM with a termination condition of "deterministic" to force the display to be updated while the encoder decides its next move.
The decoder can lengthen the duration of the frame, but by no more than the timeout. A streaming decoder could take the opportunity to wait for its input buffer to fill to a comfortable level.
After the interframe delay has expired, the decoder should wait for permission from the user (e.g., via a keypress) before proceeding, but must wait no longer than the timeout. If the decoder cannot interact with the user, this condition degenerates into "decoder-discretion".
After the interframe delay has expired, the decoder should wait for
the arrival of a signal whose number matches a sync_id, but
must wait no longer than the timeout.
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 = 0
Left subframe boundary = 0
Right subframe boundary = frame width
Top subframe boundary = 0
Bottom subframe boundary = frame height
termination = deterministic
Sync timeout = 0x7fffffff (infinite)
Sync id = <empty list>
The MOVE chunk can be used to specify the placement of each image within the layer. The CLIP chunk can be used to specify clipping boundaries for each image. The subframe boundaries are only used for clipping, not for placement. Even when the left and top frame boundaries are nonzero, the image locations are measured with respect to the {0,0} position in the display area. If the layers are transparent or do not cover the entire area defined by the subframe clipping boundaries, 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 is not recognized by the decoder. The background, as well as the images, is clipped to the subframe clipping boundaries. Any pixels outside the subframe clipping boundaries remain unchanged.
The frame_duration field gives the duration of
display, which is the minimum time that must elapse from the
beginning of displaying one frame until the beginning of displaying
the next (or between images, when framing_mode=1). It
is measured in "ticks" using the tick length determined from
ticks_per_second defined in the MHDR chunk.
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
frame_duration and the sync_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 subframe that follows the FRAM chunk. 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 frame_duration=0, sync_timeout, and
sync_id, and the second establishing the synchronization
point:
FRAM 4 0 1 1 0 1 0000 sync_timeout sync_id FRAM 0 name
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.
New location of an existing object or objects (replacing or incrementing the location given in the DEFI chunk).
The MOVE chunk gives the position, 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 (nonzero unsigned integer).
Last object: 2 bytes (nonzero 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.
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.
When an object is discarded, its object attribute set, 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: (nonzero unsigned integer).
Last object: 2 bytes: (nonzero 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. 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_fb <= x < right_fb (from the FRAM chunk) Top_fb <= y < bottom_fb 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.
When an object is discarded, its object attribute set, 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.
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: Don't 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. Cycle back 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 odd, or
when show_mode=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, 3, or 5
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=6 or 7, 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 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 in below.
When show_mode=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 2 SHOW 1 10 # Show images 1 thru 10. FRAM SHOW 1 5 # Show images 1 thru 5. SHOW 7 10 # Show images 7 thru 10. ENDL LOOP 0 0 10 FRAM 2 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 LOOP 0 0 10 FRAM 2 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 an object that has not previously been defined. In such cases, nothing is done to the nonexistent object.
If a nonviewable object is named, its do_not_show flag
is changed, but it is not displayed even if the SHOW chunk
requests it.
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 animation starting
immediately after the TERM 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
omitted if termination_action<=2, and
must be present otherwise.
Delay: 4 bytes (unsigned integer). Delay, in
ticks, before repeating the animation.
Iteration max: 4 bytes (unsigned integer). Maximum
number of times to repeat the animation.
The loop created by processing a TERM must always be treated
by the decoder as if it were a <user-discretion> loop, with
iteration_min=1.
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 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. They only need to restore information that they will use, e.g., a viewer that processes gAMA and global PLTE, but ignores iCCP and sPLT, need only restore the value of gamma and the global PLTE 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 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 and
nominal frame number.
1: Segment.
2: Subframe.
3: Exported image.
Offset: 4 or 8 bytes (unsigned integer). Omit 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 animation,
assuming that all prior segments were
played as intended on an ideal player,
ignoring any fPRI chunks. Omit 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.
Omit if entry_type > 0.
Name: 1-79 bytes (Latin-1 text). Omit for
unnamed segments.
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 are lost 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. 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.
It is the encoder's responsibility, if it changes or discards any "saved" data, to restore it to its "saved" condition (or nullifying 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, eXPI, fPRI, gAMA, iCCP, pHYs, and sRGB.
