AniPNG (Animated Portable Network Graphics) Format Version 0.93

Editor

Status of this Memo

This is a DRAFT proposal. Some version of this document may become version 1.0.

Comments on this document can be sent to the editor or 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://ftp.simplesystems.org/pub/png-group/documents/.

Abstract

This document defines the AniPNG (Animated Portable Network Graphics) format. It proposes a file extension ".apg" and an Internet Media Type "video/apg".

The AniPNG 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.

AniPNG is a multiple-image member of the PNG (Portable Network Graphics) format family. It can contain animations, slide shows, or complex still frames, comprised of multiple PNG or JNG (JPEG Network Graphics) single-image datastreams.

The AniPNG format was derived from the MNG-1.0 format [MNG] by deletion of a number of chunks, elimination of the "simplicity profile", and by addition of the RECO and PLAY chunks. The AniPNG format uses the same chunk structure that is defined in the PNG specification, and it shares other features of the PNG format. Any AniPNG decoder must be able to decode PNG and JNG datastreams.

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Table of Contents

1. Introduction

This specification defines the format of an AniPNG (Animated Portable Network Graphics) format, and proposes a file extension ".apg" and an Internet Media Type (MIME type) "video/apg".

Note: This specification depends on the PNG (Portable Network Graphics) [PNG] and the JNG (JPEG Network Graphics) [JNG] specifications. It was derived from the MNG (Multiple-image Network Graphics) specification [MNG] by deletion of a number of features and by addition of the PLAY and RECO chunks. The PNG, JNG, and MNG specifications are available at the PNG web site,

   http://www.libpng.org/pub/png/

AniPNG is a multiple-image member of the PNG format family that can contain

comprised of multiple PNG or JNG single-image datastreams.

Like PNG, an AniPNG datastream consists of an 8-byte signature, followed by a series of chunks beginning with the MHDR chunk and ending with the MEND chunk. Each chunk consists of a 4-byte data length field, a 4-byte chunk type code (e.g., "MHDR"), data (unless the length is zero), and a CRC (cyclical redundancy check value).

An AniPNG datastream describes a sequence of zero or more single frames, each of which can be composed of zero or more embedded images or directives to show previously defined images.

The embedded images can be PNG or JNG datastreams.

A typical AniPNG datastream consists of:

or a standalone PNG datastream: or a standalone JNG datastream:

AniPNG 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.

AniPNG is pronounced "ani-ping."

When an AniPNG datastream is stored in a file, it is recommended that ".apg" be used as the file suffix. In network applications, the Media Type "video/x-apg" can be used. Registration of the media type "video/apg" might be pursued at some future date.

The AniPNG datastream begins with an 8-byte signature containing

    140  65  77  71  13  10  26  10  (decimal)
     8c  41  4d  47  0d  0a  1a  0a  (hexadecimal)
   \214   A   P   G  \r  \n \032 \n  (ASCII C notation)

which is similar to the PNG signature with "\214 A P 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 AniPNG employ the same mechanism as PNG, but with rules that are explained more fully (see below, Chapter 6). An AniPNG editor is not permitted to move unknown chunks across the SAVE and SEEK chunks, across any chunks that can cause images to be created or displayed, or into or out of a IHDR-IEND or similar sequence.

Note that decoders are not required to follow any decoding models described in this specification nor to follow the instructions in this specification, as long as they produce results identical to those that could be produced by a decoder that did use this model and did follow the instructions.

Each chunk of the AniPNG datastream or of any embedded object is an independent entity, i.e., no chunk is ever enclosed in the data segment of another chunk.

AniPNG-compliant decoders are required to recognize and decode independent PNG datastreams beginning with either the 8-byte PNG signature or with the 8-byte AniPNG signature, or JNG datastreams beginning with either the 8-byte JNG signature or with the 8-byte AniPNG signature,.

Most AniPNG datastreams are valid MNG-1.0 datastreams. MNG-1.0 decoders will not recognize the RECO and PLAY chunks, which were defined in 2007 and have not been registered by the PNG Registration Authority, nor will they recognize the values 2 or 3 in the DEFI chunk's do_not_show field.

Because the embedded objects making up an AniPNG are normally in PNG format, AniPNG shares the good features of PNG:

In addition it has these features of the MNG format:

2. Terminology

See also the glossary in the PNG specification.

requirement levels
The words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", and "OPTIONAL" in this document, which are to be interpreted as described in RFC-2119. 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.

animation
A sequence of images meant to be played at a framing rate that will give the impression of motion. We use the more generic term "sequence" to include any group of images meant to be played at some specified framing rate or under user control, not necessarily an animation, such as a slide show, as well as animations.

cacheable
A loop is "cacheable" if it produces the identical set of scenes for each iteration of the loop. Thus the scenes can be placed in a cache and replayed rather than having to be rebuilt each time.

cheap transparency
Image transparency data conveyed via the PNG tRNS chunk rather than via a full alpha channel.

clipping boundaries
Limits within which a pixel must fall to be displayed. The left and top boundaries are inclusive, while the right and bottom boundaries are exclusive.

color encoding
File gamma and chromaticity values, an sRGB rendering intent, an iCCP profile, or whatever is involved in mapping between RGB values and colors.

embedded object or image
An image that appears in-line in an AniPNG datastream.

frame
A composition of zero or more layers that have zero interframe delay time followed by a layer with a specified nonzero delay time or by the MEND chunk. A frame is to be displayed as a still picture or as part of a sequence of still images or an animation. An animation would ideally appear to a perfect observer (with an inhumanly fast visual system) as a sequence of still frames.

When the layers of a frame do not cover the entire area defined by the width and height fields from the MHDR chunk, the layers are composited over the previous frame to obtain the new frame.

When the frame includes a transparent background layer the transparent background is composited against the outside world and the subsequent layers are composited against the result to obtain the new frame. If the application does not have access to the outside world, then it can use data from an embedded object's bKGD chunk or it can choose its own background.

frame origin
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 the locations in DEFI and PLAY chunks would still be measured from this offscreen origin.

framing rate
The rate, measured in frames per second, at which frames are displayed on the output device. In an AniPNG datastream, the framing rate is the interframe delay, in ticks, divided by the number of ticks per second, from the MHDR chunk.

frozen object
An object whose set of object attributes and whose object buffer are not allowed to be discarded, replaced, or modified.

image N or object N
Shorthand for "the object with the set of object attributes pointed to by `object_id=N'".

interframe delay
The amount of time a layer should be visible when a sequence of frames or an animation is played. A layer with a zero interframe delay is combined with the subsequent layer or layers to form a frame; the frame is completed by a layer with a nonzero interframe delay or by the MEND chunk. 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 interframe delay measures the time to the "start" of the next frame. There is no interframe delay prior to the implicit layer at the beginning of the sequence. Every frame has an interframe delay, including the frame that is displayed last; its delay is not really "interframe", nevertheless it is the amount of time that the frame should be visible before it is removed or replaced with something else.

iteration
One cycle of a loop. In this document, as is customary among computer programmers, the number of iterations of a loop includes the first cycle. A loop can have zero iterations, which means it is not executed at all.

layer
One of

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.

nullify
To nullify a chunk is to undo its effect, restoring the datastream to the condition it would have had if the chunk being nullified had never appeared.

object, object_id
An image. The object_id is an unsigned sixteen-bit number that serves as the identifier of a set of object attributes.

