For list of authors, see Credits (Chapter 13).
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/.
In the case of any discrepancy between this extract and the full MNG specification, the full MNG specification shall take precedence.
This document presents the MNG-LC (Multiple-image Network Graphics, Low Complexity) format, which is a proper subset of the MNG (Multiple-image Network Graphics) format.
MNG is a multiple-image member of the PNG (Portable Network Graphics) format family that can contain animations (slide shows) or complex still frames, comprised of multiple PNG single-image datastreams.
The MNG-LC format uses the same chunk structure that is defined in the PNG specification and shares other features of the PNG format. Any valid PNG datastream is also a valid MNG-LC datastream.
A MNG-LC 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 datastream.
This document includes examples that demonstrate various capabilities of MNG-LC including simple movies and composite frames.
If "231" looks like
the number "231"
instead of 2 raised to the power
31, your viewer is not
recognizing the HTML 4.0 <SUP> tag; you need
to look at the HTML 2.0, ASCII text, or PostScript
version of this document instead.
This document presents a low-complexity version (MNG-LC, which is a proper subset) of the MNG (Multiple-image Network Graphics) format.
Note: This specification depends on the PNG (Portable Network Graphics) [PNG] and, for MNG-LC applications that are enhanced with JNG support, the JNG (JPEG Network Graphics) specifications. The PNG specification is available at the PNG web site,
http://www.cdrom.com/pub/png/
and the JNG (JPEG Network Graphics) specification and the full MNG specification are available at the MNG web site,
http://www.cdrom.com/pub/mng/
A MNG-LC datastream describes a sequence of zero or more single frames, each of which can be composed of zero or more embedded images.
The embedded images can be PNG or JNG datastreams. MNG-LC datastreams do not contain JNG (JPEG Network Graphics) datastreams, which are allowed in full MNG datastreams, but MNG-LC applications can be enhanced to recognize and process those as well.
A typical MNG-LC datastream consists of:
MNG is fundamentally declarative; it describes the elements that go into an individual frame. It is up to the decoder to work out an efficient way of making the screen match the desired composition whenever a nonzero interframe delay occurs. Simple decoders can handle it as if it were procedural, compositing the images into the frame buffer in the order that they appear, but efficient decoders might do something different, as long as the final appearance of the frame is the same.
MNG is pronounced "Ming."
When a MNG datastream is stored in a file, it is recommended that ".mng" be used as the file suffix. In network applications, the Media Type "video/x-mng" can be used. Registration of the media type "video/mng" might be pursued at some future date.
The 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.
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 the full MNG specification.
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 datastream, with a PNG signature, is also a valid MNG-LC datastream that must be recognized and decoded by MNG-LC decoders. An independent JNG datastream must also be recognized and decoded by any MNG-LC decoder that has been enhanced to include JNG support. This kind of MNG-LC or enhanced MNG-LC 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 and the "terminology" section of the full MNG specification.
When the layers of a frame do not cover the entire area defined by the width and height fields from the MHDR chunk, the layers are composited over the previous frame to obtain the new frame.
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.
object_id is
an unsigned sixteen-bit number that serves as the identifier of a set of
object attributes. In MNG-LC
only object 0 is permitted.
do_not_show flag to zero.
This chapter describes chunks that can appear at the top level of a MNG datastream.
This section describes critical MNG control chunks that MNG-LC-compliant decoders must recognize and process ("processing" a chunk sometimes can consist of simply recognizing it and ignoring it; some chunks have been declared to be critical only to prevent them from being relocated by MNG editors).
The MHDR chunk is always first in all MNG datastreams except for those that consist of a single PNG or JNG datastream with a PNG or JNG signature.
The MHDR chunk contains 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).
bit 0:
1: Presence or absence of certain
features is specified by the
remaining bits of the simplicity
profile.
(must be 1 in MNG-LC datastreams)
bit 1:
0: Simple MNG features are absent.
1: Simple MNG features are present.
bit 2:
0: Complex MNG features are absent.
(must be 0 in MNG-LC datastreams)
bit 3:
0: transparency is absent or can be
ignored.
1: transparency is present and is
essential (critical) for proper
display of the images.
bit 4:
0: JNG is absent.
1: JNG is present.
(must be 0 in MNG-LC datastreams)
bit 5:
0: Delta-PNG is absent.