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, 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 the optional index is present, every segment, whether named or not, except for the prologue segment, must be listed in it. All entries must appear in the index in the same order that they appear in the MNG datastream. Only named images or subframes are permitted, and it is not an error to omit any or all named images or subframes 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 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.
The SEEK 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). 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 strings "clock(", "frame(", or "frames(" (all case-insensitive), which are reserved for use in URI queries and fragments.
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 the SEEK chunk is encountered, a decoder must restore the information that it saved when it processed the SAVE chunk.
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 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 and a name:
Snapshot id: 2 bytes (unsigned nonzero integer). Snapshot name: 1-79 bytes (Latin-1 text).
Note that the snapshot_name is associated with the
snapshot, not with the snapshot_id nor its future contents;
discarding 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.
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 strings "clock(", "frame(", or "frames(" (all
case-insensitive), which are reserved for use in URI queries and
fragments.
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, and it cannot be changed unless the prologue segment is processed again according to the new "cost."
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 immediately 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.
-[During the draft phase of the development of this specification, to specify that the datastream is written in accordance with a certain draft version of MNG, you can use "nEEDdraft nn".]
The MNG pHYs chunk is identical in syntax to the PNG pHYs chunk. When the framing mode is 2, 3, or 5 in the FRAM chunk, it applies to complete MNG layers and not to the individual images within them. When the framing mode is 1 or 4, the MNG pHYs chunk provides a default value if the subsequent PNG datastream does not contain one.
The MNG top-level pHYs chunk can be nullified by a subsequent empty pHYs 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 tEXt, tIME, and zTXt chunks in the generated PNG datastreams.
The following 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.
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.
The following 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:
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 preference 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 and pHYs data from the MNG level and simply copy any sPLT and pHYs chunks appearing within the PNG datastreams.
JNG (JPEG Network Graphics) is the lossy sub-format for MNG objects.
Note: This specification depends on the PNG Portable Network Graphics specification [PNG]. The PNG specification is available at the PNG home page,
http://www.cdrom.com/pub/png/
A JNG datastream consists of a header chunk (JHDR), JDAT chunks that contain a complete JPEG datastream, and optionally, IDAT chunks that contain a PNG-encoded grayscale image that is to be used as an alpha mask. Such a mask 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 first eight bytes of a JNG datastream are
139 80 78 74 13 10 26 10
(decimal) which is similar to the PNG signature with "\213 J N G" instead of "\211 P N G" in bytes 0-3.
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).
JDAT 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.
JDAT compression
method: 1 byte
8: ISO-10918-1 Huffman-coded baseline JPEG.
JDAT interlace
method: 1 byte.
0: Sequential JPEG, single scan.
8: Progressive JPEG.
IDAT sample
depth: 1 byte.
0, 1, 2, 4, 8, or 16.
IDAT compression
method: 1 byte.
0: Zlib DEFLATE.
IDAT filter
method: 1 byte.
0: Adaptive (see PNG spec).
IDAT interlace
method: 1 byte.
0: Not interlaced.
The width, height, JDAT_sample_depth, JDAT_compression_method, and JDAT_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 data, must be set to
zero. The IDAT_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 baseline JPEG datastream.
A JNG baseline JPEG is a baseline JPEG as defined by JPEG Part 1 (ISO IS 10918-1), using only JFIF-compatible component interpretations, with a few additional restrictions that reflect limitations of many existing JPEG implementations.
A baseline JPEG according to Part 1 is DCT-based (lossy) sequential JPEG, using Huffman entropy encoding, with the following further restrictions:
JDAT_sample_depth=20,
the datastream contains an eight-bit image followed by an twelve-bit
image, with a JSEP chunk between them. Viewers must display
only one of them; the eight-bit image is only for use by viewers that
cannot decode twelve-bit images.
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, ie, only the default SPIFF orientation is permissible.
The encoded data shall have one of the two colorspace 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 8-bit data, 2048 for 12-bit data. With these equations, Y, Cb, and Cr all have the same range as R, G, and B: 0 to 255 for 8-bit data, 0 to 4095 for 12-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 luminancey, 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 new, 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.
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.
