Target a fixed ink coverage, defaulting to 33.3%

Left alone, coverage follows whatever the photo happened to be, which came out
around 44% and printed heavier than wanted — thermal dots spread as the paper
heats, so they land fatter than they look on screen.

The brightness offset needed to hit a target is solved for rather than
searched: error diffusion preserves mean tone, so the first guess is analytic
and up to three correction passes handle the clipping. Measured 33.6% across
normal, heavily darkened and heavily brightened versions of the same photo, in
under 6 ms, so badges now print at a consistent density regardless of lighting.

Set targetCoverage to null to opt out.

src/printing/dither.js

@@ -9,7 +9,7 @@ import { createCanvas } from '@napi-rs/canvas';
  * error diffusion trades spatial resolution for apparent tone instead, which
  * is what makes a photo readable at 300 dpi.
  *
- * Two things matter for this to look like a face rather than noise:
+ * Three things matter for this to look like a face rather than noise:
  *
  *  1. Dither at the exact pixel size the photo will occupy. Rescaling a
  *     dithered image resamples the dot pattern back into greys, and the later
@@ -20,6 +20,12 @@ import { createCanvas } from '@napi-rs/canvas';
  *     produces flat mush. Normalising to the full range first gives the error
  *     diffusion something to work with.
  *
+ *  3. Aim for a fixed ink coverage. Thermal dots spread as the paper heats, so
+ *     they come out fatter on the label than they look on screen, and a
+ *     digitally "correct" image prints muddy. Targeting coverage also makes
+ *     every badge print at the same density regardless of how the visitor
+ *     happened to be lit.
+ *
  * Text is deliberately NOT dithered anywhere — dithered glyph edges look furry
  * at this resolution. Only photographs go through here.
  */
@@ -100,6 +106,58 @@ function floydSteinberg(grey, width, height) {
   }
 }
 
+/** Add a constant to every sample, clamped to the printable range. */
+function applyShift(grey, shift) {
+  if (!shift) return;
+  for (let p = 0; p < grey.length; p++) {
+    grey[p] = Math.min(255, Math.max(0, grey[p] + shift));
+  }
+}
+
+/** Fraction of dots that would burn, for a given luminance buffer. */
+function coverageOf(grey, width, height) {
+  const trial = Float32Array.from(grey);
+  floydSteinberg(trial, width, height);
+  let black = 0;
+  for (let p = 0; p < trial.length; p++) if (trial[p] < 128) black++;
+  return black / trial.length;
+}
+
+/**
+ * Find the brightness offset that lands the dithered result on a given ink
+ * coverage.
+ *
+ * Error diffusion preserves mean tone, so coverage is roughly 1 - mean/255 and
+ * the first guess can be computed directly rather than searched for. Clipping
+ * at the ends of the range spoils that slightly, so up to three cheap
+ * correction passes follow. Each pass is one dither over a few tens of
+ * thousands of samples — the whole thing runs in about 6 ms for a 24 mm photo.
+ *
+ * Capped at +/-120 so a very dark or very bright capture degrades into
+ * something faint rather than a blank square.
+ */
+function solveShiftForCoverage(grey, width, height, target, maxPasses = 3) {
+  const clamp = (v) => Math.min(120, Math.max(-120, v));
+
+  let mean = 0;
+  for (let p = 0; p < grey.length; p++) mean += grey[p];
+  mean /= grey.length;
+
+  let shift = clamp(255 * (1 - target) - mean);
+
+  for (let pass = 0; pass < maxPasses; pass++) {
+    const trial = Float32Array.from(grey);
+    applyShift(trial, shift);
+    const actual = coverageOf(trial, width, height);
+    const error = actual - target;
+    if (Math.abs(error) < 0.005) break;
+    // Coverage moves roughly linearly with the offset over this range.
+    shift = clamp(shift + error * 255);
+  }
+
+  return shift;
+}
+
 /**
  * Dither a photo to 1-bit at a given square size.
  *
@@ -109,12 +167,16 @@ function floydSteinberg(grey, width, height) {
  * @param {Object} [options]
  * @param {boolean} [options.levels=true]   Stretch contrast first
  * @param {number}  [options.brightness=0]  -100..100, nudge before dithering.
- *                                          Negative darkens; useful if badges
- *                                          print washed out on old rolls.
+ *                                          Negative darkens.
+ * @param {number}  [options.targetCoverage=0.333]
+ *        Fraction of dots to burn, 0..1. The brightness needed to hit it is
+ *        solved for, so every photo prints at the same density however it was
+ *        lit. Raise it for a heavier print, lower it for a lighter one. Pass
+ *        null to leave density alone and print whatever the photo gives.
  * @returns {import('@napi-rs/canvas').Canvas} ready to pass to drawImage
  */
 export function ditherPhoto(image, size, options = {}) {
-  const { levels = true, brightness = 0 } = options;
+  const { levels = true, brightness = 0, targetCoverage = 0.333 } = options;
 
   const canvas = createCanvas(size, size);
   const ctx = canvas.getContext('2d');
@@ -149,11 +211,10 @@ export function ditherPhoto(image, size, options = {}) {
 
   if (levels) autoLevels(grey);
 
-  if (brightness !== 0) {
-    const shift = (brightness / 100) * 255;
-    for (let p = 0; p < grey.length; p++) {
-      grey[p] = Math.min(255, Math.max(0, grey[p] + shift));
-    }
+  if (brightness !== 0) applyShift(grey, (brightness / 100) * 255);
+
+  if (targetCoverage != null) {
+    applyShift(grey, solveShiftForCoverage(grey, size, size, targetCoverage));
   }
 
   floydSteinberg(grey, size, size);