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.

object buffer
A 2D array of pixels, each of which has color and transparency information. See Object buffers, below.

potentially visible image
One of

prologue segment
The first segment, when there is more than one segment.

regular segment
Any segment other than the first (also the first segment, when there is only one).

segment
A part of an AniPNG 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.

signal
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.

tick
A "tick" is the unit of time used for measuring interframe delays and timeouts. It is defined by the ticks_per_second field of the MHDR chunk.

tile
A "tile" is a pixel array copied from an object and used in the PLAY chunk to construct a layer.

visible image
Actually drawn on a display. If an object is visible, a person looking at the display can see it.

3. Objects

An "object", which is identified by an object_id, is a PNG or JNG image. The object_id is an unsigned sixteen-bit number that serves as the identifier of a set of object attributes.

Object 0 is a special object whose pixel data is not available for later use (see below).

3.1. Embedded objects

An embedded object is:

3.2. Object attributes

Objects have object attributes that can be defined and modified by the contents of various AniPNG chunks. Decoders are responsible for keeping track of them. Object attributes include:

Existence
A nonzero object comes into existence when

A nonzero object ceases to exist when it does not have the "frozen" attribute and

Object 0 always exists.
 

Pointer to an object buffer
Every object (except for Object 0) has an object buffer. The representation of a pointer is decided by the application; pointers never appear explicitly in an AniPNG 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".

Frozen or not 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.

Potential visibility
The "potential visibility" of an object is determined by the do_not_show byte of the DEFI chunk that introduced it. When an embedded object is "potentially visible," it can be displayed "on-the-fly" as it is being decoded.

Location
The X and Y location of an object is determined by the DEFI chunk that introduced it and can be modified by the PLAY chunk. It is permitted to change the location of "frozen" objects, provided that the encoder includes a DEFI or PLAY chunk prior to the end of the segment that restores their locations to their "saved" positions.

Clipping boundaries
The clipping boundaries of an object are determined by the DEFI chunk that introduced it and can be modified by the PLAY chunk. It is permitted to change the clipping boundaries of "frozen" objects, provided that the encoder includes a DEFI chunk or PLAY chunk prior to the end of the segment that restores the boundaries to their "saved" values.

Additional information
While not required by this specification, applications may wish to store other information about the object, for error-checking or other purposes outside the scope of this specification.

3.3. Object buffers

An object buffer is created by the appearance of an embedded object in the datastream, with a nonzero object_id, or by the first layer created after the RECO chunk begins recording. The contents of an object buffer can be modified by decoding a new embedded object with the same object_id or by creating a layer while the RECO chunk is recording.

An object buffer contains a 2D array of pixel data and can contain additional information. In addition, decoders are responsible for keeping track of some properties of the data in the object buffer:

Object 0 conceptually never has an object buffer. Decoding applications can create one for their own convenience, but such an object buffer must never be made available to the rest of the AniPNG datastream.

Format of data in the object buffer
The data can be stored by the decoder in its original form or 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.

Frozen or not frozen
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.

3.4. Object 0

Object 0 is a special object that has a set of object attributes that control its location, clipping, and visibility properties, but does not have an object buffer. The object attributes, which can be modified by the DEFI chunk, are applied to subsequent embedded objects whose object_id is zero. The pixel data for Object 0 is available only for on-the-fly display and not available for later use. If at the end of any segment the attribute values are different from the default/saved values, they become undefined when a SEEK chunk appears.

3.5. Embedded images

In addition to creating MNG objects (see below, Paragraph 4.2.3), and (below, Paragraph 4.2.4), the IHDR-IEND and JHDR-IEND chunks display them when the DEFI chunk that introduced them set do_not_show to zero, to specify on-the-fly display.

A viewer can choose to display the image while it is being decoded, perhaps taking advantage of the PNG interlacing method or the JNG progressive display feature, or to display it after decoding is complete.

4. AniPNG Chunks

This chapter describes chunks that can appear at the top level of an AniPNG datastream.

Chunk structure (length, name, data, CRC) and the chunk-naming system are identical to those defined in the PNG specification [PNG]. As in PNG, all integers that require more than one byte must be in network byte order.

Unlike PNG, fields can be omitted from some AniPNG chunks with a default value if omitted. This is permitted only when explicitly stated in the specification for the particular chunk. If a field is omitted, all the subsequent fields in the chunk must also be omitted and the chunk length must be shortened accordingly.

4.1. Critical AniPNG control chunks

This section describes critical AniPNG control chunks that AniPNG-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 AniPNG editors.

4.1.1. MHDR AniPNG datastream header

The MHDR chunk is always first in all AniPNG datastreams except for those that consist of a single PNG or JNG datastream with an AniPNG, PNG or JNG signature.

The MHDR chunk contains 28 bytes, none of which can be omitted:

   Frame_width:         4 bytes (unsigned integer).
   Frame_height:        4 bytes (unsigned integer).
   Ticks_per_second:    4 bytes (unsigned integer).
   Nominal_layer_count: 4 bytes (unsigned integer).
   Nominal_frame_count: 4 bytes (unsigned integer).
   Nominal_play_time:   4 bytes (unsigned integer).
   Unused:              4 bytes (ignored).

Decoders can ignore the "informative" nominal_frame_count, nominal_layer_count, and nominal_play_time fields.

The frame_width and frame_height fields give the intended display size (measured in pixels) and provide default clipping boundaries (see Recommendations for encoders, below). It is strongly recommended that these be set to zero if the AniPNG datastream contains no visible images.

The ticks_per_second field gives the unit used by the PLAY chunk to specify interframe delay and timeout. It also provides the framing rate to be used for displaying embedded objects on-the-fly. It must be nonzero if the datastream contains a sequence of images. When the datastream contains exactly one frame, this field should be set to zero. When this field is zero, the length of a tick is infinite, and decoders will ignore any attempt to define interframe delay, timeout, or any other variable that depends on the length of a tick. If the frames are intended to be displayed one at a time under user control, such as a slide show or a multi-page FAX, the tick length can be set to any positive number and a PLAY chunk can be used to set an infinite interframe delay and a zero timeout. Unless the user intervenes, viewers will only display the first frame in the datastream.

When ticks_per_second is nonzero, and there is no other information available about interframe delay, viewers should display the sequence of frames at the rate of one frame per tick.

If the frame count field contains a zero, the frame count is unspecified. If it is nonzero, it contains the number of frames that would be displayed, ignoring the TERM chunk. If the frame count is greater than 231-1, encoders should write 231-1 (0x7FFF FFFF), representing an infinite frame count.

If the nominal_layer_count field contains a zero, the layer count is unspecified. If it is nonzero, it contains the number of layers, including all background layers, when the file is displayed 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-byte integer. If the nominal play time is greater than 231-1 ticks, encoders should write 231-1, representing an infinite nominal play time.

The final four bytes of the MHDR chunk are unused. They are present for compatibility with MNG-1.0 decoders, and must be ignored by AniPNG decoders. AniPNG encoders should write the 32-bit unsigned integer "2015" in this field to indicate that any AniPNG features might be present, or "991" if the PLAY and RECO chunks are not present. If they wish, they may write any other appropriate MNG-1.0 "simplicity profile" as defined in the MNG-1.0 specification [MNG], but this is not required.

4.1.2. PLAY Playlist

The PLAY chunk contains a compressed "play list" that provides instructions to the decoder for constructing animation layers from pieces of previously defined objects. The PLAY chunk contains a 1-byte header followed by one or more 38-byte layer structures. The header gives the compression method:

      Compression_method (1 byte)    0: deflate.
      Layer_structure_array[n]       (n > 0; n * 38 bytes
                         (compressed): layer structures.

