fix(photos): preserve tapered tips and dark live edge (#73 #74)

Lower the axial body threshold while retaining fringe rejection, minimize length-only tip inset, and make sheet sensitivity curves testable and conservative at zero.

frontend/src/mask/sheetKeep.test.ts

@@ -0,0 +1,59 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { blackKeep, greenKeep } from './sheetKeep.ts';
+
+// --- Black sheet sensitivity (#74) ---
+
+test('blackKeep: s=0 keeps dark live-edge wood (lum ~25–50), removes near-black sheet (#74)', () => {
+  // Near-black sheet (lum ≈ 5)
+  assert.equal(blackKeep(5, 5, 5, 0), false, 'near-black sheet removed at s=0');
+  assert.equal(blackKeep(0, 0, 0, 0), false, 'pure black sheet removed at s=0');
+  // Dark wood / bark / live edge
+  assert.equal(blackKeep(25, 22, 18, 0), true, 'lum~22 dark wood kept at s=0');
+  assert.equal(blackKeep(40, 35, 30, 0), true, 'lum~35 dark wood kept at s=0');
+  assert.equal(blackKeep(50, 45, 40, 0), true, 'lum~46 wood kept at s=0');
+  // Mid wood always kept
+  assert.equal(blackKeep(120, 90, 60, 0), true, 'mid wood kept at s=0');
+});
+
+test('blackKeep: s=100 removes more than s=0; curve monotonic (#74)', () => {
+  // Dark-ish pixel that sits between floors: sheet at high s, wood at low s
+  const darkish = { r: 80, g: 70, b: 60 }; // lum ≈ 72
+  assert.equal(blackKeep(darkish.r, darkish.g, darkish.b, 0), true, 'lum~72 kept at s=0');
+  assert.equal(blackKeep(darkish.r, darkish.g, darkish.b, 100), false, 'lum~72 sheet at s=100');
+
+  // Near-black always sheet
+  assert.equal(blackKeep(2, 2, 2, 0), false);
+  assert.equal(blackKeep(2, 2, 2, 100), false);
+
+  // Bright wood always kept
+  assert.equal(blackKeep(200, 160, 100, 0), true);
+  assert.equal(blackKeep(200, 160, 100, 100), true);
+
+  // Monotonic: once removed, stays removed as s increases
+  const probe = { r: 60, g: 55, b: 50 }; // lum ≈ 56
+  let prevKeep = true;
+  for (const s of [0, 25, 50, 75, 100]) {
+    const keep = blackKeep(probe.r, probe.g, probe.b, s);
+    if (!prevKeep) assert.equal(keep, false, `once sheet at lower s, stay sheet at s=${s}`);
+    prevKeep = keep;
+  }
+  // s=100 must remove more pixels than s=0 for this mid-dark band
+  assert.equal(blackKeep(probe.r, probe.g, probe.b, 0), true);
+  assert.equal(blackKeep(probe.r, probe.g, probe.b, 100), false);
+});
+
+test('greenKeep: s=0 does not eat mild wood greens; s=100 widens sheet match (#74)', () => {
+  // Neutral wood
+  assert.equal(greenKeep(140, 110, 80, 0), true);
+  assert.equal(greenKeep(140, 110, 80, 100), true);
+  // Mild green cast on wood (g - max(r,b) ≈ 5) — must keep at s=0
+  assert.equal(greenKeep(100, 105, 90, 0), true, 'mild green cast kept at s=0');
+  // Strong sheet green
+  assert.equal(greenKeep(40, 180, 50, 0), false, 'strong green sheet removed at s=0');
+  assert.equal(greenKeep(40, 180, 50, 100), false);
+  // Borderline: g-max ≈ 20 — may become sheet at high s
+  const border = { r: 80, g: 100, b: 70 }; // delta = 20
+  assert.equal(greenKeep(border.r, border.g, border.b, 0), true, 'delta20 kept at s=0');
+  assert.equal(greenKeep(border.r, border.g, border.b, 100), false, 'delta20 sheet at s=100');
+});

frontend/src/mask/sheetKeep.ts

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+/**
+ * Sheet-vs-slab keep predicates used by the mask worker.
+ * Extracted so unit tests can cover sensitivity curves without the worker.
+ */
+
+/** Keep slab pixels on a green sheet. Higher s widens the green-sheet match. */
+export function greenKeep(r: number, g: number, b: number, s: number): boolean {
+  const t = Math.max(0, Math.min(100, s)) / 100;
+  // s=0 removes only strongly green sheet; increasing sensitivity widens the match.
+  const isSheet = g - Math.max(r, b) > 45 - 35 * t;
+  return !isSheet;
+}
+
+/**
+ * Keep slab pixels on a black sheet.
+ * At s=0 only near-black sheet (very low lum) is removed so dark live-edge /
+ * bark (lum ~25–50) stays. The match widens monotonically toward s=100 (#74).
+ */
+export function blackKeep(r: number, g: number, b: number, s: number): boolean {
+  const t = Math.max(0, Math.min(100, s)) / 100;
+  const lum = 0.2126 * r + 0.7152 * g + 0.0722 * b;
+  // Old floor 40 ate dark wood at minimum sensitivity. Floor 10 isolates near-black.
+  const isSheet = lum < 10 + 140 * t;
+  return !isSheet;
+}
+
+export function autoPickSheet(
+  samples: { r: number; g: number; b: number }[],
+): 'green' | 'black' {
+  let gr = 0, gl = 0;
+  for (const p of samples) {
+    if (p.g - Math.max(p.r, p.b) > 20) gr++;
+    if (0.2126 * p.r + 0.7152 * p.g + 0.0722 * p.b < 70) gl++;
+  }
+  return gl >= gr ? 'black' : 'green';
+}