APNG Specification: Difference between revisions

APNG is backwards-compatible with PNG; any PNG decoder should be able to decode the first frame of an APNG and treat it as a normal single-frame PNG.

An APNG stream is a normal PNG stream as defined in the [PNG Specification][pngspec], with three additional chunk types describing the animation and providing additional frame data.  The first frame of an animation, frame 0, is encoded as a normal PNG.  This frame is what decoders that do not understand the APNG chunks will display.

The size of the first frame defines the boundaries of the entire animation; hence, if extra space will be needed for later frames that is unused in the first frame, the first frame should be appropriately padded with fully transparent regions.

An `fcTL` chunk may 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.

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.

The first fcTL chunk must contain sequence number 0, and the sequence numbers in the remaining fcTL and fdAT chunks must be in order, with no gaps or duplicates.

The table below illustrates the use of sequence numbers for images with more than one frame and more than one `fdAT` chunk:

     0                  `fcTL` describing frame 0

     1                  `fcTL` describing frame 1

     2                  first `fdAT` for frame 1

     3                  second `fdAT` for frame 1

or, when frame 0 is implicitly a transparent black frame:

     0                  fcTL describing frame 1

     1                  first fdAT for frame 1

     2                  second fdAT for frame 1

APNG viewers must abandon the APNG chunks in a datastream with out-of-order APNG chunks and display frame 0 instead. APNG-aware PNG editors should restore them to correct order using the sequence numbers.

`num_frames` indicates the total number of frames in the animation. 0 is not a valid value. 1 is a valid value for a single-frame APNG. In case this number does not match the actual number of frames, behaviour of the implementation is not specified.

`num_iterations` indicates the number of iterations that this animation should play; if it is 0, the animation should play indefinitely.  If nonzero, the animation should come to rest on the final non-skipped frame at the end of the last iteration.

* For the first frame, if the fcTL chunk is present it 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 after the `IDAT` chunks from the first frame and before the fdAT chunks for the second frame.

* For any subsequent frames, the `fcTL` chunk for the frame N must appear after the `fdAT` chunks from the frame N-1 and before the fdAT chunks for the frame N.

* Other ancillary chunks are allowed to appear among the APNG chunks, including between fdAT chunks.

The fcTL chunk is mandatory for every frame except 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 chunk per frame is not allowed.

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 must be greater than zero and must not be bigger than (respectively) the width and height specified in the IHDR chunk.

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 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.

* APNG_DISPOSE_OP_NONE: no disposal is done on this frame before rendering the next; its contents are left on the output buffer.  This is the default.

* APNG_DISPOSE_OP_BACKGROUND: the frame's region is to be cleared to the background color. If no bKGD chunk is specified, the result is fully transparent black (r, g, b, and a all 0).

* APNG_DISPOSE_OP_PREVIOUS: the frame's region is to be reverted to the previous contents.

`blend_op` specifies whether the frame is to be alpha blended into the current output buffer content, or whether it should completely replace its region in the output buffer. Valid `blend_op` values are:

`blend_op` must be APNG_BLEND_OP_SOURCE for frame 0.

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. 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, filter method, interlace method, and palette (if any) as the main image.

Each frame inherits every property except the width and the height specified by any critical or ancillary chunks before the first IDAT in the file.

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.