Each layer is a tile combined with a transparent background to form a layer of size (frame_width, frame_height), from the MHDR chunk. Each layer is initialized to a transparent (RGBA(0,0,0,0)) rectangle

The layer structure array is always compressed according to the specified compression method.

A layer is described by a structure of 38 bytes:

      Object     (2 bytes unsigned int)  Object ID of source image.
      Delta mode (1 byte unsigned int)   0: location and boundaries
                                            are absolute.
                                         1: location and boundaries
                                            are deltas from the DEFI
                                            values for the object.
      Update mode (1 byte unsigned int)  0: Do not update DEFI
                                            values of location and
                                            boundaries.
                                         1: Update DEFI values.
      Left       (4 byte signed integer) Left of tile source.
      Top        (4 byte signed integer) Top of tile source.
      Right      (4 byte signed integer) Right of tile source.
      Bottom     (4 byte signed integer) Bottom of tile source.
      X_location (4 byte signed integer) X location of object
                                         destination.
      Y_location (4 byte signed integer) Y location of object
                                         destination.
      Delay    (4 byte unsigned integer) Interframe delay.
      Timeout  (4 byte unsigned integer) Interframe timeout.
      Compose mode (1 byte unsigned int) 0: Over
                                         1: Replace
      Dispose mode (1 byte unsigned int) 0: None
                                         1: Restore to background
                                         2: Restore to previous

If the object_id is non-zero, the tile is taken from the already existing object buffer as it exists when the PLAY chunk is encountered. If it is zero, the tile is taken from the next instance of Object 0 following the PLAY chunk. If any layer of the playlist refers to Object 0, all of the layers in the playlist must refer to Object 0, and all of them are taken from the single immediately following instance of Object 0.

The stored values of the boundaries and location are taken from the DEFI chunk. These are used as the basis for computing current values to be used when delta_mode is 1, and the results, whether from a delta calculation or from supplied absolute values, are used to update the stored values when update_mode is 1.

The layer data identifies a rectangle (left,right, top,bottom) in the object containing pixels of a tile. The boundaries are measured from the upper left corner of the source object. The resulting rectangle is relocated to the position (x_location,y_location). and composed against the existing frame according to the compose_mode.

Any part of the tile outside the object must be treated as transparent. Any part of the tile which extends outside the layer must be ignored. The tile width or height may be zero, in which case the layer contains no pixels.

The location and clipping boundaries from the DEFI chunk for the object are used to initialize and store these values. These are used as the basis for computing current values to be used when delta_mode is 1, and the results, whether from a delta calculation or from supplied absolute values, are used to update the stored values in the object attributes when update_mode is 1.

The delay is measured in ticks_per_second from the MHDR chunk. Even if a layer's tile is entirely outside the layer, the delay for the resulting empty layer must be handled in the same way as if it were inside the frame.

The timeout field can be a number or <infinity>. Infinity is represented by 0x7FFF FFFF. When the delay is nonzero and the timeout is greater than the interframe delay, the application can adjust the interframe delay, provided that it is not greater than the greater of the specified interframe delay and the timeout nor smaller than the smaller of the specified interframe delay and the timeout. If the decoder can interact with the user, it should wait for permission from the user (e.g., via a keypress) before proceeding, but must wait no less than the smaller of the timeout and the interframe delay nor no longer than the greater of the timeout and the interframe delay.

After the interframe delay or timeout has elapsed, the layer is either left in place, replaced with the background, or restored to its previous condition, before proceeding to the next layer, according to its dispose mode

A playlist containing references to both Object 0 and a non-zero object, and two or more PLAY chunks referring to the same instance of Object 0 are error conditions, and decoders must reject such PLAY chunks.

4.1.3. LOOP, ENDL Define a loop

The LOOP chunk provides a "shorthand" notation that can be used to avoid having to repeat identical chunks in an AniPNG datastream. Its contents are the first two or more of the following fields. If any field is omitted, all subsequent fields must also be omitted:

   Nest_level:      1 byte (unsigned integer).
   Iteration_count: 4 bytes (unsigned integer),
                      range [0..2^31-1].
   Termination_condition:
                    1 byte (unsigned integer).
                      Must be omitted if
                      termination_condition=0, which
                      means Deterministic, not cacheable,
                      or if iteration_count=0.
                      1: Decoder discretion, not cacheable.
                      2: User discretion, not cacheable.
                      3: External signal, not cacheable.
                      4: Deterministic, cacheable.
                      5: Decoder discretion, cacheable.
                      6: User discretion, cacheable.
                      7: External signal, cacheable.
   Iteration_min:   4 bytes(unsigned integer).  Must be
                      present if termination_condition is
                      3 or 7.  If omitted, the default
                      value is 1.
   Iteration_max:   4 bytes (unsigned integer).  Must be
                      present if termination_condition is 3
                      or 7; must be omitted if iteration_min
                      is omitted; if omitted, the default
                      value is infinity.
   Signal_number:   4 bytes (unsigned integer).  Must be
                      present if termination_condition is 3
                      or 7.  Must not be present otherwise.
   Additional
     signal_number: 4 bytes.  May be present only if
                      termination_condition is 3 or 7.
   ...etc...

Decoders must treat the chunks enclosed in a loop exactly as if they had been repeatedly spelled out. The termination_condition field can be used to inform decoders that it is safe to change the number of loop iterations.

Simple decoders can ignore all fields except for the iteration_count.

When the LOOP chunk is present, an ENDL chunk with the same nest_level must be present later in the AniPNG 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 can take the following values:

Deterministic
This is the default behavior, when the termination_condition field is omitted or has a value that is unrecognized by the decoder. The loop terminates after exactly the number of iterations specified by the iteration count. This value must be used if altering the number of repetitions would mess up the AniPNG datastream, but can be used merely to preserve the author's intent.

Decoder-discretion
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.

User-discretion
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.

External-signal
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.

When the value of the termination_condition field is 4 or more, the loop is guaranteed to be "cacheable", which means that each iteration of the loop produces the same sequence of frames, and that all objects and object buffers are left in the same condition at the end of each iteration. Decoders can use this information to select a different strategy for handling the loop, such as storing the composited frames in a cache and replaying them rather than decoding them repeatedly.

The iteration_min and iteration_max can be omitted. If the condition is <deterministic>, the values are not used. Otherwise, defaults of 1 and <infinity> are used. The iteration_count, iteration_min, and iteration_max can be any non-negative integers or <infinity>, but they must satisfy iteration_min <= iteration_count <= iteration_max. Infinity is represented by 0x7FFF FFFF.

If iteration_count is zero, the termination_condition, the subsequent fields must be omitted, and the loop is done zero times. Upon encountering a LOOP chunk whose iteration_count is zero, decoders simply skip chunks until the matching ENDL chunk is found, and resume processing with the chunk immediately following it.

The signal_number can be omitted only if the termination condition is not <external-signal>. There can be any number of signal_number fields. Signal_number=0 is reserved to represent any input from a keyboard or pointing device, and 1-255 are reserved to represent the corresponding character code, received from a keyboard or simulated keyboard, and values 256-1023 are reserved for future definition by this specification.

