fdAT

fdAT is a PNG extension chunk type used in Animated Portable Network Graphics (APNG). It carries the image data for frames beyond the first frame in an APNG animation. In APNG, the first frame is stored as a standard PNG image using the usual IHDR, IDAT, and other chunks. Additional frames use a combination of fcTL (frame control) to describe timing and disposal, and one or more fdAT chunks to provide the actual image data for those frames.

Each fdAT chunk contains a 4-byte sequence number followed by payload data that is effectively the same as the data traditionally carried in IDAT chunks. The sequence number serves to verify the correct order and integrity of the frame data across the animation. A single animation frame may be represented by multiple fdAT chunks, which must appear in increasing sequence order.

The APNG file-wide header acTL indicates the number of frames and plays, while per-frame control is defined by fcTL. fdAT chunks are thus integral to delivering the compressed image data for frames after the first, enabling smooth playback of the animated sequence alongside the initial PNG image data.

Compatibility notes: APNG and its fdAT chunks are not universally supported by all PNG-reading software. Some image viewers and web browsers support APNG and render animations with fdAT data, whereas others may display only the first frame or fail to render the animation. fdAT remains one mechanism among several used to encode animated frames within APNG files.