# Lossless WebP, with levels and alpha quantised to 16 levels

Slice images are encoded as **lossless WebP**, with the levels adjustment applied
and the alpha channel quantised to 16 levels. About 7 KB per slice, ~450 KB for a
65-system song. Every lossy option and every alternative format measured
*larger* for this content, which is the opposite of the usual intuition — hence
this record.

## The measurement

20 slices of one real song, levels applied throughout, relative to plain lossless
WebP:

| | vs baseline | |
|---|---|---|
| **WebP lossless + alpha quantised to 16** | **68%** | chosen |
| AVIF q60 | 90% | lossy, for 10% |
| WebP lossless | 100% | baseline |
| WebP lossy q85 (alpha) | 107% | |
| AVIF q85 | 114% | |
| JXL lossless | 130–133% | |
| WebP lossy q85 (opaque ink-on-white) | 158% | |
| PNG grayscale + alpha | 165% | |
| AVIF lossless | 188% | |

Separately, before levels: applying levels alone takes 338 KB → 211 KB, a 38%
reduction.

## Four results that contradict an instinct

- **Lossy is bigger than lossless here.** Not a quality problem — the measured
  difference between q85 and lossless is max 12/255, mean 0.33, i.e. invisible.
  Lossy VP8 simply spends more bits on sharp black/white edges than VP8L's
  palette and predictor transforms do, and notation is nothing but sharp edges.
  The "q85 looks fine" intuition comes from photographs and inverts here.
- **AVIF and JXL both lose**, AVIF lossless by nearly 2×. Their lossless modes
  are afterthoughts on photo codecs. WebP's VP8L is close to purpose-built for
  flat two-tone line art — sheet music is the content type it is best at. JXL
  additionally has no path forward in Chrome.
- **Alpha costs nothing.** Opaque ink-on-white and black-plus-alpha are within
  0.1% at lossless, so paper-tint removal and future non-rectangular slices are
  free.
- **Levels is the single biggest lever** — 38%, as a side effect of a control
  that exists for quality reasons anyway. Pushing the white point below the
  paper's luminance sets vast regions to exactly `alpha = 0`, which costs almost
  nothing to encode.

Alpha quantisation to 16 levels is imperceptible: antialiased edges span 2–3 px
at 1920, and 16 steps across that is below notice. 8 levels starts to gamble on
thin strokes.

## Rejected as not worth it

- **Encoder effort tuning** — Pillow's `method=6` buys 3% and a dependency.
- **`alpha_quality=60`** — 24%, for less control than quantisation gives.
- **Grayscale WebP** — no such mode exists. It wouldn't help anyway: the RGB
  channels are constant black and compress to nearly nothing, so alpha is the
  entire payload.