The APNG chunk that marks an animated PNG and stores its frame count is acTL.
The name means animation control. An acTL chunk appears before the default image’s IDAT data and contains two important unsigned integers: the number of animation frames and the number of times the animation should loop. A loop count of zero means that playback repeats indefinitely.
The other choices have different jobs. IDAT carries the compressed image data for the default PNG image. fcTL, or frame control, describes an individual frame’s dimensions, position, delay, and disposal or blending behavior. fdAT carries compressed data for animation frames beyond the default image, with a sequence number added to the chunk’s payload.
APNG was developed by Mozilla in 2004 as an animation extension that retained PNG’s lossless compression, broad color support, and alpha transparency. Its structure is deliberately backward-compatible: a PNG decoder that does not understand APNG can usually display the default image, while an APNG-aware decoder reads acTL, fcTL, and fdAT to reconstruct the animation.