(must be 0 in MNG-LC datastreams)
bits 6-15:
Reserved for public expansion. Must
be zero in this version.
bits 16-30:
Available for private or experimental
expansion. Undefined in this version
and can be ignored.
bit 31:
Must be zero.
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
the
default
clipping boundaries.
The ticks_per_second field gives the
unit used by the FRAM chunk to specify interframe delay and 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 interframe
delay and a zero timeout. Unless the user intervenes, viewers will only
display the first frame in the datastream.
When ticks_per_second is nonzero,
and there is no other
information available about interframe delay,
viewers should display
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
TERM chunk. If the frame count is greater
than 231-1,
encoders should write 231-1, 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
TERM chunk.
If the layer count is greater than 231-1, encoders
should
write 231-1, 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
TERM chunk.
Authors who write this field should choose a
value of "ticks_per_second" that will allow the nominal play time
to be expressed in a four-bit integer. If the nominal play time is greater
than 231-1 ticks, encoders should write 231-1,
representing an infinite nominal play time.
When bit 0 of the simplicity profile is zero, the simplicity (or complexity) of the MNG datastream is unspecified, and all bits of the simplicity profile must be zero. The simplicity profile must be nonzero in MNG-LC datastreams.
If the simplicity profile is nonzero, it can be regarded as a 32-bit profile, with bit 0 (the least significant bit) being a "profile-validity" flag, bit 1 being a "simple MNG" flag, bit 2 being a "complex MNG" flag, bit 3 being a "transparency" flag, bit 4 being a "JNG" flag, and bit 5 being a "Delta-PNG" 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.
When bit 1 is zero ("simple" MNG features are absent), the datastream does not contain the DEFI, FRAM, or global PLTE and tRNS chunks.
"Transparency is absent or can be ignored" means that the MNG or PNG tRNS chunk is not present and no PNG or JNG image has an alpha channel, or that if they are present they are not essential (or critical) for displaying the images.
A MNG-LC (i.e., a "low-complexity MNG") datastream must have a simplicity profile with bit 0 equal to 1 and all other bits except possibly for bits 1 and 3 ("simple MNG" MNG features and transparency) equal to zero. If bit 4 (JNG) is 1, the datastream is a "MNG-LC with JNG" datastream. MNG-LC decoders are allowed to reject such datastreams unless they have been enhanced with JNG capability.
Encoders that write a nonzero simplicity profile should endeavor to be accurate, so that decoders that process it will not unnecessarily reject datastreams. For example, the simplicity profile 31 indicates that JNG, critical transparency, and at least one "complex" MNG feature are all present, but Delta-PNG is not. This example would not qualify as a MNG-LC datastream because a "complex" MNG feature might be present. If the simplicity profile promises that certain features are absent, but they are actually present in the MNG datastream, the datastream is invalid.
The MEND chunk's data length is zero. It signifies the end of a MNG datastream.
The LOOP chunk can be ignored by MNG-LC decoders, along with the ENDL chunk.
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,
or JHDR-IEND datastreams.
Bit 1 of the MHDR simplicity profile can be used to promise that the DEFI chunk is not present.
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.
This field must be zero in MNG-LC
files.
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
cannot 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).
MNG-LC decoders can ignore this flag.
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).
Negative values are permitted for the X and Y location and clipping boundaries. The positive directions are downward and rightward from the frame origin.
The DEFI chunk values are discarded after the object's IEND chunk is processed. If no DEFI chunk precedes an object, the decoder must use the following default values:
Object_id = 0
Do_not_show = 0
Concrete_flag = 0
X_location = 0
Y_location = 0
Left_cb = 0
Right_cb = frame_width
Top_cb = 0
Bottom_cb = frame_height
The PLTE chunk has the same format as a PNG PLTE chunk. It provides a global palette that is inherited by PNG datastreams that contain an empty PLTE chunk.
The tRNS chunk has the same format as a PNG tRNS chunk. It provides a global transparency array that is inherited along with the global palette by PNG datastreams that contain an empty PLTE chunk.
If a PNG datastream is present that does not contain an empty PLTE chunk, neither the global PLTE nor the global tRNS data is inherited by that datastream.
If the global PLTE chunk is not present, each indexed-color PNG in the datastream must supply its own PLTE (and tRNS, if it has transparency) chunks.
