effectcraft/crates/opusenc/README.md

effectcraft-opusenc

Clean-room, pure-Rust Opus encoder for EffectCraft's WebM export audio (layer L0, std only, no unsafe).

• CELT-only packets (TOC configuration 31: fullband 20 kHz, 20 ms, one frame per packet), 48 kHz input, mono or stereo (stereo coded as dual L/R stereo), constant packet size from the requested bitrate; all-zero frames become 3-byte silence packets.
• Forward MDCT with the low-overlap window, band energies with inter/intra coarse prediction (cheaper of the two per frame), fine and final energy bits, the decoder's exact bit allocation, spreading and recursive band splitting, greedy PVQ search and codeword indexing.
• Not implemented: SILK/hybrid modes, transient (short-block) frames, TF changes, pitch post-filter, intensity stereo, dynamic allocation boosts, VBR rate control.
• OpusEncoder::pre_skip() is 120 samples (the MDCT overlap); opus_head() returns the RFC 7845 identification header.

let mut enc = effectcraft_opusenc::OpusEncoder::new(2, 128_000);
let packet = enc.encode_float(&vec![0.0; effectcraft_opusenc::OpusEncoder::FRAME_SIZE * 2]);

Specifications

• RFC 6716, Definition of the Opus Audio Codec (September 2012), with the RFC 8251 updates (October 2017) as implemented by the decoder we round-trip against.
• RFC 7845, Ogg Encapsulation for the Opus Audio Codec (April 2016), §5.1 OpusHead and §4.2 pre-skip.

Implemented from the RFC text. Constant tables (band edges, eMeans, the coarse-energy Laplace model, the allocation table, ICDFs) and the decoder-side allocation arithmetic are mirrored from FilmCraft's first-party, spec-derived filmcraft-opus decoder (MIT OR Apache-2.0), which is also the round-trip oracle in crates/export/tests/opus_roundtrip.rs (ffmpeg is a second, external oracle when installed). No third-party encoder or decoder source was consulted.