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. The alpha channel, if
present, can have sample depths 1, 2, 4, 8, or 16. The IDAT
chunks can be interleaved with the JDAT chunks. 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^IDAT_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 12-bit JDAT chunks must read the 8-bit IDAT chunks and apply the alpha samples to the 8-bit image that is contained in the JDAT chunks that precede the JSEP chunk. Viewers that skip the 12-bit JDAT chunks must read the 8-bit IDAT chunks chunks that precede the JSEP chunk and apply the alpha samples to the 12-bit image that is contained in the JDAT chunks that follow the JSEP chunk. JDAT chunks must read the 8-bit IDAT chunks and apply the alpha samples to the 8-bit image that is contained in the JDAT chunks that precede the JSEP chunk.
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.
The JSEP chunk is empty.
A JSEP chunk must appear between the JDAT
chunks of an 8-bit datastream and those of a 12-bit datastream, when
JDAT_sample_depth=20 in the JHDR chunk. The 8-bit
datastream must appear first. Both images must have the same width,
height, color type, compression method, and interlace type. 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:
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 format as given in the PNG specification: cHRM, gAMA, iCCP, sRGB, pHYs, oFFs, tEXt, tIME, and zTXt.
The PNG PLTE, hIST, pCAL, sBIT, and tRNS chunks are not defined in JNG.
When cHRM, gAMA, iCCP, or sRGB are present, they provide information about the colorspace of the decoded JDAT image, and they have no effect on the decoded alpha samples from the IDAT 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 JDAT and IDAT chunks can be interleaved.
A Delta-PNG datastream describes a single image, by giving the changes from a previous PNG (Portable Network Graphics), a JNG (JPEG Network Graphics), 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 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_type, filter_type, and
interlace_type are also the same as defined in the PNG
specification.
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.
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 or 3.
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)
color type 4 or color type 6.
3: Block pixel replacement.
4: Block alpha replacement.
5: No change to pixel data.
Block width: 4 bytes (unsigned integer). Omit when
delta_type=5.
Block height: 4 bytes (unsigned integer). Omit when
delta_type=5.
Block
X_location: 4 bytes (unsigned integer),
measured in pixels from the left edge
of the parent object. Omit when
delta_type=0 or when delta_type=5.
Block
Y_location: 4 bytes (unsigned integer),
measured in pixels from the top edge
of the parent object. Omit when
delta_type=0 or when delta_type=5.
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, 2, 3, or 4, 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.
When 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.
When 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.
The compression method, filter method, and interlace method need not be the same as those of the parent object.
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 actual pixel values are calculated using the method defined in the
delta_type field of the DHDR chunk. 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.
An encoder calculates the new 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 by bytes.
When color_type=3, the deltas are differences between
index values, not between color samples.
When 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 sample_depth of the data must match that of the parent
object, and 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.
When delta_type=3 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.
The sample_depth and color_type of the data
must match that of the parent object. The compression method, filter
method, and interlace method need not be the same.
When delta_type=4 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 must match that of the parent object, and 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.
When delta_type=5 in the DHDR chunk, there is
no change to the pixel data, and it is an error for IDAT or
JDAT to appear.
End of Delta-PNG datastream. An IEND chunk must be present for each DHDR chunk in a MNG datastream.
The IEND chunk is empty.
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)
Don't change the gray values. Set all the alpha values to fully opaque, except for pixels marked transparent by cheap transparency--set their alpha values to fully transparent. Discard the cheap transparency information (the PNG tRNS chunk data).
color_type 2 -> 6)
Don't change the RGB values. Convert the tRNS chunk data to alpha values as in the G -> GA promotion.
color_type 0 -> 2)
Set R, G, and B equal to the gray value. Apply the same operation to the cheap transparency data (if any).
color_type 4 -> 6)
Set R, G, and B equal to the gray value. Don't change the alpha values.
color_type 0 -> 6)
Set R, G, and B equal to the gray value. Handle transparency as in the G -> GA promotion.
color_type 3 -> 2)
Set R, G, and B according to the palette entry corresponding to the index. Discard the cheap transparency information (if any).
color_type 3 -> 6)
Set R, G, and B as in indexed -> RGB. Set the alpha value according to the cheap transparency information (if any). Discard the cheap transparency information.
JNG color_type 8 -> 12)
Don't change the gray values. Set all the alpha values to fully opaque. The given sample depth is the new sample depth for the alpha channel.