An infinite or just overly long loop could give the appearance of having locked up the machine. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by abandoning the AniPNG entirely or just proceeding to the next SEEK chunk (the SEEK chunk makes it safe for a viewer to resume processing after it has jumped out of the interior of a segment).

AniPNG editors that extract a series of PNG or JNG files from an AniPNG datastream are expected to execute the loop only iteration_min times, when the termination condition is not <deterministic>.

The ENDL chunk ends a loop that begins with the LOOP chunk. It contains a single one-byte field:

   Nest_level: 1 byte (unsigned integer), range [0..255].

When the ENDL chunk is encountered, the loop iteration count is decremented, if it is not already zero. If the result is nonzero, processing resumes at the beginning of the loop. Otherwise processing resumes with the chunk immediately following the ENDL chunk.

When the ENDL chunk is present, a LOOP chunk with the same nest_level must be present earlier in the AniPNG datastream. See below. Loops must be properly nested: if a LOOP chunk with higher nest_level appears inside a LOOP/ENDL pair, a matching ENDL chunk must also appear to close it.

The SAVE and SEEK chunks are not permitted inside a LOOP-ENDL pair. To rerun an entire datastream that includes these chunks, use the TERM chunk instead. See below (Paragraph 4.1.4).

4.1.4. TERM Termination action

The TERM chunk suggests how the end of the AniPNG 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 indefinitely.
                              3: Repeat the sequence starting
                                 immediately after the TERM
                                 chunk and ending with the
                                 MEND chunk.
   
   Action_after_iterations: 1 byte
                              0: Show the last frame
                                 indefinitely after
                                 iteration_max iterations
                                 have been done.
                              1: Cease displaying anything.
                              2: Show the first frame after
                                 the TERM chunk indefinitely.
   
                             This and the subsequent fields
                             must be present if
                             termination_action is 3, and
                             must be omitted otherwise.
   
   Delay:                  4 bytes (unsigned integer) Delay,
                             in ticks, before repeating the
                             sequence.
   
   Iteration_max:          4 bytes (unsigned integer) Maximum
                           number of times to execute the
                           sequence.  Infinity is represented
                           by 0x7FFF FFFF.

The final frame of the sequence endures at least for the maximum of:

before the sequence is repeated or before ceasing displaying anything.

If no TERM chunk is present, viewers should assume termination_action=0 as the default.

The loop created by processing a TERM chunk must always be treated by the decoder as if it were a cacheable <user-discretion> loop, with iteration_min=1.

Applications must not depend on anything that has been drawn on the output buffer or device during the previous iteration. Its contents become undefined when the TERM loop restarts.

AniPNG editors that extract a series of PNG or JNG files from an AniPNG datastream are expected to execute the TERM loop only once, rather than emitting the files repeatedly.

The TERM chunk, if present, must appear either immediately after the MHDR chunk or immediately prior to a SEEK chunk. The TERM chunk is not considered to be a part of any segment for the purpose of determining the copy-safe status of any chunk. Only one TERM chunk is permitted in an AniPNG datastream.

Simple viewers and single-frame viewers can ignore the TERM chunk. It has been made critical only so AniPNG editors will not inadvertently relocate it.

4.1.4. MEND End of AniPNG datastream

The MEND chunk's data length is zero. It signifies the end of an AniPNG datastream that began with the MHDR chunk. AniPNG datastreams that begin with the IHDR or JHDR chunk are terminated by the IEND chunk instead.

4.2. Critical AniPNG image defining chunks

The chunks described in this section create objects and may cause them to be immediately displayed.

4.2.1. DEFI Define an object

The DEFI chunk sets the default set of object attributes (object_id, do_not_show flag, location, and clipping boundaries) for any subsequent images that are defined with IHDR-IEND or JHDR-IEND datastreams.

The DEFI chunk contains 2, 3, 4, 12, or 28 bytes. If any field is omitted, all subsequent fields must also be omitted.

   Object_id:     2 bytes (unsigned integer) identifier to be
                    given to the objects that follow the DEFI
                    chunk.
   
   Do_not_show:   1 byte (unsigned integer)
                    0:  Make the existing object and
                        the following objects potentially
                        visible.
                    1:  Make the existing object and
                        the following objects not
                        potentially visible.
                    2:  Discard existing object; make the
                        following objects potentially
                        visible.
                    3:  Discard existing object; make the
                        following objects not potentially
                        visible.
   
   Concrete_flag: 1 byte (unsigned integer)
                    0:  Make the objects "abstract" .
                    AniPNG decoders can ignore this flag.
                    AniPNG encoders should write "0".
   
   X_location:    4 bytes (signed integer).
                    The X_location and Y_location fields can
                    be omitted as a pair.
   
   Y_location:    4 bytes (signed integer).
   
   Left_cb:       4 bytes (signed integer).  Left clipping
                    boundary.  The left_cb, right_cb, top_cb,
                    and bottom_cb fields can be omitted as a
                    group.
   
   Right_cb:      4 bytes (signed integer).
   
   Top_cb:        4 bytes (signed integer).
   
   Bottom_cb:     4 bytes (signed integer).

If the object number for an object is nonzero, subsequent chunks can use this number to identify it.

When the object number for an object is zero, its object buffer can be discarded immediately after it has been processed. Its object attributes must be retained.

If fields are omitted, their values are as defined by a previous class=cn>DEFI chunk, if that chunk is still in effect.

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 for any omitted fields:

              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 do_not_show byte can be used to inform the decoder that it can discard existing object data associated with the object identifier, but does not cause the object attributes to become undefined. Whether the decoder actually discards the data or not, it must not use it after encountering a DEFI chunk whose do_not_show byte is 2 or 3. It is an error to attempt to discard Object 0 or any "frozen" object in this manner.

Negative values are permitted for the X and Y location and clipping boundaries. The left and top boundaries are inclusive, while the right and bottom boundaries are exclusive. The positive directions are downward and rightward from the frame origin (see Recommendations for encoders, below).

Multiple IHDR-IEND and JHDR-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. The object_id can only be changed by using another DEFI chunk.

The object attributes for all existing unfrozen objects except for object 0 become undefined when a SEEK chunk is encountered.

The object attributes for Object 0 become undefined when a SEEK chunk is encountered, only if they have been reset to values other than these defaults. It is the encoder's responsibility to reset them explicitly to these values prior to the end of every segment in which they have been changed, or to include a full DEFI chunk prior to embedding Object 0 in any segment.

These default values are also used to fill any fields that were omitted from the DEFI chunk, when an object with the same object_id has not been previously defined or a SEEK chunk has caused it to be discarded.

A set of object attributes is created or an existing one is modified when the DEFI chunk appears, but an object buffer is neither created nor discarded. If object_id is an identifier that already exists when a DEFI chunk appears, the set of object attributes (except for the pointer to the object buffer) is immediately replaced. The contents of the object buffer do not change, however, until and unless an IHDR or JHDR chunk is encountered. When one of these chunks appears, all of the contents of the object buffer previously associated with the identifier are discarded and the new data is stored in the object buffer.

4.2.2. PLTE and tRNS Global palette

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.

4.2.3. IHDR, PNG chunks, IEND

A PNG (Portable Network Graphics) datastream.

See the PNG specification [PNG] and the Extensions to the PNG Specification document [PNG-EXT] for the format of the PNG chunks.

The IHDR and IEND chunks and any chunks between them are written and decoded according to the PNG specification, except as extended in this section. These extensions do not apply to standalone PNG datastreams that have the PNG signature, but only to PNG datastreams that begin with the AniPNG signature or are embedded in an AniPNG datastream that begins with an AniPNG signature.

