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* Vladimir Vukicevic <vladimir@pobox.com>
* Andrew Smith <asmith15@learn.senecac.on.ca>
= Big Important Note =
The chunk names in this specification will be changed to aCTl, fCTl and fDAt. Please consider any APNGs with acTl, fcTl and fdAt to be useless after that change happens.
= Overview =
To be recognized as an APNG, an `acTl` chunk must appear in the stream before any `IDAT` chunks. The `acTl` structure is described in the next section.
An `fcTl` chunk must also appear before `IDAT`, providing frame information for the first frame encoded in the PNG stream's `IDAT` chunks, known as frame 0. If there is no fCTl chunk before IDAT, then frame 0 is intialized to RGBA(0,0,0,0) and the IDAT chunks are not used.
Subsequent frames are encoded in `fdAt` chunks containing almost the same structure of content as IDAT chunks. Information for each frame about placement and rendering is stored in `fcTl` chunks. The full layout of `fdAt` and `fcTl` chunks is described below.
Note: For purposes of chunk descriptions, an "unsigned int" shall be a 32-bit unsigned integer in network byte order limited to the range 0 to (2^31)-1; an "unsigned short" shall be a 16-bit unsigned integer in network byte order; a "byte" shall be an 8-bit unsigned integer.
== Terminology ==
The "canvas" is the area on the output device on which the images are to be written. The contents of the canvas are not available to the decoder.
The "output buffer" is a pixel array with dimensions specified by the width and height parameters of the PNG IHDR chunk. Conceptually, the images are constructed in the output buffer and then written to the canvas. The contents of the output buffer are available to the decoder. The corner pixels of the output buffer are mapped to the corners of the canvas.
== Chunk Sequence Numbers ==
The `fcTl` and `fdAt` chunks have a 4 byte sequence number. Both chunk types share the sequence. The purpose of this number is to detect (and optionally correct) sequence errors in an Animated PNG, since the PNG specification does not impose orderng restrictions on ancillary chunks.
The table below illustrates the use of sequence numbers for images with more than one frame and more than one `fdAt` chunk:
....
== `acTl`: The Animation Control Chunk ==
The `acTl` chunk is an ancillary chunk as defined in the PNG Specification. It must appear before the first `IDAT` chunk within a valid PNG stream. If the color type of the image is 3 (indexed-color) the `acTl` chunk must appear after the `PLTE` chunk.
The `acTl` chunk contains:
* For the first frame, the `fcTl` chunk must appear before the first `IDAT` chunk. Position relative to the `acTl` chunk is not specified.
* For the second frame, the `fcTl` chunk must appear immediately after the `IDAT` chunks from the first frame and immediately before the fdAt chunks for the second frame.* For any subsequent frames, the `fcTl` chunk for the frame N must appear immediately after the `fdAt` chunks from the frame N-1 and immediately before the fdAt chunks for the frame N.* Other ancillary chunks are allowed to appear among the APNG chunks, including between fDAt chunks. Decoders must ignore such chunks.
The `fcTl` fCTl chunk is mandatory for every frameexcept for frame 0. When it is present for frame 0, decoders must render frame 0 from the data in the IDAT chunks. When it is not present for frame 0, frame 0 is a transparent black frame (all pixels are RGBA(0,0,0,0)) and the IDAT chunks are ignored. More than one `fcTl` fCTl chunk per frame is not allowed.
Format:
24 render_op (byte) Type of canvas area disposal to be done after rendering this frame
The frame must be rendered within the region defined by the `width`, `height`, `x_offset` and `y_offset` from the `fcTl`, and the width and height from the `IHDR`chunk.
The width and height cannot must be greater than zero and must not be bigger than (respectively) the width and height specified in the `IHDR` chunk. The value of both the width and the height must be greater than 0.
For frame 0 the width and height fields must equal the width and height from the `IHDR` chunk. Also for frame 0 the `x_offset` and `y_offset` fields must be 0. No part of the region may fall outside the canvas area defined by frame 0.
The `delay_num` and `delay_den` parameters together specify a fraction indicating the delay after the current frame, in seconds. If the denominator is 0, it is to be treated as if it were 100 (that is, delay_num then specifies 1/100ths of a second). If the the value of the numerator is 0 the decoder should render the next frame as quickly as possible, though viewers may impose a reasonable lower bound on the delay.
+-------------------------------+
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+-------------|----|---|---|---|-+ | | +---+---+------ bits 0-2: dispose_op | | | +------------------ bit 3: APNG_RENDER_OP_BLEND_FLAG | +---------------------- bit 4: APNG_RENDER_OP_SKIP_FRAME
Bits 5 4 through 7 are reserved and must be 0.
Valid values for `dispose_op` are:
`APNG_RENDER_OP_BLEND_FLAG` is not valid for color types 0 (greyscale without alpha) or 2 (truecolor without alpha). It is valid for type 3 (indexed) images; however, the resulting pixel values may not be present in the specified palette. It is always valid for type 4 (greyscale with alpha) and type 6 (truecolor with alpha) images.
APNG_RENDER_OP_BLEND_FLAG must be 0 for frame 0.
If `APNG_RENDER_OP_SKIP_FRAME` is not 0, then the decoder should not render the current frame as part of the animation. Though this flag can be set on any frame and must be honored, it is most useful for frame 0, to prevent the frame that would be visible to PNG viewers not supporting animation from being part of the animated frame set. If animation in the viewer is not desired or is explicitly disabled by the user, the viewer should display frame 0 even if `APNG_RENDER_OP_SKIP_FRAME` is not 0 on frame 0. This provides content authors with a means to provide a still image to be used in lieu of the full animation.
The `fdAt` chunk has the same purpose as an `IDAT` chunk. It has the same data structure as an `IDAT` chunk, except a sequence number is appended in the beginning.
Each frame must contain at least one fDAt chunk. There may be multiple `fdAt` chunks for any one frame; if so, they shall appear consecutively with no other intervening chunks. The compressed datastream is then the concatenation of the contents of the data fields of all the fDAt chunks within a frame. When decompressed, the datastream is a complete PNG image, including the filter byte at the beginning of each scanline. It utilizes the same bit depth, color type, compression method, filtermethod, interlace method, and palette (if any) as the main image.
Format:
If the PNG oFFs chunk is present, it supplies offsets of the canvas that are in addition to the frame's "x_offset" and "y_offset".
If the PNG pHYs chunk is present, the APNG images and their "x_offset" and "y_offset" values must be scaled in the same way as the main image. Conceptually, such scaling occurs while mapping the output buffer onto the canvas.
= Revisions to this Specification =
* Reintroduced the width and height fields in fcTl
== From 0.7 ==
* Removed 'hidden' flag, instead only the first frame can be hidden and it is signaled with a missing fcTl
* IDAT, fcTl and fdAt are no longer required to have no other chunks in between them
= Test Encoder and Sample Images =
