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web-image-prep: chroma-contracted quantiser for newsprint illusion

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Previous version pulled accent ink too aggressively (accentPull 0.85)
so anything red-leaning collapsed to solid red. Real photographic feel
from a 3-ink palette comes from W/B halftones simulating greys with
the chromatic ink only sparsely overlaid - the newsprint illusion.

Three knobs now make this work:
  - L_WEIGHT 3.5 (was 1.6): lightness completely dominates the metric
  - CHROMA_CAP: source a/b axes pre-scaled to the achievable accent
    chroma so the dither isn't asked for more colour than one ink can
    deliver. Excess chroma gets dropped via FS error diffusion through
    B+W neighbours.
  - accentPull 0.06 (was 0.85): only a tiebreaker bonus, used in the
    most strongly hue-aligned pixels.
This commit is contained in:
i12bp8
2026-04-26 21:03:10 +02:00
parent ad9b6dd2fe
commit 99bcc162b8
1 changed files with 44 additions and 26 deletions
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web-image-prep/index.html
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@@ -722,36 +722,42 @@ function quantize(img, ACC) {
buf[i*3+2] = srgb2lin(src[i*4+2]);
}
/* Hue-aligned, lightness-weighted nearest-palette search.
/* Modern e-paper tri-colour quantiser - the goal is the "newsprint
* illusion" where the eye fuses W/B halftone density into apparent
* grey shades and the single chromatic ink only appears as a faint
* accent over warm regions, NOT as a solid colour fill.
*
* The naive Oklab nearest treated red and black as equidistant from a
* dark-red pixel, so accent ink got smeared across shadow areas, washing
* out depth. Two changes fix that:
* Three knobs make this work, all tuned for the Oklab metric used
* below (typical photo chroma magnitude ~0.05..0.25, never anywhere
* near the accent's own ~0.20):
*
* 1. Distance weights L (lightness) ~1.6x more than chroma, so shadows
* always pick black and highlights always pick white before colour
* ever wins. That keeps tonal structure of the photo intact.
* 1. L_WEIGHT - lightness dominates the distance so shadows pick
* black and highlights pick white before chroma is even a
* tie-breaker. This is what keeps the tonal structure intact.
*
* 2. Accent gets a bonus proportional to the *signed projection* of the
* pixel's chroma onto the accent direction. Red ink is only pulled
* toward pixels whose hue actually leans red (positive a*); a blue
* or green pixel never picks red. Squaring the projection gives a
* smooth ramp so faintly-red regions get sparse red dither and
* saturated reds get dense red, matching how a real risograph layers
* a single chromatic ink under W/B halftones.
* 2. CHROMA_CAP - the source image's a/b axes are pre-scaled
* toward grey BEFORE dithering. With only one chromatic ink
* to spend, we don't want to "owe" the error diffuser more
* red than the paper can deliver - that's the bug that made
* everything red. Treating the source as low-saturation lets
* FS spread the residual through black and white naturally.
*
* The result is a photo-grade tri-colour quantisation where red/yellow
* lives where it belongs and depth/shading is carried by W+B mixing.
* 3. accentPull - very small bonus for hue-aligned pixels, just
* enough to break ties in favour of accent in the most-saturated
* regions. The bonus uses the *signed projection* of the pixel's
* chroma onto the accent direction so opposite-hue pixels (a
* blue sky) never pick red.
*/
const L_WEIGHT = 3.5;
/* Map source chroma 1.0 -> accent's chroma * 1.1 so even the most
* saturated source pixel never sits beyond what one ink can render. */
const accentLab = useColor ? paletteLab[2] : null;
const accentMag = accentLab ? Math.hypot(accentLab[1], accentLab[2]) || 1e-9 : 1;
/* Strength of the accent attraction. Detail slider can tilt -50%..+50%. */
const accentPull = 0.85 + tilt * 0.6;
const L_WEIGHT = 1.6;
const CHROMA_CAP = useColor ? Math.min(1, (accentMag * 1.1)) : 1;
/* Detail slider tilts attraction +/- ~30%. */
const accentPull = 0.06 * (1 + tilt * 0.5);
function nearest(r, g, b) {
const lab = rgb2oklab(r, g, b);
const L = lab[0], a = lab[1], bp = lab[2];
function nearest(L, a, bp) {
let best = 0, bd = Infinity;
for (let p = 0; p < paletteRGB.length; p++) {
const dl = L - paletteLab[p][0];
@@ -759,8 +765,6 @@ function quantize(img, ACC) {
const db = bp - paletteLab[p][2];
let d = L_WEIGHT * dl*dl + da*da + db*db;
if (useColor && p === 2) {
/* Signed projection of pixel chroma onto accent chroma direction.
* Positive only when the pixel hue is aligned with the accent. */
const proj = (a * accentLab[1] + bp * accentLab[2]) / accentMag;
const align = Math.max(0, proj);
d -= align * align * accentPull;
@@ -770,6 +774,18 @@ function quantize(img, ACC) {
return best;
}
/* Sample the (already-tone-mapped) linear-RGB working pixel into the
* Oklab the dither operates on, with chroma contracted toward grey
* so we don't ask for more colour than three inks can deliver. */
function sampleLab(r, g, b) {
const lab = rgb2oklab(r, g, b);
if (useColor) {
lab[1] *= CHROMA_CAP;
lab[2] *= CHROMA_CAP;
}
return lab;
}
if (S.dither === "thr" || S.dither === "bayer8") {
const m = S.dither === "bayer8" ? BAYER8 : null;
for (let y = 0; y < H; y++) for (let x = 0; x < W; x++) {
@@ -786,7 +802,8 @@ function quantize(img, ACC) {
continue;
}
}
idx[i] = nearest(r, g, b);
const lab = sampleLab(r, g, b);
idx[i] = nearest(lab[0], lab[1], lab[2]);
}
} else {
const k = KERNELS[S.dither] || KERNELS.fs;
@@ -803,7 +820,8 @@ function quantize(img, ACC) {
for (let x = xStart; x !== xEnd; x += xStep) {
const i = y*W + x;
const r = buf[i*3], g = buf[i*3+1], b = buf[i*3+2];
const p = nearest(r, g, b);
const lab = sampleLab(r, g, b);
const p = nearest(lab[0], lab[1], lab[2]);
idx[i] = p;
const er = r - paletteRGB[p][0];
const eg = g - paletteRGB[p][1];