If object_id is zero, there is no need to store the pixel data after decoding it and perhaps displaying it.

If an object already exists with the same object_id, the contents of its object buffer are replaced with the new data.

4.2.4. JHDR, JNG chunks, IEND

A JNG (JPEG Network Graphics) datastream.

See the JNG specification [JNG] for the format of the JNG datastream.

The JHDR and IEND chunks and any chunks between them are written and decoded according to the JNG specification.

The remaining discussion in the previous paragraph about PNG datastreams also applies to JNG datastreams.

4.2.5. RECO Record current frame

The RECO chunk is useful for accomplishing the equivalent of the GIF "restore-to-previous" disposal method.

This chunk is currently not a registered MNG chunk. Registration is being pursued with the PNG Registration Authority. In the meantime, the private form of the chunk name (RECO, with a lowercase "e"), must be used in any public datastreams. It has two fields

      Object_id  (16-bit unsigned int):  Existing object to be
                        used for storing image of frame
      Mode (byte)    0: Initialize object to transparent and start
                        recording offscreen copy of current frame
                        (including any background layers).
                     1: Stop recording.
                     2: Restart recording with existing contents.

The object_id must already have been the subject of a DEFI chunk and must still exist. The dimensions and location of the area to be recorded are specified in the DEFI chunk.

Each layer that is defined while the RECO chunk is in recording mode is composited over whatever is in the object.

It is permitted to have more than one RECO object open at the same time. The first RECO chunk for each object in a segment must have mode 0, and the last RECO chunk for each object in a segment must have mode 1.

Once a recording is completed (by the appearance of a RECO chunk with mode == 1), the object_id can be used like any other defined object. It is an error to attempt to use the object_id as the source of any operation while recording is in progress.

4.3. SAVE and SEEK chunks

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 AniPNG datastream where a restart is possible, and where the decoder must restore the "saved" information, if they have jumped or skipped to a SEEK point from the interior of a segment. They only need to restore information that they will use, e.g., a viewer that processes gAMA and global PLTE and tRNS, but ignores iCCP and sPLT, need only restore the value of gamma and the global PLTE and tRNS data from the prologue segment but not the values of the iCCP and sPLT data.

Simple decoders that only read AniPNG 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 chunk is processed.

4.3.1. SAVE Save information

The SAVE chunk marks a point in the datastream at which objects are "frozen" and other chunk information is "saved"; a decoder skipping or jumping to a SEEK chunk from the interior of a segment must restore the "saved" chunk information if it has been redefined or discarded. In addition, the SAVE chunk can contain an optional index to the AniPNG datastream.

The SAVE chunk can be empty, or it can contain an index consisting of the following:

   Offset_size: 1 byte (unsigned integer).
                  4: Offsets and nominal start times are
                     expressed as 32-bit integers.
                  8: Offsets and nominal start times are
                     expressed as 64-bit integers.

plus zero or more of the following index entries:

   Entry_type:  1 byte (unsigned integer).
                  0: Segment with nominal start time, nominal
                     layer number, and nominal frame number.
                  1: Segment.
                  2: Subframe.
                  3: Exported image.
   Offset:     4 or 8 bytes (unsigned integer).  Must be
                 omitted if entry_type > 1, set equal to zero
                 if the offset is unknown.
   Nominal_start_time:
               4 or 8 bytes: (unsigned integer).  Start time
                 of the segment, measured in ticks from the
                 beginning of the sequence, assuming that all
                 prior segments were played as intended on an
                 ideal player.  Must be omitted if
                 entry_type > 0.
   Nominal_layer_number:
              4 bytes (unsigned integer).  Sequence number of
                 the first layer in the segment, assuming
                 that all prior segments were played as
                 intended on an ideal player; the first layer
                 of the first segment being layer 0.  Must be
                 omitted if entry_type > 0.
   Nominal_frame_number:
             4 bytes (unsigned integer).  Sequence number of
                 the first frame in the segment, assuming
                 that all prior segments were played as
                 intended on an ideal player; the first
                 frame of the first segment being frame 0.
                 Must be omitted if entry_type > 0.
   Name:       1-79 bytes (Latin-1 text).  Must be omitted
                 for unnamed segments.  The contents of this
                 field must be the same as the name field in
                 the corresponding SEEK or eXPI chunk.
   Separator:  1 byte (null) (must be omitted after the final
                 entry).

The SAVE chunk must be present when the SEEK chunk is present. It appears after the set of chunks that define information that must be retained for the remainder of the datastream. These chunks, collectively referred to as the prologue segment, are no different from chunks in other segments. They can be chunks that define objects, or they can be chunks that define other information such as gAMA, cHRM, and sPLT. If any chunks appear between the SAVE chunk and the first SEEK chunk, these chunks also form a part of the prologue segment, but their contents become undefined when the SEEK chunk appears.

Only one instance of the SAVE chunk is permitted in an AniPNG 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.

A chunk like gAMA that overwrites a single current value is permitted after the SAVE chunk, even if the chunk has appeared ahead of the SAVE chunk. Decoders are responsible for saving a copy of the chunk data (in any convenient form) when the SAVE chunk is encountered and restoring it when skipping or jumping to a SEEK chunk from the interior of a segment. If no instance of the chunk appeared ahead of the SAVE chunk, the decoder must restore the chunk data to its original "unknown" condition when it skips or jumps to a SEEK chunk from the interior of a segment.

It is the encoder's responsibility, if it changes or discards any "saved" data, to restore it to its "saved" condition (or to nullify it, if it was unknown) prior to the end of the segment. This makes it safe for simple decoders to ignore the SAVE/SEEK mechanism.

Known chunks in this category include DEFI, PLTE, cHRM, tRNS, gAMA, iCCP, bKGD, sBIT, pHYg, pHYs, and sRGB. In addition, it is the responsibility of the encoder to include chunks that restore the potential visibility, location, and clipping boundaries of any "frozen" objects to their "saved" condition.

In the case of chunks like sPLT that can occur multiple times, with different "purpose" fields, additional instances of the chunk are permitted after the SAVE chunk, but not with the same keyword as any instances that occurred ahead of the SAVE chunk. The decoder is required to forget such additional instances when it skips or jumps to a SEEK chunk from the interior of a segment, but it must retain those instances that were defined prior to the SAVE chunk. Encoders are required to nullify such additional instances prior to the end of the segment. Known chunks in this category include only sPLT.

If an entry for a segment (entry type 0 or 1) appears in the optional index, there must also be an entry for every segment, whether named or not, except for the prologue segment, that precedes it. All entries must appear in the index in the same order that they appear in the AniPNG datastream. There must never be a segment entry (type 0 or 1) for the prologue segment, but there can be entries for named images in the prologue, placed ahead of the first segment entry. Only named images are permitted, and it is not an error to omit any or all named images. Nor is it an error to omit a contiguous set of segments at the end of the datastream from the index.

Offsets are calculated from the first byte of the AniPNG 8-byte signature, which has offset=0. This is true even if the AniPNG 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 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 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 an AniPNG 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.

4.3.2. SEEK Seek point

The SEEK chunk marks positions ("seek points") in the AniPNG datastream where a restart is possible, and where the decoder must restore certain information to the condition that existed when the SAVE chunk was processed, if it has skipped or jumped to the SEEK chunk from the interior of a segment.

The SEEK chunk can be empty, or it can contain a segment name.