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 and tRNS 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, along with the global tRNS data if it is present. The global tRNS data can be subsequently overridden by a tRNS chunk in the PNG datastream. 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.
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.
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.
A JNG (JPEG Network Graphics) datastream.
See the JNG specification 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.
MNG-LC applications are not expected to process JNG datastreams unless they have been enhanced with JNG capability.
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 is advisory.
Applications can use it if they
choose to.
1: Background color is mandatory.
Applications must use it.
This byte can be omitted. If so,
the background color is advisory.
Viewers are expected to composite the first layer in the MNG-LC datastream against a background layer that fills the entire frame. The BACK chunk provides a background that the viewer can use for this purpose. If it is not "mandatory" the viewer can choose another background if it wishes. If the BACK chunk is not present, or if it has been clipped to less than full frame, the viewer must provide its own background for the first subframe. Each subframe after the first must be composited over the layers that precede it.
Viewers are expected, however, to composite every subframe in the MNG datastream against a fresh copy of the background, when the framing mode given in the FRAM chunk is 3 or 4. The images and the background are both clipped to the frame boundaries given in the FRAM chunk. Anything outside these boundaries is inherited from the previous subframe.
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.
When the BACK chunk appears between FRAM chunks, it applies to the upcoming frame, not to the current one. When the framing_mode is 3, it takes effect immediately prior to the next IHDR or JHDR chunk in the datastream.
Multiple instances of the BACK chunk are permitted in a MNG datastream.
The BACK chunk can be omitted. If a background is needed and the BACK chunk is omitted, then the viewer must supply its own background. For the purpose of counting layers, such a viewer-supplied background layer is counted the same as a background supplied by the BACK chunk.
In practice, most applications that use MNG as part of a
larger composition should ignore the BACK data if
mandatory_background=0 and the application already has
its own background definition. This will frequently be the case in
World Wide Web pages, to achieve nonrectangular transparent animations
displayed against the background of the page.
The FRAM chunk provides information that a decoder needs for generating layers, subframes, and frames. The FRAM parameters govern how the decoder is to behave when it encounters a FRAM chunk, or an embedded image. The FRAM chunk also delimits subframes.
Bit 1 of the MHDR simplicity profile can be used to promise that the FRAM chunk is not present.
An empty FRAM chunk is just a subframe delimiter. A nonempty one is a subframe delimiter, and it also changes FRAM parameters, either for the upcoming subframe or until reset. 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: No background layer is inserted.
The interframe delay occurs before
each image (except the very first
in the datastream) is displayed.
2: No background layer is inserted.
The interframe delay occurs before
each "FRAM" chunk. No delay occurs
before or between the images.
3: The background layer is displayed
before each image is displayed.
The interframe delay occurs before
displaying the background, and no
delay occurs between the background
and the image.
4: The background layer is inserted
after each "FRAM" chunk. The
interframe delay occurs before
the background is displayed, and no
delay occurs before or between the
images.
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 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 the next 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.
Timeout: 4 bytes (unsigned integer). Omit if
change_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:
The framing mode provides information to the decoder that it uses whenever it is about to display an image, and when it is processing the next FRAM chunk.
The following events trigger the display of an image:
Any of these events generates a display update, even if the visible image is outside the clipping boundaries and no pixels are actually changed.
When a decoder is ready to perform a display update, it must check the framing mode, to decide whether it should restore the background (framing modes 3 and 4) or not (framing modes 1 and 2), and whether it needs to wait for the interframe delay to elapse before continuing (framing modes 1 and 3) or not (framing modes 2 and 4).
When the interframe delay is zero, viewers are not required actually to update the display but can continue to process the remainder of the frame and composite the next image over the existing frame before displaying anything. The final result must appear the same as if each image had been displayed in turn with no delay.
Regardless of the framing mode, encoders must insert a background layer, with a zero delay, ahead of the first image layer in the datastream, even when the BACK chunk is not present. This layer must be included in the layer count but not in the frame count.
framing_mode=1, the decoder must wait until the
interframe delay has elapsed before displaying each image.
The background layer is inserted, with no interframe delay, before the
first image in the datastream. If the interframe delay is nonzero,
the first frame in the datastream consists of two layers and the rest
consist of a single layer.
