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Brandon ThomasandClaude Opus 5.5
b26fbb6247
effectcraft-vp9enc gains inter frames: single LAST reference, 16x16/8x8 blocks, candidate + diamond search refined to quarter samples with the regular 8-tap filters, NEARESTMV/NEARMV/ZEROMV/NEWMV with spec MV prediction, skip blocks, the in-loop deblocking filter, per-frame forward probability updates, a key frame interval and target-bitrate rate control. Decoded output matches the encoder's reconstruction exactly through FilmCraft's decoder and ffmpeg. WebM export now encodes sequentially with a key frame every two seconds. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
48 lines
3.7 KiB
Markdown
48 lines
3.7 KiB
Markdown
# effectcraft-vp9enc
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A small VP9 encoder for WebM export (profile 0, 8-bit 4:2:0, BT.709).
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* **Key frames**: 8×8 intra blocks (DC / V / H / TM prediction chosen per block), 4×4 DCT/ADST
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transforms, or the Walsh–Hadamard transform at quality 100 (mathematically lossless).
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* **Inter frames**: one reference (the previous frame, LAST), 16×16 or 8×8 blocks chosen by a
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cost comparison; motion search from the predicted vectors (NearestMv / NearMv / last frame's
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co-located vector) with a whole-sample cross/diamond search refined to half and quarter
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samples with the regular 8-tap interpolation filters; NEARESTMV / NEARMV / ZEROMV / NEWMV
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chosen by bit cost; blocks with no coefficients worth coding are skipped; 4×4 DCT residual
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with a rate–distortion check per transform block. Inter frames are coded error resilient
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(`error_resilient_mode` = 1: no dependence on earlier probability contexts or motion vectors),
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so `UsePrevFrameMvs` is never needed. No high-precision (1/8) vectors, no compound prediction,
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no intra blocks inside inter frames.
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* **Loop filter**: the in-loop deblocking filter (§8.8) on every lossy frame; the level is the
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best of three trials against the source (sharpness 0, no mode/reference deltas).
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* **Probabilities**: per-frame forward updates in the compressed header (`diff_update_prob` /
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`update_mv_prob`) of the coefficient, skip, inter-mode, is-inter, reference, partition and
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motion-vector probabilities. Tiles are recorded as symbols first, the frame's own counts pick
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the updates that pay for their signalling, then the symbols are entropy coded with the updated
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probabilities. There is no backward adaptation (`refresh_frame_context` = 0).
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* **Rate control**: constant quality (quality → `base_q_idx`) or a target bitrate: a per-frame
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q index from a bits ≈ complexity / step model per frame type, key frames getting a larger share
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of each key-frame interval, with the running over/undershoot paid back over about a second.
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* **Key frame interval** is configurable (1 = every frame a key frame, the old intra-only mode).
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The encoder reconstructs exactly what a decoder reconstructs (inter prediction with the
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decoder's vector clamping and edge-clamped reference fetches, integer inverse transforms, the
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loop filter) and exposes it (`Vp9Encoder::reconstruction`); the tests require FilmCraft's VP9
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decoder (and ffmpeg, when installed) to output exactly that reconstruction, frame by frame, for
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moving synthetic sequences at several sizes and qualities, with tile columns, and lossless.
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On a moving-gradient sequence (192×108, 24 frames) inter coding is 3.9–7× smaller than all-intra
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at the same quality setting with equal or better PSNR.
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Written from the **VP9 Bitstream & Decoding Process Specification, version 0.6 (31 March
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2016)**: the encoder emits the uncompressed and compressed headers (§6.2, §6.3), partition, mode
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info, motion vector and token syntax (§6.4), derives motion vector candidates with the §6.4
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`find_mv_refs` / `find_best_ref_mvs` processes, and reconstructs with the specification's intra
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and inter prediction (§8.5), integer inverse transforms (§8.7) and loop filter (§8.8) so it stays
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in step with any conformant decoder. The boolean encoder is the inverse of the §9.2 decoder in
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the form given in RFC 6386 §7.3. Spec tables (§10) are the specification's; the key-frame tables
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were transcribed from FilmCraft's first-party decoder `filmcraft-vp9` (MIT OR Apache-2.0), which
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is also the decoder the tests round-trip through and whose behaviour (loop filter edge rules,
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vector candidate order) the encoder was checked against; ffmpeg is used only as an external test
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oracle. No libvpx or other third-party encoder/decoder source was consulted.
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