   Segment_name: 1-79 bytes (Latin-1 string).

The segment name is optional. It must follow the format of a tEXt keyword: It must consist only of printable Latin-1 characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the keyword. There is no null byte terminator within the segment name, nor is there a separate null byte terminator. Segment names are case-sensitive. Use caution when printing or displaying keywords (Refer to Security considerations, below, Chapter 14). 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 eXPI chunk. Segment names should not begin with the case-insensitive strings "CLOCK(", "FRAME(", or "FRAMES(", which are reserved for use in URI queries and fragments (see Uniform Resource Identifier below).

Applications must not use any information preceding the SEEK chunk, except for:

They also must not depend on anything that has been drawn on the output buffer or device. Its contents become undefined when the SEEK chunk is encountered. Viewers that make random access to a seek point from the interior of a segment must insert a background layer before processing the segment. Encoders must ensure that simple viewers do not need to do this.

When the SEEK chunk is encountered, the decoder can discard any objects appearing after the SAVE chunk.

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.

When a decoder jumps to a seek point from the interior of a segment, it must restore the information that it saved when it processed the SAVE chunk, and it must reset the object attributes for Object 0 to their default values. When it encounters a SEEK chunk during normal sequential processing of an AniPNG datastream, it need not restore anything, because the encoder will have written chunks that restore all saved information.

Multiple instances of the SEEK chunk are permitted. The SEEK chunk must not appear prior to the SAVE chunk. The SAVE chunk must also be present if the SEEK chunk is present. The SEEK chunk must not appear between a LOOP chunk and its ENDL chunk.

4.4. Ancillary AniPNG chunks

This section describes ancillary AniPNG chunks. AniPNG-compliant decoders are not required to recognize and process them.

4.4.1. eXPI Export image

The eXPI chunk takes a snapshot of an object, 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).
   Snapshot_name: 1-79 bytes (Latin-1 text).

When the snapshot_id is zero, the snapshot is the first instance of an embedded image with object_id=0 following the eXPI chunk. When the snapshot_id is nonzero, the snapshot is an already-defined object with that object_id as it already exists when the eXPI chunk is encountered.

Note that the snapshot_name is associated with the snapshot, not with the snapshot_id nor its subsequent contents; changing the image identified by snapshot_id will not affect the snapshot. The snapshot_name means nothing inside the scope of the AniPNG 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 SEEK chunk. A decoder that knows of no "outside world" can simply ignore the eXPI chunk. This chunk could be used in AniPNG 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 AniPNG. Thumbnail images are regular PNG or JNG images, but they would normally have smaller dimensions and fewer colors than the AniPNG frames. They can be defined with the potential visibility field set to "invisible" if they are not intended to be shown as a part of the regular display.

The snapshot_name string must follow the format of a tEXt keyword: It must consist only of printable Latin-1 characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the keyword. Keywords are case-sensitive. There is no null byte terminator within the snapshot_name string, nor is there a separate null byte terminator. Snapshot names should not begin with the case-insensitive strings "CLOCK(", "FRAME(", or "FRAMES(" which are reserved for use in URI queries and fragments (see Uniform Resource Identifier below).

Multiple instances of the eXPI chunk are permitted in an AniPNG datastream, and they need not have different values of snapshot_id.

4.4.2. pHYg Physical pixel size (global)

The AniPNG pHYg chunk is identical in syntax to the PNG pHYs chunk. It applies to complete full-frame AniPNG layers and not to the individual images within them.

Conceptually, an AniPNG viewer that processes the pHYg chunk will first composite each image into a full-frame layer, then apply the pHYg scaling to the layer, and finally composite the scaled layer against the frame. AniPNG datastreams can include both the PNG pHYs chunk (either at the AniPNG top level or within the PNG and JNG datastreams) and the AniPNG pHYg chunk (only at the AniPNG top level), to ensure that the images are properly displayed either when displayed by an AniPNG viewer or when extracted into a series of individual PNG or JNG datastreams and then displayed by a PNG or JNG application. The pHYs and pHYg chunks would normally contain the same values, but this is not necessary.

The AniPNG top-level pHYg chunk can be nullified by a subsequent empty pHYg chunk appearing in the AniPNG top level.

4.5. Ancillary PNG chunks

The namespace for AniPNG chunk names is separate from that of PNG. Only those PNG chunks named in this paragraph are also defined at the AniPNG top level. They have exactly the same syntax and semantics as when they appear in a PNG datastream:

5. Extension and Registration

New public chunk types, and additional options in existing public chunks, can be proposed for inclusion in this specification by contacting the PNG/AniPNG specification maintainers at , or .

New public chunks and options will be registered only if they are of use to others and do not violate the design philosophy of PNG and AniPNG. Chunk registration is not automatic, although it is the intent of the authors that it be straightforward when a new chunk of potentially wide application is needed. Note that the creation of new critical chunk types is discouraged unless absolutely necessary.

Applications can also use private chunk types to carry data that is not of interest to other applications.

Decoders must be prepared to encounter unrecognized public or private chunk type codes. If the unrecognized chunk is critical, then decoders should abandon the segment, and if it is ancillary they should simply ignore the chunk. Editors must handle them as described in the following section, Chunk Copying Rules.

6. Chunk Copying Rules

The chunk copying rules for AniPNG are the same as those in PNG, except that an AniPNG editor is not permitted to move unknown chunks across any of the following chunks, or across any critical chunk in a future version of this specification that creates or displays an image:

The copy-safe status of an unknown chunk is determined from the chunk name, just as in PNG. If bit 5 of the first byte of the name is 0 (Normally corresponding to an uppercase ASCII letter), the unknown chunk is critical and cannot be processed or copied. If it is 1 (usually corresponding to a lowercase ASCII letter), the unknown chunk is ancillary and its copy-safe status is determined by bit 5 of the fourth byte of the name, 0 meaning copy-unsafe and 1 meaning copy-safe.

If an editor makes changes to the AniPNG 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 AniPNG 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 AniPNG datastream can have ordering rules only with respect to critical chunks. Safe-to-copy ancillary chunks in the top-level AniPNG datastream can have ordering rules only with respect to the SAVE, SEEK, PLAY, IHDR-IEND, JHDR-IEND sequences, or with respect to any other critical "header-end" sequence that might be defined in the future that could contain IDAT or similar chunks.

The copying rules for unknown chunks inside IHDR-IEND, 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 AniPNG chunks.

7. Minimum Requirements for AniPNG-Compliant Decoders

This section specifies the minimum level of support that is expected of AniPNG-compliant decoders, and provides recommendations for viewers that will support slightly more than the minimum requirements. All critical chunks must be recognized, but some of them can be ignored after they have been read and recognized. Ancillary chunks can be ignored, and do not even have to be recognized.

Anything less than this level of support requires subsetting.

We are allowing conformant decoders to skip twelve-bit JNGs because those are likely to be rarely encountered and used only for special purposes.

7.1. Required AniPNG chunk support

MHDR
The ticks_per_second must be supported by animation viewers. The frame count, layer count, and nominal play time can be ignored.

MEND
The MEND chunk must be recognized but does not require any processing other than completing the last frame.

Global PLTE and tRNS
Must be fully supported.

LOOP, ENDL
The iteration_count must be supported. The nest_level should be used as a sanity check but is not required. When iteration_min=1 either explicitly or when it is omitted and the termination_condition is not 0 or 4, the LOOP chunk and its ENDL chunk can be ignored.