JNG color_type 10 -> 14)
Don't change the color values. Set all the alpha values to fully opaque. The given sample depth is the new sample depth for the alpha channel.
color_type
Only the sample depth is changed. The new sample depth must be larger than the old one.
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 only the alpha channel
is affected, if one is present. If the color_type has been
promoted from indexed, the original bit depth is always considered to be
8. See the PNG specification [PNG]
for
further information on these filling methods.
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,
height, sample_depth, and color_type
fields are ignored. The values for these parameters are inherited from
the parent object or from the PROM chunk.
The compression_method, interlace_type, and
filter_type 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.
See the PNG specification for the format of the PNG chunks. 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_type, or
interlace_type. 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 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 JNG JHDR chunk.
If the JHDR chunk is present, its width,
height, JDAT_sample_depth,
JDAT_color_type, JDAT_Filter_type, and
JDAT_interlace_type fields are ignored. The values for these
parameters are inherited from the parent object.
The IDAT_compression_method,
IDAT_interlace_type, and IDAT_Filter_type 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.
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 IHDR 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 chunk. Note: Be sure that the first JDAT chunk
precedes the first IDAT 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 chunk, then either the IJNG or JHDR 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.
Chunk name: 4 bytes (ASCII text). etc.
If multiple names appear in the DROP chunk, it is shorthand for multiple DROP chunks.
Chunk name: 4 bytes (ASCII text).
Polarity: 1 byte (unsigned integer).
0: Only.
1: All-but.
Keywords (null-separated Latin-1 text strings).
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).
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.
2: Before IDAT and/or JDAT.
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 datastream, when the framing mode is 2, 3, or 5 in the MNG FRAM chunk. When the framing mode is 1 or 4 and when the FRAM chunk is not present, viewers must treat these chunks in the same manner as they would treat them in single-image PNG files. MNG editors are expected to treat them as unknown chunks that will be handled as described in above.
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 in above.
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:
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, IHDR-IEND, DHDR-IEND, JHDR-IEND, and BASI-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-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 would require subsetting. We are allowing minimal decoders to skip 12-bit JNGs because those are likely to be rarely encountered and used only for special purposes. Applications that provide less than minimal support should check the MHDR "simplicity profile" for the presence of features that they are unable to support.
All PNG critical chunks must be fully supported. All
values of 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.
The PNG tRNS chunk, while it is an ancillary chunk, must be supported in MNG-compliant viewers, at least to the degree that fully opaque pixels are opaque and fully transparent ones are transparent. It is recommended that alpha data from the tRNS chunk be fully supported in the same manner as alpha data from an RGBA image or a JNG with an alpha channel contained in IDAT chunks.
Ancillary chunks other than PNG tRNS can be ignored, and do not even have to be recognized.
It is highly recommended that decoders support at least the gAMA chunk to allow platform-independent color rendering.
All JNG critical chunks must be fully supported. All
values of 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.
All JNG ancillary chunks can be ignored, and do not even have to be recognized.
Only sample_depth=8 must be supported. The JSEP
chunk must be recognized and must be used by minimal decoders to select
the 8-bit version of the image, when both 8-bit and 12-bit versions
are present, as indicated by JDAT_sample_depth=20 in the
JHDR chunk. When JDAT_sample_depth=12, minimal
decoders are not obligated to display anything, but should display an
empty transparent 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
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.
The MEND chunk must be recognized but does not require any processing other than completing the last frame.
The repeat_count must be supported. The
nest_level should be used as a sanity check but is not
required.
Must be fully supported. All objects can be treated as "concrete" if the decoder does not wish to take advantage of the distinction between "abstract" and "concrete".
Must be fully supported.
The framing_mode and clipping parameters must be
supported. The interframe_delay must be supported
except by single-frame viewers. The sync_id and
sync_timeout data can be ignored.
Must be fully supported.
Partial support is required: All existing objects must be marked "frozen" when the SAVE chunk is processed, so that unneeded objects can be discarded when the SEEK chunk or an empty DISC chunk is processed. The SEEK chunk must be processed as if it were an empty DISC chunk, as a minimum. Other information need only be "saved" and "restored" when the viewer is able to skip or jump to random SEEK chunk locations. The optional index can be ignored. Slide-show controllers may wish to support SAVE and SEEK fully.
