gamut-bitstream

gamut-bitstream holds the low-level bit writers and entropy coders the gamut codecs share — the encoder-side mirror of the parsing processes defined in the codec specs.

Goals

Part of the gamut workspace, this crate exists to:

Provide the bit-level building blocks once. The pieces here are the encoder-side counterpart of the AV1 Bitstream & Decoding Process Specification (../../references/av1/):
- [BitWriter] — most-significant-bit-first fixed-width fields (f(n)) and byte alignment, used by the AV1 uncompressed sequence/frame headers (AV1 §4, §8.1).
- [write_leb128] / [leb128_len] — unsigned LEB128 for OBU sizes (AV1 §4.10.5, Annex B).
- [SymbolEncoder] — the AV1 multi-symbol arithmetic (range) coder, derived by inverting the symbol decoder of AV1 §8.2; the entropy back-end for coded tile data.
Stay byte-exact with decoders. Each writer is validated to produce streams real decoders (dav1d, avifdec) accept, since it is derived directly from the decode process.
Stay allocation-conscious. Writers append into caller buffers and avoid per-symbol heap churn.
Stay memory-safe. #![forbid(unsafe_code)].

Usage

use gamut_bitstream::{leb128_len, write_leb128};

// Unsigned LEB128, as used for AV1 OBU sizes: 300 -> 0xac 0x02 (2 bytes).
let mut out = Vec::new();
write_leb128(&mut out, 300);
assert_eq!(out, [0xac, 0x02]);
assert_eq!(leb128_len(300), 2);

Status

M0 provides the AV1 encoder-side primitives: [BitWriter], LEB128 ([write_leb128] / [leb128_len]), and the [SymbolEncoder] range coder. The forward-looking ANS / Huffman coders named in the workspace plan (for AV2 / JPEG XL) are not implemented yet.

Roadmap

ANS and Huffman entropy coders (for gamut-av2 / gamut-jxl), each behind its own module.

License