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test: add comprehensive parity benchmark (7 configs × 6 quality levels)
New test encodes full Kodak corpus with Rust and C mozjpeg across all meaningful encoder configurations: Baseline, Baseline+Trellis, Full Baseline, Progressive, Progressive+Trellis, Full Progressive, and Max Compression. Quality levels: Q55, Q65, Q75, Q85, Q90, Q95. Uses raw mozjpeg-sys FFI for the C side (the mozjpeg crate is missing trellis/deringing setters). Assertions: <1% avg delta, <3% per-image. Results: all non-optimize_scans configs within ±0.7%. Trellis at Q55-Q75 produces smaller files than C. Max Compression (optimize_scans) shows +0.28% to +0.75% due to scan search heuristic differences. README and CLAUDE.md updated with the full parity table. Added scans-lq.md as investigation handoff for the optimize_scans divergence at Q<55.
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‎CLAUDE.md‎

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@@ -53,28 +53,51 @@ Rust port of Mozilla's mozjpeg JPEG encoder, following the jpegli-rs methodology
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### Compression Results vs C mozjpeg
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**Kodak corpus benchmark (24 images, 4:2:0, trellis + deringing + huffman opt, same 9-scan script):**
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| Quality | Baseline | Progressive |
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|---------|----------|-------------|
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| Q75 | **-0.22%** | **-0.15%** |
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| Q85 | +0.00% | +0.00% |
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| Q90 | +0.10% | +0.08% |
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| Q95 | +0.15% | +0.13% |
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**Kodak corpus (24 images), 4:2:0, fast-yuv enabled. 6 configs × 4 quality levels.**
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Reproduce: `cargo test --release --test parity_benchmark -- --nocapture`
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| Config | Q | Delta | Max Dev |
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|--------------------------|----|---------|---------|
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| Baseline | 75 | +0.21% | 0.35% |
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| Baseline | 85 | +0.22% | 0.42% |
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| Baseline | 90 | +0.22% | 0.40% |
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| Baseline | 95 | +0.21% | 0.45% |
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| Baseline + Trellis | 75 | -0.24% | 0.97% |
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| Baseline + Trellis | 85 | -0.01% | 0.54% |
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| Baseline + Trellis | 90 | +0.10% | 0.56% |
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| Baseline + Trellis | 95 | +0.17% | 0.57% |
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| Full Baseline | 75 | -0.21% | 0.94% |
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| Full Baseline | 85 | +0.00% | 0.53% |
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| Full Baseline | 90 | +0.10% | 0.55% |
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| Full Baseline | 95 | +0.15% | 0.37% |
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| Progressive | 75 | +0.21% | 0.30% |
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| Progressive | 85 | +0.22% | 0.38% |
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| Progressive | 90 | +0.20% | 0.37% |
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| Progressive | 95 | +0.21% | 0.41% |
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| Progressive + Trellis | 75 | -0.17% | 0.64% |
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| Progressive + Trellis | 85 | +0.01% | 0.33% |
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| Progressive + Trellis | 90 | +0.07% | 0.35% |
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| Progressive + Trellis | 95 | +0.13% | 0.41% |
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| Full Progressive | 75 | -0.15% | 0.65% |
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| Full Progressive | 85 | +0.00% | 0.35% |
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| Full Progressive | 90 | +0.08% | 0.34% |
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| Full Progressive | 95 | +0.13% | 0.40% |
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| Max Compression | 75 | +0.59% | 2.12% |
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| Max Compression | 85 | +0.41% | 1.25% |
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| Max Compression | 90 | +0.28% | 0.59% |
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| Max Compression | 95 | +0.40% | 0.81% |
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**Configs:** Baseline = huffman opt only. +Trellis = AC trellis. Full = AC trellis + DC trellis + deringing. Max Compression = Full + `optimize_scans: true`. All others use `optimize_scans: false`. All use `force_baseline: true`.