DEFI
Must be fully supported. All objects can be treated as "abstract".

SAVE and SEEK
Must be recognized but can be ignored. However, partial support is recommended: All existing objects should be marked "frozen" when the SAVE chunk is processed, so that unneeded objects can be discarded when the SEEK chunk is processed. Chunk information need only be "saved" and "restored" when the viewer is able to skip or jump to random SEEK chunk locations from the interior of a segment, such as when recovering from a corrupted datastream or from a segment containing an unknown critical chunk, or when escaping from a deterministic loop in response to a user request. The optional index can be ignored. Slide-show controllers may wish to support SAVE and SEEK fully.

TERM
Must be recognized but can be ignored.

7.2. Required PNG chunk support

IHDR, PLTE, IDAT, IEND
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.

tRNS
The PNG tRNS chunk, although it is an ancillary chunk, must be supported in AniPNG-compliant viewers, at least to the degree that fully opaque pixels are opaque and fully transparent ones are transparent. It is required that alpha data from the tRNS chunk be supported in the same manner as alpha data from an RGBA image.

Other PNG ancillary chunks
Ancillary chunks other than PNG tRNS can be ignored, and do not even have to be recognized.

Color management
It is highly recommended that decoders support at least the gAMA chunk to allow platform-independent color rendering. If they support the gAMA chunk, they must also support the sRGB chunk, at least to the extent of interpreting it as if it were a gAMA chunk with gamma value 0.45455.

7.3. Required JNG chunk support

AniPNG-compliant decoders must support JNG.
JHDR, JDAT, IDAT, JDAA, JSEP, IEND
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.

JNG ancillary chunks
All JNG ancillary chunks can be ignored, and do not even have to be recognized.

JNG image sample depth
Only image_sample_depth=8 must be supported. The JSEP chunk must be recognized and must be used by minimal decoders to select the eight-bit version of the image, when both eight-bit and twelve-bit versions are present, as indicated by image_sample_depth=20 in the JHDR chunk. When image_sample_depth=12, minimal decoders are not obligated to display anything. Such decoders can choose to display nothing or an empty rectangle of the width and height specified in the JHDR chunk.

8. Recommendations for Encoders

The following recommendations do not form a part of the specification.

8.1. Use a common color space

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 AniPNG, using either an sRGB chunk or the gAMA and cHRM chunks and perhaps the iCCP chunk, and removing any color space chunks from the individual images after converting them to the common color space.

When the encoder converts all images to a single color space before putting them in the AniPNG datastream, decoders can improve the speed and consistency of the display.

For single-frame and slide-show AniPNG datastreams, however, decoding speed is less important and exact color rendition might be more important. Therefore, it is best to leave the images in their original color space, as recommended in the PNG specification, retaining the individual color space chunks if the images have different color spaces. This will avoid any loss of data due to conversion.

8.2. Embedded images in LOOPs

Embedded images should not be enclosed in loops unless absolutely necessary. It is better to store them ahead of time and then use PLAY chunks inside the loops.

8.3. Including optional index in SAVE chunk

Authors of AniPNG 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 AniPNG datastream contains all of the information needed to build the index. If an application does build an index, and the file is going to be kept as a local file, the application should replace the empty SAVE chunk with one containing the index. See above (Paragraph 4.3.1).

8.4. Interleaving JDAT, JDAA, and IDAT chunks

When a JNG datastream contains an alpha channel, and the file is intended for transmission over a network, it is useful to interleave the IDAT or JDAA and the JDAT chunks. In the case of sequential JPEG, the interleaving should be arranged so that the alpha data arrives more or less in sync with the color data for the scanlines. In the case of progressive JPEG, the alpha data should be interleaved with the first JPEG pass, so that all of the alpha data has arrived before the beginning of the second JPEG pass.

8.5. Use of the JDAA chunk

It is recommended that the JDAA chunk be used only to convey smoothly varying alpha channels and not to convey binary transparency which is more precisely and efficiently conveyed in IDAT chunks.

9. Recommendations for Decoders

9.1. ENDL without matching LOOP

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 AniPNG datastream. See above.

9.2. Note on compositing

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.

9.3. Retaining object data

The decoder must retain information about each object (except for objects with object_id=0) for possible redisplay with the PLAY chunk.

The following information must be retained, for each nonzero object that is defined and not subsequently discarded:

When the encoder knows that data in the object buffer will not be needed later, it can help decoders operate more efficiently by using object_id=0 or by using the SEEK chunk.

9.4. Decoder handling of fatal errors

When a fatal error is encountered, such as a bad CRC or an unknown critical AniPNG chunk, minimal viewers that do not implement the SAVE/SEEK mechanism should simply abandon the AniPNG datastream. More capable AniPNG 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 AniPNG 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.

AniPNG editors, on the other hand, should be more strict and reject any datastream with errors unless the user intervenes.

9.5. Decoder handling of interlaced images

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 AniPNG authors should not assume that the frames will be displayed in other than their final form.

9.6. Decoder handling of palettes

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. Encoders can supply a top-level sPLT chunk with a suggested reduced global palette to help decoders build an appropriate palette when necessary.

9.7. Behavior of single-frame viewers

Viewers that can only display a single frame must display the first frame that they encounter.

9.8. Clipping

AniPNG provides two 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
The frame_width and frame_height are defined in the MHDR chunk and cannot be changed by any other AniPNG chunk.

Decoders can use these parameters to establish the size of a window in which to display the AniPNG 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 an AniPNG datastream that consists of only a PNG or JNG datastream, with the AniPNG, PNG or JNG signature, the frame_width and frame_height are defined by the width and height fields of the IHDR (or JHDR) chunk.

Image clipping boundaries
The image clipping boundaries are defined in the DEFI chunk and can be changed by the PLAY chunk. They are associated with individual objects, not with the layers, and they can be changed within a seqence of layers. They are useful for exposing only a portion of an image in a frame, to achieve effects such as scrolling, panning, or gradual exposure.

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_cb  <= x < right_cb     (from the DEFI chunk)
   Top_cb   <= y < bottom_cb

10. Recommendations for Editors

10.1. Editing datastreams with optional index

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 AniPNG segment, so changing it has no effect on the copy-safe status of unknown chunks in any other part of the AniPNG 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.

10.2. Handling LOOP and TERM chunks

Editors that create a series of PNG or JNG datastreams from an AniPNG datastream should check the termination condition of any LOOP chunks and execute loops only iteration_min times. The loop created by the TERM chunk should be executed only once.

11. Miscellaneous Topics

11.1. File name extension

On systems where file names customarily include an extension signifying file type, the extension .mng is recommended for AniPNG files. Lowercase .mng is preferred if file names are case-sensitive.

11.2. Internet media type

When and if the AniPNG format becomes finalized, the AniPNG authors intend to register video/apg as the Internet Media Type for AniPNG [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-apg.

11.3. Uniform Resource Identifier (URI)

Segments and objects are externally accessible via named SEEK and eXPI chunk names. They can be referred to by URI, as in

   SRC=file.apg#segment_name
   SRC=file.apg#snapshot_name
   SRC=file.apg?segment_name#segment_name
   SRC=file.apg?snapshot_name#snapshot_name

When the URI specializer ("#" or "?") is "#", and the fragment identifier (the string following the specializer) is the name of a segment, i.e., a named SEEK chunk, the viewer should display the sequence from the beginning of the named segment up to the next segment. When it refers to an image, i.e., a named eXPI chunk, it should display the single image that is identified by the fragment identifier. The client can find the needed segment quickly if the SAVE chunk is present and contains the optional index.