Can be ignored.
Must be fully supported.
Can be recognized and ignored. These are only of concern to MNG editors and to MNG viewers that handle private chunks or chunks that can be selected by keyword, such as pCAL and iCCP. If you decide to support such chunks, then you will also have to support these three chunks.
Ancillary chunks appearing in Delta-PNG datastreams must be treated in the same manner as if they appeared in a PNG or JNG datastream. See the recommendations, above. Note that the PNG tRNS chunk must be supported, despite its being an ancillary chunk in PNG.
The following recommendations do not form a part of the specification.
It is a good idea to use a single colorspace 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 colorspace at the top level of MNG, using gAMA and cHRM chunks, and either an sRGB or iCCP chunk, and removing any colorspace chunks from the individual images after converting them to the common colorspace.
When the encoder converts all images to a single colorspace before putting them in the MNG datastream, this will allow decoders to improve the speed and consistency of the display.
For single-frame MNG datastreams, however, where decoding speed is less important and exact color rendition might be more important, it is best to leave the images in their original colorspace, as recommended in the PNG specification, to avoid any loss of data due to conversion, and to retain the individual colorspace chunks if the images have different colorspaces.
Always use framing mode 4 or 5 when all of the images are opaque. This avoids unnecessary screen clearing, which can cause flickering.
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 in below.
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.
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 by subsequent frames, it can make life easier for decoders 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.
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 or Delta-PNG datastream that includes it, and any subsequent Delta-PNG datastreams that depend on it.
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 without a PLTE chunk, that 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, 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.
The frame_width and frame_height are defined in
the MHDR chunk and cannot be changed.
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.
The subframe clipping boundaries are optionally defined in the
FRAM chunk, and cannot be changed within a subframe. When
the framing mode is 1, 2, or 3, viewers must, prior to displaying
each subframe, clear the area within the subframe clipping boundaries
to the background color, and display the background image if one has
been defined. Viewers must not change any pixels outside the subframe
boundaries; encoders must be able to rely on the fact that the part of
the display that is outside the subframe clipping boundaries (but inside
the area defined by 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 image clipping boundaries are defined in the DEFI and CLIP chunks. They are associated with individual objects, not with the subframes, and they can be changed within a subframe. They are useful for exposing only a portion of an image in a frame, to achieve effects such as scrolling, panning, or gradual exposure.
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.
On systems where file names customarily include an extension
signifying file type, the extension .mng is recommended for
MNG files. Lower case .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 do not intend to register an Internet Media Type for JNG files.
Segments, subframes, and objects are externally accessible via named SEEK, FRAM, and eXPI 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 animation 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. If the SAVE chunk is present and contains the optional index, this can help the client find the needed segment quickly.
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, subframe 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).
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 animation 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.
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, 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 strings "clock(", "frame(", or "frames(" (all case-insensitive), which are reserved for use in URI queries and fragments.
See [RFC-2396], Uniform Resource Identifiers (URI): Generic Syntax and the W3C SMIL recommendation.
Boutell, T., et. al., PNG (Portable Network Graphics Format Version
1.0), RFC 2083,
ftp://ds.internic.net/rfc/rfc2083.txt
also available at
ftp://swrinde.nde.swri.edu/pub/png/documents/.
This
specification has also been published as a W3C Recommendation, which is
available at
http://www.w3.org/TR/REC-png.html.
PNG Special-Purpose Public Chunks,
ftp://swrinde.nde.swri.edu/pub/png/documents/.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119/BCP 14, Harvard University, March 1997.
Freed, N., and N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies", RFC 2045, Innosoft,
First Virtual, November 1996.
ftp://ds.internic.net/rfc/rfc2045.txt.
Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
Innosoft, MCI, USC/Information Sciences Institute, November 1996.
ftp://ds.internic.net/rfc/rfc2048.txt
Berners-Lee, T., Masinter, L., McCahill, M., "Uniform Resource
Locators (URL)", RFC 1738, CERN, Xerox Corporation, University of
Minnesota, December 1994.
ftp://ds.internic.net/rfc/rfc1738.txt
Security considerations are addressed in the basic 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
sync_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. (The SEEK chunk makes it safe for a
viewer to resume processing after it encounters a corrupted portion of a
MNG datastream.)