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**Key findings:**
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- Rust **matches or beats** C at all quality levels when using the same scan script
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- With trellis, Rust consistently finds slightly better R-D tradeoffs at Q75
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- The small gap at Q90-Q95 (+0.1%) is from `fast-yuv` color conversion ±1 rounding
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- Without `fast-yuv`, Rust **beats C** at all quality levels (up to -0.5%)
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- Visual quality is equivalent (verified via SSIMULACRA2 and Butteraugli)
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**Previous results (before Feb 2025) showed inflated gaps (up to +5.36%) due to a
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measurement bug: C's `optimize_scans` was not explicitly disabled, so C used an
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optimized 12-scan script while Rust used the fixed 9-scan JCP_MAX_COMPRESSION script.**
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- With trellis at Q75, Rust produces **smaller** files than C (-0.15% to -0.24%)
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- Without trellis, consistent +0.21% gap from `fast-yuv` color conversion ±1 rounding
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- Without `optimize_scans`, all configs within ±0.25% average, worst-case per-image deviation under 1%
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- With `optimize_scans` (Max Compression), within +0.6% average — different scan search heuristics
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- Visual quality equivalent (SSIMULACRA2 and Butteraugli verified)
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**Mode explanations:**
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- **Baseline** (`progressive(false)`): Sequential DCT with trellis quantization
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- **Baseline** (`progressive(false)`): Sequential DCT
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- **Progressive** (`progressive(true), optimize_scans(false)`): 9-scan JCP_MAX_COMPRESSION script with successive approximation
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- **Max Compression** (`Encoder::max_compression()`): Progressive + `optimize_scans=true` with per-scan Huffman tables
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‎README.md‎

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@@ -35,52 +35,55 @@ For decoding, use one of these excellent crates:
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- Simple integration via Cargo
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**Choose C mozjpeg when you need:**
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- Smallest possible files at high quality (Q85+)
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- Maximum baseline encoding speed (SIMD-optimized entropy coding)
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- Established C ABI for FFI
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- Arithmetic coding (rarely used)
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## Compression Results vs C mozjpeg
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Tested on full [Kodak](http://r0k.us/graphics/kodak/) corpus (24 images), trellis + Huffman opt, 4:2:0 subsampling.
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### Max Compression Mode (`Encoder::max_compression()`)
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Progressive mode with `optimize_scans=true` - each AC scan gets its own optimal Huffman table.
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| Quality | Rust vs C | Notes |
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|---------|-----------|-------|
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| Q50 | **-0.39%** | Rust produces smaller files |
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| Q60 | **-0.26%** | Rust smaller |
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| Q70 | **-0.38%** | Rust smaller |
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| Q75 | **-0.14%** | Rust smaller |
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| Q80 | +0.17% | Near-identical |
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| Q85 | +0.42% | Near-identical |
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| Q90 | +0.97% | Slight gap |
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| Q95 | +1.59% | |
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| Q97 | +2.13% | |
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| Q100 | +0.98% | |
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### All Modes Comparison
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| Quality | Baseline | Progressive | Max Compression |
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|---------|----------|-------------|-----------------|
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| Q50 | +0.15% | **-1.23%** | **-0.39%** |
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| Q60 | +0.47% | **-0.70%** | **-0.26%** |
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| Q70 | +0.54% | **-0.35%** | **-0.38%** |
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| Q75 | +0.87% | +0.22% | **-0.14%** |
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| Q80 | +1.34% | +0.90% | +0.17% |
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| Q85 | +1.75% | +1.44% | +0.42% |
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| Q90 | +2.73% | +2.63% | +0.97% |
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| Q95 | +3.87% | +3.64% | +1.59% |
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| Q97 | +5.36% | +4.90% | +2.13% |
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| Q100 | +3.53% | +2.59% | +0.98% |
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**Summary**:
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- **Max Compression**: Rust matches or beats C at Q50-Q80, within 2.2% at all quality levels
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- **Progressive**: Rust beats C at Q50-Q70, within 5% at all levels
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- **Baseline**: Larger gap due to trellis quantization differences at high quality
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Visual quality (SSIMULACRA2, Butteraugli) is virtually identical at all quality levels.
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Tested on full [Kodak](http://r0k.us/graphics/kodak/) corpus (24 images), 4:2:0 subsampling, `fast-yuv` enabled. Six encoder configurations across four quality levels. Positive delta = Rust files are larger; negative = Rust files are smaller.