When the URI specializer is "?" (server side query), the "query component" is the string following the "?" specializer and up to but not including the "#" if the "#" specializer is also present. The server should find the segment that is named in the query component or the segment that contains the 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 AniPNG datastream. Servers that are unwilling to parse the AniPNG datastream and are unconcerned about bandwidth can return the entire AniPNG 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 AniPNG datastream, either displaying the entire segment, if no fragment identifier is present, or extracting the segment 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 an image is being requested).

A part of the AniPNG datastream can also be requested by timecode, as in

   SRC=file.apg#clock(10s-20s)
   SRC=file.apg#clock(0:00-0:15)
   SRC=file.apg?clock(0:00-0:15)#clock(0:00-0:15)

or by frame number, as in

   SRC=file.apg#frame(10)
   SRC=file.apg#frames(30-60)
   SRC=file.apg?frames(30-60)#frames(30-60)

The timecode must consist of starting and ending clock values, as defined in the W3C SMIL recommendation, separated by a hyphen (ASCII code 45).

When the URI specializer is "#", the viewer should play that part of the sequence beginning and ending at the requested times, measuring from zero time at the beginning of the AniPNG datastream, or beginning and ending with the specified frame numbers. To do this it must start with the segment containing the requested time and decode any part of the segment up to that time, composing but not displaying the frames. This will provide the background against which the desired frames are displayed.

When the URI specializer is "?", the server can send the entire AniPNG datastream, or, preferably, it should construct a complete AniPNG file containing:

If the server does not send the entire AniPNG datastream, and the first segment after the SAVE chunk is not sent but a later segment is sent, the optional index must be written even if it does not exist in the source file. The index must contain at least one "type 0" entry that gives the nominal start time and frame number for the first segment that is sent after the SAVE chunk. The offset field can be set to zero and the segment name can be omitted.

The query component should always be repeated as a fragment identifier, so clients can find the requested item in case the server sends more than what was requested.

AniPNG datastreams should not contain segment or image names that begin with the case-insensitive strings "CLOCK(", "FRAME(", or "FRAMES(", which are reserved for use in URI queries and fragments (see Uniform Resource Identifier below).

See [RFC-2396] and the W3C SMIL recommendation at http://www.w3.org/TR/.

12. Revision History

12.1. Version 0.93

Released 27 Sept 2015

12.2. Version 0.92

Released 27 June 2013

12.3. Version 0.91

Released 14 July 2010

12.4. Version 0.9

Released 17 October 2007

12.5. Version 0.1

Released 9 September 2007

13. References

[ISO/IEC-10918-1]
International Organization for Standardization and International Electrotechnical Commission, "Digital Compression and Coding of Continuous-tone Still Images, Part 1: Requirements and guidelines," ISO/IEC IS 10918-1, ITU-T T.81.

See also Pennebaker, William B., and Joan L. Mitchell, "JPEG: Still Image Data Compression Standard," Van Nostrand Reinhold, ISBN:0442012721, September 1992

[JFIF]
C-Cube Microsystems, "JPEG File Interchange Format, Version 1.02," September 1992.

[JNG]
Randers-Pehrson, G., et al, "JNG (JPEG Network Graphics) Format,"
ftp://ftp.simplesystems.org/pub/png/documents/.

[LOCO]
Weinberger, Marcelo J., Gadiel Seroussi, and Guillermo Sapiro, "The LOCO-I Lossless Image Compression Algorithm: Principles and Standardization into JPEG-LS" Hewlett Packard Report HPL-98-193R1, November 1998, revised October 1999,, available at http://www.hpl.hp.com/loco/.

[MNG]
Randers-Pehrson, G., et al, "MNG (Multiple-image Network Graphics Format,"
ftp://ftp.simplesystems.org/pub/png/documents*.

[PNG]
Boutell, T., et. al., "PNG (Portable Network Graphics Format) Version 1.0," RFC 2083,
ftp://ftp.isi.edu/in-notes/rfc2083.txt also available at
ftp://ftp.simplesystems.org/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.

See also the PNG-1.2 specification:
Randers-Pehrson, G., et. al., "PNG (Portable Network Graphics Format) Version 1.2," which is available at
ftp://ftp.simplesystems.org/pub/png/documents/.

[PNG-EXT]
Randers-Pehrson, G., et al, "Extensions to the PNG 1.2 Specification,"
ftp://ftp.simplesystems.org/pub/png/documents/pngext-*.

[RFC-2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels," RFC 2119/BCP 14, Harvard University, March 1997.

[RFC-2045]
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://ftp.isi.edu/in-notes/rfc2045.txt

[RFC-2048]
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://ftp.isi.edu/in-notes/rfc2048.txt

[RFC-2396]
Berners-Lee, T., R. Fielding, U. C. Irvine, and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax," RFC 2396, MIT/LCS, Xerox Corporation, University of Minnesota, August 1998.
ftp://ftp.isi.edu/in-notes/rfc2396.txt

14. Security Considerations

Security considerations are addressed in the PNG specification.

An infinite or just overly long loop could give the appearance of having locked up the machine, as could an unreasonably long interframe delay or a long timeout value. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by abandoning the MNG entirely or just proceeding to the next SEEK chunk. Decoders should check for user input after each loop iteration (not just after each frame) in case of infinite loops that are empty or that generate layers with zero interframe delay. The SEEK chunk makes it safe for a viewer to resume processing after it encounters a corrupted portion of a MNG datastream or jumps out of the interior of a segment for any reason.

Some people may experience epileptic seizures when they are exposed to certain kinds of flashing lights or patterns that are common in everyday life. This can happen even if the person has never had any epileptic seizures. All graphics software and file formats that support animation and/or color cycling make it possible to encode effects that may induce an epileptic seizure in these individuals. It is the responsibility of authors and software publishers to issue appropriate warnings to the public in general and to animation creators in particular.

No known additional security concerns are raised by this format.

15. Credits

Acknowledgments

The following persons contributed to the development of the PLAY chunk or its predecessor, the proposed PNG aNIM chunk:

Thanks to the following person for carefully proofreading several versions of this document:

Trademarks

Document source

This document was built from the file anipng-0.92-20150927.html on 27 September 2015.

Copyright Notice

Copyright © 2013 by Glenn Randers-Pehrson

This specification is being provided by the copyright holder under the following license. By obtaining, using and/or copying this specification, you agree that you have read, understood, and will comply with the following terms and conditions:

Permission to use, copy, and distribute this specification for any purpose and without fee or royalty is hereby granted, provided that the full text of this NOTICE appears on ALL copies of the specification or portions thereof, including modifications, that you make.

THIS SPECIFICATION IS PROVIDED "AS IS," AND COPYRIGHT HOLDER MAKES NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED. BY WAY OF EXAMPLE, BUT NOT LIMITATION, COPYRIGHT HOLDER MAKES NO REPRESENTATIONS OR WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF THE SPECIFICATION WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS. COPYRIGHT HOLDER WILL BEAR NO LIABILITY FOR ANY USE OF THIS SPECIFICATION.

The name and trademarks of copyright holder may NOT be used in advertising or publicity pertaining to the specification without specific, written prior permission. Title to copyright in this specification and any associated documentation will at all times remain with copyright holder.

End of AniPNG Specification.