oxideav-gif

Pure-Rust GIF (87a / 89a) decoder + encoder, hand-written from CompuServe's Graphics Interchange Format Version 89a (July 1990) and the original Graphics Interchange Format (June 1987) specs.

No third-party GIF library code (giflib, libnsgif, Pillow, etc.) was read or referenced during implementation — the codec was written from the spec PDFs alone, with cross-validation against magick and giftext as black-box subprocess validators.

What's implemented

• File header (§17), Logical Screen Descriptor (§18), Global / Local Color Tables (§19, §21).
• Image Descriptor (§20) + Table-Based Image Data (§22) — including the four-pass interlace order (§E).
• Variable-Length-Code LZW compression and decompression (Appendix F) with the deferred-clear-code rule from the cover sheet.
• Graphic Control (§23), Comment (§24), Plain Text (§25), and Application (§26) extensions.
• Trailer (§27).
• Multi-image / animation files: all <Table-Based Image> blocks in the data stream are surfaced through GifImage::frames. A naive per-frame compositor lives at GifImage::composite_frame_rgba and respects transparency + Disposal Method.

Not (yet) implemented

• Real-time animation playback machinery — the per-frame data and the composite_frame_rgba helper are exposed, but there's no render loop.
• NETSCAPE2.0 looping extension parsing as a structured concept — it's preserved verbatim through GifBlock::Application for callers that want to interpret it (the byte container is spec'd at §26; the loop-count payload semantics are not).
• On-line Capabilities Dialogue (§G) — that's an interactive protocol layer, not part of any GIF file.

Standalone vs registry-integrated

oxideav-core is gated behind the default-on registry Cargo feature. Image-library consumers can depend on the crate with default-features = false to get a decode_gif / encode_gif API plus crate-local GifImage / GifError types and never reference oxideav-core.

# In an oxideav-framework consumer (default):
oxideav-gif = "0.0.10"

# In a standalone image-library consumer:
oxideav-gif = { version = "0.0.10", default-features = false }

Spec references

The implementation cites section numbers from the W3C-mirrored CompuServe spec at every block's parse / emit site. Fetched copies of the spec text live in oxideav-workspace/docs/image/gif/{gif87a-spec.txt,gif89a-spec.txt}.

The LZW state-machine implementation references the Welch (1984) paper A Technique for High-Performance Data Compression (IEEE Computer 17(6)) for the algorithm framing — the GIF spec itself describes only the GIF-specific deviations (§F items 1–4) and the bit packing layout, not the underlying dictionary algorithm.

License

Apache-2.0. The spec text grants royalty-free use of the GIF format in computer software; the codec implementation is original work licensed under Apache-2.0.