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. IDAT ... IEND
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.
30 # 30 ticks per second.
tERm 2 0 120 10 # When done, repeat from SAVE 10 times.
FRAM 1 0 2 0 0 0 30 # Set framing_mode=1 (because the
# images are opaque) and frame_duration to 1 sec.
DEFI 1 IHDR ... IDAT ... IEND # Eight PNG datastreams
DEFI 2 IHDR ... IDAT ... IEND # are read and stored as
DEFI 3 IHDR ... IDAT ... IEND # abstract images, and
DEFI 4 IHDR ... IDAT ... IEND # are displayed as they
DEFI 5 IHDR ... IDAT ... IEND # are read.
DEFI 6 IHDR ... IDAT ... IEND
DEFI 7 IHDR ... IDAT ... IEND
DEFI 8 IHDR ... IDAT ... IEND
SAVE
SHOW 1 8
MEND
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
1 # 1 tick per second.
FRAM 1 0 2 2 0 2 0 600 0 # Set frame_duration to 0,
# sync_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 # 1 tick per second.
DEFI 1 1 1 # Define image 1, invisible, concrete.
IHDR ... IDAT ... IEND
FRAM 1 0 2 2 0 2 0 600 0 # set frame_duration to 0,
# sync_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 # 30 ticks per second.
tEXtTitle\0Sample Movie
fPRI 0 128 # Default frame priority is "medium".
FRAM 1 0 2 0 0 0 3 # Set frame_duration 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, ticklength. BACK 16448 16448 52800 1 # Must use sky blue background. DEFI 1 1 # Define invisible abstract thumbnail image. IHDR 64 64 4 3 0 0 0 IDAT IEND eXPI 1 "thumbnail 1" DEFI 1 1 # Also define a larger thumbnail. IHDR 96 96 4 3 0 0 0 IDAT IEND eXPI 1 "thumbnail 2" DISC # Discard the thumbnail image. FRAM 5 "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. gAMA 50000 IDAT ... IEND # End of image. CLON 1 2 0 0 1 0 518 6 # Make a full "concrete" clone. DHDR 2 1 5 # Modify it (no change to pixels). ORDR faLT 2 # Establish chunk placement. gAMA 100000 # 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 ... gAMA 50000 # We need a new gAMA because PLTE ... # this is not a Delta-PNG datastream. 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 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame
# sets frame_duration=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 0 1 # make a full clone of it as image 3.
0 400 500 # Location for image 3.
DHDR 3 1 3 0 0 0 0 # Modify image 3.
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 # 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 # Width, height, ticklength.
BACK 52800 52800 52800 # "Browser gray" default background.
FRAM 4 0 2 0 0 0 3 # Set frame_duration=3 ticks. Use
# framing mode 4 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 1 # 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 5 # 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 don't show it now.
LOOP 0 0 15
DHDR 2 1 5 # An image delta.
# Delta-type is 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 8-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.
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 subtraction,
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. 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.
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 and height on screen.
BACK 50000 50000 50000 0 # advisory gray background
DEFI 1 1 0 0 256 # Define image 1 but don't 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 # Frame_duration = 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 # Frame_duration = 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 4:
(Same as above down to the LOOP chunk.)
BACK 50000 50000 50000 2 1 # Advisory gray background.
# Mandatory image background.
FRAM 4 0 2 0 0 0 1 # Frame_duration = 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
Outline of a program to convert GIF animations to MNG format:
begin
write "MHDR" and "BACK" chunks
saved_images := 0
Frame_duration := 0
First_frame := TRUE
if(loops>1) "write tERm 2 0 0 loops" chunk
for subimage in gif89a file do
if(Frame_duration != gif_duration) then
Frame_duration := gif_duration
write "FRAM 5 0 2 2 0 2 0 Frame_duration 0" chunk
First_frame := FALSE
else if(First_frame == TRUE)then
write "FRAM 5" 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.
Contributors' names are presented in alphabetical order: [some addresses need to be updated from Greg's list]
Trademarks:
Glenn Randers-Pehrson
611 Rivershore Court
Edgewood, MD 21040-3603
Phone: (410) 278-6554
EMail: randeg @ alumni.rpi.edu
End of MNG Specification.
This document expires on 09 May 1999