In the usual case, the interframe delay is nonzero, so each
subframe is a composite frame. When framing_mode=2, viewers are
expected to display all of the images in a frame at once, if possible, or as
fast as can be managed, without clearing the display or restoring the
background.
The next FRAM chunk delimits the subframe. A subframe
also ends when a SEEK chunk or the MEND chunk appears,
and the final frame ends when the MEND chunk appears.
framing_mode=3,
a background layer is displayed before each image layer is displayed,
without a delay between the background and the image. The decoder must
wait until the interframe delay has elapsed before displaying the background.
If the interframe delay is nonzero, each frame consists of two
layers. A subframe boundary occurs after each image appears. Otherwise,
framing_mode=3 is identical to framing_mode=1.
framing_mode=4, the background layer is displayed
before each frame, i.e., after each FRAM chunk, with no interframe
delay before each image. The decoder must wait until the
interframe delay has elapsed before displaying the background layer.
Otherwise, framing_mode=4 is identical to framing_mode=2.
The subframe name must conform to the same formatting rules as
those for a PNG tEXt keyword: It must consist only of printable
Latin-1 characters and must not have leading or trailing blanks, but
can have single embedded blanks. There must be at least one (unless
the subframe name is omitted) and no more than 79 characters in the
keyword. Keywords are case-sensitive. There is no null byte within
the keyword.
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
case-insensitive
strings "CLOCK(", "FRAME(", or "FRAMES(",
which are reserved for use in URI queries and
fragments, as explained in the full MNG specification.
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 and completed. Decoders are not obligated to display such subframes individually; they can composite them offscreen and only display the complete frame.
There is no interframe delay before the first layer (the implicit background layer) in the datastream nor after the final frame, regardless of the framing mode.
The timeout field can be a number or <infinity>. Infinity can be represented by 0x7fffffff.
The termination condition given in the
change_timeout_and_termination field specifies how much
longer, after the normal interframe delay has elapsed, the frame will
endure, and whether the interframe delay can be shortened. It can take
the following values:
sync_id, but
must wait no longer than the timeout.
When the timeout is zero, the arrival of a signal can shorten the duration
of the frame to less than the interframe delay.
The sync_id list can be omitted if the termination
condition is not "external-signal".
When the sync_id list is changed, the number of
sync_id entries is determined by the remaining length of the
chunk data, divided by four. This number can be zero, which either
inactivates the existing sync_id list for one frame or
deletes it.
The initial values of the FRAM parameters are:
Framing mode = 1
Subframe name = <empty string>
Interframe delay = 1
Left subframe boundary = 0
Right subframe boundary = frame width
Top subframe boundary = 0
Bottom subframe boundary = frame height
Termination = deterministic
Timeout = 0x7fffffff (infinite)
Sync id = <empty list>
The subframe boundaries from the FRAM chunk are only used for clipping, not for placement. The DEFI chunk can be used to specify the placement of each image within the layer. The DEFI chunk can be used to specify clipping boundaries for each image. Even when the left and top subframe boundaries are nonzero, the image locations are measured with respect to the {0,0} position in the display area.
If the layers are transparent or do not cover the entire area defined by the subframe clipping boundaries, they are composited against whatever already occupies the area, when the framing mode is 1 or 2. When the framing mode is 3 or 4, they are composited against the background defined by the BACK chunk, or against an application-defined background, if the BACK chunk is not present or does not define a mandatory background. The images, as well as the background, are clipped to the subframe clipping boundaries. Any pixels outside the subframe clipping boundaries remain unchanged from the previous subframe.
The interframe_delay field gives the duration of
display, which is the minimum time that must elapse from the
beginning of displaying one subframe until the beginning of displaying
the next. It
is measured in "ticks" using the tick length determined from
ticks_per_second defined in the MHDR chunk.
When the interframe delay is zero, it indicates that
all subframes are to be combined into a single frame, until
a nonzero interframe delay is specified or the MEND chunk is reached.
A viewer does not actually have to follow the procedure of erasing the screen, redisplaying the background, and recompositing the images against it, but what is displayed when the frame is complete must be the same as if it had. It is sufficient to redraw the parts of the display that change from one frame to the next.
The sync_id list provides a point at which the processor
must wait for all pending processes to reach the synchronization
point having the same sync_id before resuming, perhaps
because of a need to synchronize a sound datastream (not defined
in this specification) with the display, to synchronize stereo
images, and the like. When the period defined by the sum of the
interframe_delay and the timeout fields
elapses, processing can resume even though the processor has not
received an indication that other processes have reached the
synchronization point.
