APNG

APNG, or Animated Portable Network Graphics, is an extension of the PNG image format that adds support for animation while preserving PNG’s lossless compression and color capabilities. An APNG file remains a PNG at its core, but includes additional chunks that define and carry multiple frames of animation.

Technical overview: The first frame is stored as a regular PNG image within the file. Additional frames are represented by a Frame Control Chunk (fcTL) and one or more Frame Data Chunks (fdAT). The Animation Control Chunk (acTL) indicates the total number of frames and the number of times the animation should play. Each frame’s fcTL specifies timing (delay_num and delay_den), as well as the frame’s dimensions and offsets, dispose_op (how the previous frame should be removed) and blend_op (how the new frame is blended with the current canvas). The actual image data for later frames is carried in the fdAT chunks. All frames share the same overall canvas size, and APNG supports full 24-bit color with an alpha channel.

Compatibility and usage: APNG is designed to be backward compatible with PNG decoders that do not understand the animation chunks; such decoders may display only the first frame. APNG is supported by many modern web browsers and image-editing tools, though support can vary by platform and application. It is commonly used for web graphics where animation without relying on proprietary formats is desirable, and it remains an accessible alternative to animated GIF in lossless environments.