Editing APNG (section)

== File format ==

The APNG specification follows the [[Portable Network Graphics#File format|PNG File format]] introducing three new ancillary chunks:<ref name="spec">{{Cite web |title=APNG Specification - MozillaWiki |url=https://wiki.mozilla.org/APNG_Specification}}</ref>

* The animation control chunk (acTL) precedes the IDAT(s) of the default image and is a kind of "marker" that this is an animated PNG file. It also contains the number of frames and the number of times to loop the animation (0 meaning infinite).
* The frame control chunk (fcTL) precedes each frame and contains its metadata : dimensions; position (relative to the default image); duration; if once over it is cleared to black, replaced by the previous frame or drawn over by the next frame; and if its transparency applies.
* The frame data chunk (fdAT) storing frame's content. It starts with a sequence number, then has the same structure as the default image's IDAT chunk(s).

Sequence numbers apply to both frame control and frame data chunks, which together follow a common sequence, thus enabling the order and timing of frames to be recovered should an APNG-unaware PNG editor re-order them as allowed by PNG chunk ordering rules.<ref name="PNG chunk">[http://www.libpng.org/pub/png/spec/1.2/PNG-Ordering.html Chunk Ordering Rules] (PNG 1.2 spec)</ref>

Frames utilize the same bit depth, color type, compression method, filter method, interlace method, and palette (if any) as the default image.

An application reading a PNG file is meant to ignore any chunks which it does not understand,<ref name="pngspec">{{Cite web |title=Portable Network Graphics (PNG) Specification (Second Edition) |url=https://www.w3.org/TR/PNG/}}</ref> making APNG [[Backward compatibility|backwards compatible]]. Applications without support for the APNG extension show only the first frame, disregarding additional animation frames.

[[Image:Apng assembling.svg|frame|none|Diagram illustrating a possible way to assemble an animated PNG from three individual PNG files]]

=== Compression and optimization ===
A number of optimization techniques make APNG files as small as possible: [[Inter frame|Inter-frame]] optimization <ref>{{Cite web |title=Inter-frame Optimization in APNG |url=http://littlesvr.ca/apng/inter-frame.html |url-status=live |archive-url=https://web.archive.org/web/20140426100136/http://littlesvr.ca:80/apng/inter-frame.html |archive-date=2014-04-26 |access-date=2021-10-21 |website=littlesvr.ca}}</ref> utilizing [[Alpha Blend|alpha-blend]] and alpha dispose operations, smaller than the full-size subframes, dirty transparency, color type and [[Palette (computing)|color palette]] optimizations, and various compression options: [[zlib]], [[7-Zip]], [[Zopfli]].<ref>{{Cite web |title=APNG Assembler |url=https://apngasm.sourceforge.net/ |access-date=2021-10-21 |website=apngasm.sourceforge.net}}</ref>

===Derived formats===
Animated [[sticker (messaging)|sticker]]s for [[Signal (messaging app)|Signal]] are APNG with some restrictions (the size of the file is limited to 300kb, the length of the animation is limited to 3 seconds and, visibly (this last point is unclear), the resolution must be 512x512px).{{Which|date=January 2024}}<ref name="Signal">{{Cite web |title=Stickers |url=https://support.signal.org/hc/en-us/articles/360031836512-Stickers#sticker_reqs |access-date=28 December 2021 |website=Signal Support |publisher=Signal |language=en}}</ref>