Note that the synchronization point does not occur immediately, but at the end of the subframe that follows the FRAM chunk.
The identifier sync_id=0 is reserved to represent
synchronization with a user input from a keyboard or pointing device.
The sync_id values 1-255 are reserved to represent the
corresponding ASCII letter, received from the keyboard (or a simulated
keyboard), and values 256-1023 are reserved for future definition
by this specification. If multiple channels (not defined in this
specification) are not present, viewers can ignore other values
appearing in the sync_id list.
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.
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.
This and the remaining fields must be
present if termination_action is 3, and
must be omitted otherwise.
Delay: 4 bytes (unsigned integer). Delay, in
ticks, before repeating the animation.
Iteration max: 4 bytes (unsigned integer). Maximum
number of times to repeat the animation.
The TERM chunk, if present, must appear either immediately after the MHDR chunk or immediately prior to a SEEK 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.
Simple decoders that only read MNG datastreams sequentially can safely ignore the SAVE and SEEK chunks.
This section describes ancillary MNG chunks. MNG-compliant decoders are not required to recognize and process them.
The eXPI chunk takes a snapshot of an image, associates the name with that snapshot, and makes the name available to the "outside world" (like a scripting language).
The chunk contains an object identifier (snapshot id) and a name:
Snapshot id: 2 bytes (unsigned integer).
(must be zero in MNG-LC
datastreams)
Snapshot name: 1-79 bytes (Latin-1 text).
When the snapshot_id is zero, the snapshot is the first instance of an embedded image following the eXPI chunk.
Note that the snapshot_name is associated with the
snapshot, not with the snapshot_id nor its future contents;
changing the image identified by snapshot_id will not
affect the snapshot.
The snapshot_name means nothing inside the scope of the
MNG-LC specification.
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
chunk. A decoder that knows of no
"outside world" can simply ignore the eXPI chunk. This chunk
could be used in MNG datastreams that define libraries of related
images, rather than animations, to allow applications to extract
images by their snapshot_id.
Names beginning with the word "thumbnail" are reserved for snapshot images that are intended to make good icons for the MNG. Thumbnail images are regular PNG images, but they would normally have smaller dimensions and fewer colors than the MNG frames. They can be defined with the potential visibility field set to "invisible" if they are not intended to be shown as a part of the regular display.
The snapshot_name string must follow the format of a
tEXt keyword: It must consist only of printable Latin-1
characters and must not have leading or trailing blanks, but can have
single embedded blanks. There must be at least one and no more than
79 characters in the keyword. Keywords are case-sensitive. There is
no null byte terminator within the snapshot_name string,
nor is there a separate null byte terminator. Snapshot names should
not begin with the case-insensitive strings
"CLOCK(", "FRAME(", or "FRAMES("
which are reserved for use in URI queries and
fragments.
Multiple instances of the eXPI chunk are permitted
in a MNG datastream, and they need not have different values of
snapshot_id.
The MNG pHYs chunk is identical in syntax to the PNG pHYs chunk. It applies to complete MNG layers and not to the individual images within them.
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 iTXt, 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.
JNG (JPEG Network Graphics) is the lossy sub-format for MNG objects. It is described in the full MNG specification and is also available as a separate extract from the full MNG specification. Both documents are available at the MNG home page,
http://www.cdrom.com/pub/mng/
MNG-LC applications can choose to support JNG or not. Those that do not can check bit 4 (JNG is present/absent) of the MHDR simplicity profile to decide whether they can process the datastream.
The chunk copying rules for MNG are similar to those in PNG. Authors of MNG editing applications should consult the full MNG specification for details.
This section specifies the minimum level of support that is expected of MNG-LC-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.
Applications that provide less than minimal MNG support should check the MHDR "simplicity profile" for the presence of features that they are unable to support. A specific subset, in which "complex MNG features" and JNG are absent, is called "MNG-LC". In MNG-LC datastreams, bit 0 of the simplicity profile must be 1 and bits 2 and 4 must be 0. Another subset is called "MNG-VLC". In MNG-VLC datastreams, "simple MNG features" are also absent, and bit 1 must therefore also be 0.