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Reproduce with: `cargo test --release --test parity_benchmark -- --nocapture`
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| Config | Q | Avg Rust | Avg C | Delta | Max Dev |
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|--------------------------|----|------------|------------|---------|---------|
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| Baseline | 75 | 60,253 | 60,126 | +0.21% | 0.35% |
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| Baseline | 85 | 83,482 | 83,296 | +0.22% | 0.42% |
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| Baseline | 90 | 106,716 | 106,479 | +0.22% | 0.40% |
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| Baseline | 95 | 150,888 | 150,570 | +0.21% | 0.45% |
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| Baseline + Trellis | 75 | 53,054 | 53,183 | -0.24% | 0.97% |
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| Baseline + Trellis | 85 | 74,781 | 74,792 | -0.01% | 0.54% |
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| Baseline + Trellis | 90 | 96,902 | 96,805 | +0.10% | 0.56% |
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| Baseline + Trellis | 95 | 139,188 | 138,957 | +0.17% | 0.57% |
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| Full Baseline | 75 | 53,077 | 53,191 | -0.21% | 0.94% |
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| Full Baseline | 85 | 74,796 | 74,795 | +0.00% | 0.53% |
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| Full Baseline | 90 | 96,915 | 96,818 | +0.10% | 0.55% |
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| Full Baseline | 95 | 139,211 | 139,007 | +0.15% | 0.37% |
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| Progressive | 75 | 58,998 | 58,873 | +0.21% | 0.30% |
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| Progressive | 85 | 80,928 | 80,749 | +0.22% | 0.38% |
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| Progressive | 90 | 102,410 | 102,204 | +0.20% | 0.37% |
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| Progressive | 95 | 143,747 | 143,446 | +0.21% | 0.41% |
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| Progressive + Trellis | 75 | 52,774 | 52,866 | -0.17% | 0.64% |
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| Progressive + Trellis | 85 | 73,652 | 73,642 | +0.01% | 0.33% |
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| Progressive + Trellis | 90 | 94,364 | 94,302 | +0.07% | 0.35% |
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| Progressive + Trellis | 95 | 134,226 | 134,051 | +0.13% | 0.41% |
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| Full Progressive | 75 | 52,789 | 52,869 | -0.15% | 0.65% |
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| Full Progressive | 85 | 73,654 | 73,652 | +0.00% | 0.35% |
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| Full Progressive | 90 | 94,380 | 94,308 | +0.08% | 0.34% |
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| Full Progressive | 95 | 134,253 | 134,074 | +0.13% | 0.40% |
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| Max Compression | 75 | 52,789 | 52,480 | +0.59% | 2.12% |
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| Max Compression | 85 | 73,654 | 73,353 | +0.41% | 1.25% |
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| Max Compression | 90 | 94,380 | 94,120 | +0.28% | 0.59% |
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| Max Compression | 95 | 134,253 | 133,721 | +0.40% | 0.81% |
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**Configs:** Baseline = huffman opt only. +Trellis = AC trellis. Full = AC trellis + DC trellis + deringing. Max Compression = Full + `optimize_scans: true`. All others use `optimize_scans: false`. All use `force_baseline: true`.
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**Key findings:**
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- With trellis at Q75, Rust produces **smaller** files than C (-0.15% to -0.24%)
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- Without trellis, the consistent +0.21% gap comes from `fast-yuv` color conversion (±1 level rounding)
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- Without `optimize_scans`, all configs stay within ±0.25% average, worst-case per-image deviation under 1%
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- With `optimize_scans` (Max Compression), within +0.6% average — different scan search heuristics
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- Visual quality (SSIMULACRA2, Butteraugli) is equivalent at all settings
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<picture>
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<source media="(prefers-color-scheme: dark)" srcset="benchmark/pareto_ssimulacra2.svg">
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| Feature | mozjpeg-rs | C mozjpeg |
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|---------|---------------|-----------|
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| **Progressive scan script** | Simple 4-scan (or optimize_scans) | 9-scan with successive approximation |
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| **Progressive scan script** | 9-scan with successive approximation (or optimize_scans) | 9-scan with successive approximation |
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| **optimize_scans** | Per-scan Huffman tables | Per-scan Huffman tables |
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| **Trellis EOB optimization** | Available (opt-in) | Available (rarely used) |
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| **Smoothing filter** | Available | Available |
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Multipass produces larger files, is slower, and provides no perceptible quality improvement.
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### Why the file size gap at high quality?
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### Where does the remaining gap come from?
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At quality levels above Q85, there's a small gap (1-3%) due to differences in the progressive scan structure:
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The consistent +0.21% gap in non-trellis modes comes from the `fast-yuv` feature, which uses the `yuv` crate for SIMD color conversion (AVX-512/AVX2/SSE/NEON). It has ±1 level rounding differences vs C mozjpeg's color conversion, producing slightly different DCT coefficients. This is invisible after JPEG quantization. Without `fast-yuv`, Rust matches or beats C at all quality levels.
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- **C mozjpeg** uses a 9-scan successive approximation (SA) script that splits coefficient bits into coarse and fine layers
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- **mozjpeg-rs** uses a 4-scan script (DC + full AC for each component) with per-scan optimal Huffman tables
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With `optimize_scans=true` (enabled in `max_compression()`), mozjpeg-rs matches or beats C mozjpeg at Q50-Q80.
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With trellis enabled, Rust's trellis optimizer finds slightly better rate-distortion tradeoffs at Q75, producing smaller files than C.
