Animcodec

Animcodec decodes and encodes editable raster animation sequences on top of Imcodec. Its first supported container is GIF89a, including frame offsets, local and global palettes, transparency, delays, loop metadata and disposal methods.

Decoded frames are complete straight-alpha RGBA canvases. This deliberately keeps GIF palette and delta-frame details out of editors: modify, reorder or replace the frames, then encode the resulting sequence again.

import 'dart:typed_data';

import 'package:animcodec/animcodec.dart';

final RasterAnimation animation = decodeGifAnimation(bytes);
final RasterAnimation edited = RasterAnimation(
  width: animation.width,
  height: animation.height,
  loopCount: 0,
  frames: animation.frames.reversed.toList(),
);
final Uint8List encoded = encodeGifAnimation(edited);

// Or encode a long sequence without holding every frame in memory.
final GifAnimationStreamEncoder stream = GifAnimationStreamEncoder(
  width: animation.width,
  height: animation.height,
  loopCount: 0,
);
edited.frames.forEach(stream.add);
final Uint8List streamed = stream.close();

All inputs are bounded by encoded size, canvas pixels, frame count and aggregate decoded bytes. An AnimationCodecException reports whether the data was malformed, cut short, or valid but beyond a configured limit. A file that stops early still yields every complete frame, as in browsers, and GifAnimationInfo.isTruncated tells the caller it happened. Canvases start transparent and "restore to background" clears to transparency, matching Skia and browsers rather than the rarely honoured logical-screen colour.

Each frame after the first is encoded as the smallest rectangle holding its changes, with unchanged pixels inside it left transparent. Frames can be added one at a time through GifAnimationStreamEncoder, which retains only two canvases whatever the length of the sequence. GIF durations are rounded to the nearest 10 milliseconds, the smallest interval the container can represent.

Flutter's own multi-frame codec cannot draw a partial frame that follows a "restore to background" frame. Animcodec only uses that disposal when pixels must become transparent again, which the format cannot express otherwise.

The test suite includes the public-domain imazen GIF conformance corpus, covering valid, malformed and edge-case files. It also checks decoded pixels against Flutter's independent Skia-backed decoder where both implementations use the same logical-background convention.

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