Subsets are useable when the set of MNG datastreams to be processed is known to be (or is very likely to be) limited to the feature set in MNG-LC. Limiting the feature set in a widely-deployed WWW browser to anything less than MNG with 8-bit JNG support would be highly inappropriate.
Some subsets of MNG support are listed in the following table, more or less in increasing order of complexity.
Level of support MHDR Profile bits Profile
31 30-6 5 4 3 2 1 0 value
MNG-VLC without transparency 0 0 0 0 0 0 0 1 1
MNG-VLC 0 0 0 0 1 0 0 1 9
MNG-VLC with JNG 0 0 0 1 0 0 0 1 17
MNG-LC 0 0 0 0 1 0 1 1 11
MNG-LC with JNG 0 0 0 1 1 0 1 1 19
One reasonable path for an application developer to follow might be to develop and test the application at each of the following levels of support in turn:
An equally reasonable development path might be
On the other hand, a developer working on an application for storing multi-page fax documents might have no need for more than "MNG-VLC without transparency".
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.
framing_mode and clipping parameters must be
supported. The interframe_delay must be supported
except by single-frame viewers. The sync_id and
timeout data can be ignored.
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported (interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded). The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported. Alpha can be ignored if bit 3 of the simplicity profile is
0.
Bit 4 of the simplicity profile can be used to promise that JNG chunks are not present. Viewers that choose not to support JNG can check this bit before deciding to proceed. MNG-LC decoders are not required to support JNG.
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported (interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded). The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported.
sample_depth=8 must be supported. The JSEP
chunk must be recognized and must be used by minimal decoders to select
the eight-bit version of the image, when both eight-bit and twelve-bit versions
are present, as indicated by JDAT_sample_depth=20 in the
JHDR chunk. When JDAT_sample_depth=12, minimal
decoders are not obligated to display anything. Such decoders can
choose to display nothing or an empty rectangle of the width and height
specified in the JHDR chunk.
The following recommendations do not form a part of the specification.
It is a good idea to use a single color space for all of the layers in an animation, where speed and fluidity are more important than exact color rendition. This is best accomplished by defining a single color space at the top level of MNG, using and either an sRGB chunk or the gAMA and cHRM chunks and perhaps the iCCP chunk, and removing any color space chunks from the individual images after converting them to the common color space.
When the encoder converts all images to a single color space before putting them in the MNG datastream, 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 color space, as recommended in the PNG specification, to avoid any loss of data due to conversion, and to retain the individual color space chunks if the images have different color spaces.
Always use framing mode 1 or 2 when all of the images are opaque. This avoids unnecessary screen clearing, which can cause flickering.
If it
is necessary to establish a synchronization point immediately, this can
be done by using two consecutive FRAM chunks, the first setting
a temporary interframe_delay=0, timeout, and
sync_id, and the second establishing the synchronization
point:
FRAM 2 0 1 1 0 1 0000 timeout sync_id FRAM 0 name
When a fatal error is encountered, such as a bad CRC or an unknown critical MNG chunk, minimal viewers should simply abandon the MNG datastream.
Decoders are required to be able to interpret datastreams that contain interlaced PNG images, but are only required to display the completed frames; they are not required to display the images as they evolve. Viewers that are decoding datastreams coming in over a slow communication link might want to do that, but MNG authors should not assume that the frames will be displayed in other than their final form.
When a PLTE chunk is received, it only affects the display of the PNG datastream that includes or inherits it. Decoders must take care that it does not retroactively affect anything that has already been decoded.
If a frame contains two or more images, the PLTE chunk in one image does not affect the display of 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.
Viewers that can only display a single frame must display the first frame that they encounter.
MNG-LC provides three types of clipping, in addition to any clipping that might be required due to the physical limitations of the display device.
frame_width and frame_height are defined in
the MHDR chunk and cannot be changed by any other MNG chunk.
Decoders can use these parameters to establish the size of
a window in which to display the MNG frames. When the frame_width
or frame_height exceeds the physical dimensions of the
display hardware, the contents of the area outside those dimensions is
undefined. If a viewer chooses, it can create "scroll bars" or the
like, to enable persons to pan and scroll to the offscreen portion
of the frame. If this is done, then the viewer is responsible for
maintaining and updating the offscreen portion of the frame.