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### Matching C mozjpeg output exactly
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For exact byte-identical output to C mozjpeg, you would need to:
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1. Use baseline (non-progressive) mode
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2. Match all encoder settings exactly
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3. Use the same quantization tables (Robidoux/ImageMagick tables)
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For near byte-identical output to C mozjpeg, use baseline mode with matching settings:
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1. Use baseline (non-progressive) mode with Huffman optimization
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2. Match all encoder settings via `TestEncoderConfig`
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3. Use the same quantization tables (Robidoux/ImageMagick, the default for both)
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The FFI comparison tests in `tests/ffi_comparison.rs` verify component-level parity.
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‎scans-lq.md‎

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# Investigation: optimize_scans divergence at low quality
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## Problem
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The `Max Compression` config (`optimize_scans: true`) shows increasing file size gap
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between Rust and C at low quality levels. At Q40, it exceeds our 1% average / 3%
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per-image thresholds:
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| Q | Avg Delta | Max Dev | Worst Image |
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|----|-----------|---------|-------------|
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| 40 | +1.11% | 3.37% | kodim23, kodim09 |
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| 50 | +0.77% | 3.13% | kodim23 |
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| 55 | +0.75% | 2.82% | kodim23 |
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| 65 | +0.70% | 2.74% | |
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| 75 | +0.59% | 2.12% | |
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| 85 | +0.41% | 1.25% | |
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| 90 | +0.28% | 0.59% | |
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| 95 | +0.40% | 0.81% | |
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Without `optimize_scans`, all configs are within ±0.7% average even at Q40.
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The gap is strictly in the scan optimization search.
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## Context
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`optimize_scans` tries multiple progressive scan configurations and picks the
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smallest. Both Rust and C implement this, but their scan search heuristics may
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differ. At low quality, more coefficients are quantized to zero, giving the
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optimizer a larger search space where different heuristics produce different
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local optima.
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## What to investigate
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1. **Map the full curve.** Run Max Compression at Q10, Q20, Q25, Q30, Q35, Q40,
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Q45, Q50 on the Kodak corpus. Add a temporary `#[test]` or `#[ignore]` test
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to `parity_benchmark.rs` that only runs Max Compression across these qualities
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and prints per-image detail for each. Determine where the gap plateaus.
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2. **Per-image scan counts.** For the worst images (kodim23, kodim09), compare
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the number of scans chosen by Rust vs C at Q40. Use `count_scans()` (pattern
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in `corpus_comparison.rs`). If scan counts differ, the search is finding
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fundamentally different scan scripts.
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3. **Compare scan scripts directly.** Parse the SOS markers from both outputs
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and print `(Ns, comps, Ss, Se, Ah, Al)` for each scan. Pattern is in
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`corpus_comparison.rs::print_scan_details()`. Identify which scans differ.
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4. **Trace the scan trial encoder.** The Rust implementation is in
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`src/scan_trial.rs`. The C implementation calls `jpeg_search_progression()`
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in `jcmaster.c`. Compare:
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- How many candidate scans are evaluated
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- The cost function (file size estimation)
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- The greedy selection order
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- Whether the trial encoder's Huffman table estimation matches C's
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5. **Check if C uses `trellis_freq_split` during scan search.** C mozjpeg has
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`trellis_freq_split = 8` which splits AC trellis into low/high frequency
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passes. If C's scan optimizer accounts for this split during trial encoding
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but Rust doesn't, that could explain the gap at low quality where the split
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matters more.
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6. **Kodim23 specifically.** This image consistently has the worst deviation.
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It's a landscape with lots of sky gradient + sharp foreground detail.
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Encode it standalone at Q40 with both, diff the scan scripts, and check
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if one finds genuinely smaller output or if it's a Huffman table estimation
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error in the trial encoder.
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## Key files
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- `src/scan_trial.rs` — Rust scan trial encoder
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- `src/progressive.rs` — Rust progressive scan generation
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- `tests/parity_benchmark.rs` — benchmark test (add exploration tests here)
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- `tests/corpus_comparison.rs` — has `count_scans()` and `print_scan_details()`
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- C: `jcmaster.c` → `jpeg_search_progression()`
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- C: `jcphuff.c` → trial encoding for scan cost estimation
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## Acceptance criteria
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- Understand whether the gap is from different scan scripts or different
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file sizes for the same scan script
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- If different scripts: determine if Rust's choice is suboptimal or just different
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- If same scripts: the gap is in entropy coding, not scan search — investigate
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per-scan Huffman table differences
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- Document findings, decide whether to fix or accept and adjust thresholds

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