Esa Kataja
46ae7e813a
The handoff was written as a message to relay information; several decisions lived only there. Split into permanent homes: - docs/spec.md — scope, geometry model, pipeline order, detection, levels, editor, project file, markers, bundle format, noteman's changes, reference values - ADR 0002 — raster only in release 1; SVG slices measured at 40x WebP naive, 2.6x with a bounding-box cull, deferred on risk not size - ADR 0003 — lossless WebP with levels and 16-level alpha; every lossy option and alternative format measured larger for line art - ADR 0004 — detection proposes, the human disposes; no unattended mode - ADR 0005 — PyMuPDF for all PDF access, accepting AGPL - ADR 0006 — systems are found by vertical brackets, not row-darkness gaps, which miscount every page of a 6-voice score Also from testing against the hardest score in the repertoire: scanned PDFs carry their scan as an embedded image and must be extracted at native resolution rather than re-rendered at 600 DPI, and per-page deskew is mandatory (skew varies -2.6 to +1.2 within one PDF).
59 lines
2.6 KiB
Markdown
59 lines
2.6 KiB
Markdown
# Lossless WebP, with levels and alpha quantised to 16 levels
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Slice images are encoded as **lossless WebP**, with the levels adjustment applied
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and the alpha channel quantised to 16 levels. About 7 KB per slice, ~450 KB for a
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65-system song. Every lossy option and every alternative format measured
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*larger* for this content, which is the opposite of the usual intuition — hence
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this record.
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## The measurement
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20 slices of one real song, levels applied throughout, relative to plain lossless
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WebP:
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| | vs baseline | |
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|---|---|---|
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| **WebP lossless + alpha quantised to 16** | **68%** | chosen |
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| AVIF q60 | 90% | lossy, for 10% |
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| WebP lossless | 100% | baseline |
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| WebP lossy q85 (alpha) | 107% | |
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| AVIF q85 | 114% | |
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| JXL lossless | 130–133% | |
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| WebP lossy q85 (opaque ink-on-white) | 158% | |
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| PNG grayscale + alpha | 165% | |
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| AVIF lossless | 188% | |
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Separately, before levels: applying levels alone takes 338 KB → 211 KB, a 38%
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reduction.
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## Four results that contradict an instinct
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- **Lossy is bigger than lossless here.** Not a quality problem — the measured
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difference between q85 and lossless is max 12/255, mean 0.33, i.e. invisible.
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Lossy VP8 simply spends more bits on sharp black/white edges than VP8L's
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palette and predictor transforms do, and notation is nothing but sharp edges.
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The "q85 looks fine" intuition comes from photographs and inverts here.
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- **AVIF and JXL both lose**, AVIF lossless by nearly 2×. Their lossless modes
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are afterthoughts on photo codecs. WebP's VP8L is close to purpose-built for
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flat two-tone line art — sheet music is the content type it is best at. JXL
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additionally has no path forward in Chrome.
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- **Alpha costs nothing.** Opaque ink-on-white and black-plus-alpha are within
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0.1% at lossless, so paper-tint removal and future non-rectangular slices are
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free.
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- **Levels is the single biggest lever** — 38%, as a side effect of a control
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that exists for quality reasons anyway. Pushing the white point below the
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paper's luminance sets vast regions to exactly `alpha = 0`, which costs almost
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nothing to encode.
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Alpha quantisation to 16 levels is imperceptible: antialiased edges span 2–3 px
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at 1920, and 16 steps across that is below notice. 8 levels starts to gamble on
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thin strokes.
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## Rejected as not worth it
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- **Encoder effort tuning** — Pillow's `method=6` buys 3% and a dependency.
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- **`alpha_quality=60`** — 24%, for less control than quantisation gives.
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- **Grayscale WebP** — no such mode exists. It wouldn't help anyway: the RGB
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channels are constant black and compress to nearly nothing, so alpha is the
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entire payload.
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