In the case of a MNG datastream that consists of a PNG or JNG
datastream, with the PNG or JNG signature, the frame_width
and frame_height are defined by the width and
height fields of the IHDR (or JHDR) chunk.
frame_width and frame_height)
will remain on the display from frame to frame without being explicitly
redisplayed.
On systems where file names customarily include an extension
signifying file type, the extension .mng is recommended for
MNG (including MNG-LC)
files. Lowercase .mng is
preferred if file names are case-sensitive. The extension .jng is
recommended for JNG files.
See also the PNG-1.1 draft:
Randers-Pehrson, G., et. al., "PNG (Portable Network Graphics
Format) Version 1.1", which is available at
ftp://swrinde.nde.swri.edu/pub/png/documents/.
Security considerations are addressed in the basic PNG specification.
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.
1 # 1 tick per second.
7 6 6 # Layers, frames, play time
1 # MNG-VLC simplicity
TERM 3 0 120 10 # When done, repeat animation 10 times.
IHDR ... IDAT ... IEND # Six PNG datastreams
IHDR ... IDAT ... IEND # are read and displayed.
IHDR ... IDAT ... IEND
IHDR ... IDAT ... IEND
IHDR ... IDAT ... IEND
IHDR ... IDAT ... IEND
MEND
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
1 6 5 0 # 1 tick per second.
6 5 5 # Layers, frames, play time.
1 # Simplicity profile (MNG-VLC)
FRAM 1 0 2 2 0 2 1 600 0 # Set interframe_delay to 1,
# timeout to 600 sec, and sync_id list to {0}.
SAVE
SEEK "Briefing to the Workforce"
IHDR ... IDAT ... IEND # DEFI 0, visible, abstract
SEEK "Outline" # is implied.
IHDR ... IDAT ... IEND
SEEK "Our Vision" IHDR ... IDAT ... IEND
SEEK "Our Mission" IHDR ... IDAT ... IEND
SEEK "Downsizing Plans" IHDR ... IDAT ... IEND
MEND
These examples in the full MNG specification use features that are not available in MNG-LC.
Outline of a program to convert simple GIF animations that do not use the "restore-to-previous" disposal method to "simple" MNG (or "MNG-LC") format:
begin
write "MHDR" and "mandatory BACK" chunks
Interframe_delay := 0
Previous_mode := 1
Framing_mode := 1
if(loops>1) "write TERM 3 0 0 loops"
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 0 0 2 2 0 2 0 interframe_delay 0"
endif
if(X_loc != 0 OR Y_loc != 0) then
write "DEFI 0 0 0 X_loc Y_loc" chunk
endif
write "<image>"
if (gif_disposal_method < 1) then
/* (none or keep) */
Framing_mode := 1
else if (gif_disposal_method == 2) then
/* (restore background) */
Framing_mode := 3
else if (gif_disposal_method == 3) then
/* (restore previous) */
error ("can't do gif_disposal method = previous.")
endif
if(Framing_mode != Previous_mode) then
write "FRAM Framing_mode" chunk
Previous_mode := Framing_mode
endif
end
write "MEND" chunk
end
Where "<image>" represents a PNG datastream containing a GIF frame that has been converted to PNG format.
Caution: if you write such a program, you might have to pay royalties in order to convey it to anyone else.
Here is an example of storing a library of icons in a MNG-LC datastream. All of the icons use the same palette, transparency, and colorspace, so these are put in global chunks at the beginning. Empty PLTE chunks in the embedded images are used to import the global palette and transparency data.
MHDR 96 96 1 6 5 5 11 # Profile 11 is MNG-LC sRGB 2 # Global sRGB PLTE ... # Global PLTE tRNS 0 # Global tRNS eXPI 0 "thumbnail" IHDR 32 32 ... PLTE IDAT ... IEND eXPI 0 "left arrow" IHDR 96 96 ... PLTE IDAT ... IEND eXPI 0 "right arrow" IHDR 96 96 ... PLTE IDAT ... IEND eXPI 0 "up arrow" IHDR 96 96 ... PLTE IDAT ... IEND eXPI 0 "down arrow" IHDR 96 96 ... PLTE IDAT ... IEND MEND
Contributors' names are presented in alphabetical order:
This document was built from the file mng-master-19990412 on 12 April 1999.
Copyright © 1998, 1999 by: Glenn Randers-Pehrson
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