Animated PNG Graphics

“APNG is designed to allow incremental display of frames before the entire image has been read. This implies that some errors may not be detected until partway through the animation. It is strongly recommended that when any error is encountered decoders should discard all subsequent frames, stop the animation, and revert to displaying the default image. A decoder which detects an error before the animation has started should display the default image. An error message may be displayed to the user if appropriate.

Structure

An APNG stream is a normal PNG stream as defined in the PNG Specification, with three additional chunk types describing the animation and providing additional frame data.

To be recognized as an APNG, an ‘acTL’ chunk must appear in the stream before any ‘IDAT’ chunks. The ‘acTL’ structure is described below.

Conceptually, at the beginning of each play the output buffer must be completely initialized to a fully transparent black rectangle, with width and height dimensions from the ‘IHDR’ chunk.

The default image may be included as the first frame of the animation by the presence of a single ‘fcTL’ chunk before ‘IDAT’. Otherwise, the default image is not part of the animation.

Subsequent frames are encoded in ‘fdAT’ chunks, which have the same structure as ‘IDAT’ chunks, except preceded by a sequence number. Information for each frame about placement and rendering is stored in ‘fcTL’ chunks. The full layout of ‘fdAT’ and ‘fcTL’ chunks is described below.

The boundaries of the entire animation are specified by the width and height parameters of the PNG ‘IHDR’ chunk, regardless of whether the default image is part of the animation. The default image should be appropriately padded with fully transparent pixels if extra space will be needed for later frames.

Each frame is identical for each play, therefore it is safe for applications to cache the frames.”

Read the full article at the Mozilla Developer’s web page.