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== Terminology ==
The "placeholder image" is the image described by the standard `IDAT` chunks, and is the image that displayed by decoders that do not support APNG.
The "canvas" is the area on the output device on which the frames are to be displayed. The contents of the canvas are not necessarily 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, each frame is constructed in the output buffer before being composited onto 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.
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.
Swap Structure and Terminology sections
APNG is backwards-compatible with PNG; any PNG decoder should be able to ignore the APNG-specific chunks and display a single image.
== Terminology ==
The "placeholder image" is the image described by the standard `IDAT` chunks, and is the image that displayed by decoders that do not support APNG.
The "canvas" is the area on the output device on which the frames are to be displayed. The contents of the canvas are not necessarily 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, each frame is constructed in the output buffer before being composited onto 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.
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.
= Structure =
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.
== Chunk Sequence Numbers ==
