Grains riding a belt or filter cast no shadow (t37)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PROJECT_MAP.md

@@ -352,9 +352,9 @@ Drawing (src/render/): browser only. This file: the renderer state R, colour tab
 - functions: glowIndex, thermIdx, pipeCellColor, pipeBodyCell, thinPipeColor, beltLike, buildSky, initRender, resizeRender, viewCells, updateCamera, snapCamera, screenToWorld, worldToScreenPx, renderMonoFont
 - variables: R, GLOW_W, GLOW_R, GLOW_G, GLOW_B, THERM, PC, TREE_R, TREE_G, TREE_B, BG, SKY_R, SKY_G, SKY_B
 
-### src/render/light.js (95 lines)
+### src/render/light.js (103 lines)
 Propagated coloured light from glowing and burning cells.
-- functions: builtForLight, computeLight
+- functions: builtForLight, onBeltForLight, computeLight
 
 ### src/render/world.js (332 lines)
 Scale colour without changing the smooth-look class.

docs/agent-log.md

@@ -4,6 +4,14 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-25: t37 follow-up: belt loads cast no shadows (Claude Code, Codex)
+
+- The owner saw grains riding a belt overhead still shading the ground. `onBeltForLight(i)` in
+  `src/render/light.js` walks down through up to 12 loose cells to a belt or filter; such cells
+  are skipped by the light roof and pass light like air. `builtForLight` is unchanged, so the t33
+  loose-grain shading still applies to belt loads. Heaps on rock, silos and wall floors still shade.
+- `test/t_light.js` (56 checks); `run_all.py --browser` ALL PASSED.
+
 ## 2026-10-25: t35: diagonal screw tubes (Claude Code, Codex)
 
 - Parts `P_TUR`..`P_TUL` (11-14), `TUBE_PARTS`, and an `IS_TUBE` table replacing every

src/render/light.js

@@ -5,6 +5,14 @@
 function builtForLight(i, x, y) {
   return (PH[mat[i]] === PH_SOLID && !(flags[i] & F_NAT)) || blockP[(y >> 2) * BW + (x >> 2)] || mach[i] || part[i];
 }
+function onBeltForLight(i) {
+  for (let depth = 0; depth <= 12 && i < N; depth++, i += W) {
+    const m = mat[i], ph = PH[m];
+    if (m === BELTL || m === BELTR || part[i] === P_FILTER) return true;
+    if (ph !== PH_POWDER && ph !== PH_LIQUID) return false;
+  }
+  return false;
+}
 function computeLight() {
   const res = SET.lighting === 'fast' ? 4 : 2;
   const M = Math.ceil(28 / res);
@@ -32,7 +40,7 @@ function computeLight() {
     let y = roofY[x];
     for (; y < roofEnd; y++) {
       const i = y * W + x, ph = PH[mat[i]];
-      if ((ph === PH_SOLID || ph === PH_POWDER || ph === PH_LIQUID) && !builtForLight(i, x, y)) break;
+      if ((ph === PH_SOLID || ph === PH_POWDER || ph === PH_LIQUID) && !builtForLight(i, x, y) && !((ph === PH_POWDER || ph === PH_LIQUID) && onBeltForLight(i))) break;
     }
     lightRoof[lx] = y;
   }
@@ -42,7 +50,7 @@ function computeLight() {
       const wx = (lx0 + lx) * res, k = ly * lw + lx;
       if (wx < 0 || wy < 0 || wx >= W || wy >= H) { la[k] = 0; lr[k] = lg[k] = lb[k] = 0; continue; }
       const i = wy * W + wx, m = mat[i], ph = PH[m];
-      la[k] = builtForLight(i, wx, wy) ? aAir : ph === PH_EMPTY ? (bgm[i] ? aGas : aAir) : ph === PH_GAS ? aGas : ph === PH_LIQUID ? aLiq : m === GLASS || m === ICE ? aGas : aSol;
+      la[k] = builtForLight(i, wx, wy) || ((ph === PH_POWDER || ph === PH_LIQUID) && onBeltForLight(i)) ? aAir : ph === PH_EMPTY ? (bgm[i] ? aGas : aAir) : ph === PH_GAS ? aGas : ph === PH_LIQUID ? aLiq : m === GLASS || m === ICE ? aGas : aSol;
       let r = 0, g = 0, b = 0;
       if (wy < lightRoof[lx]) { r = skyK; g = skyK * 0.98; b = skyK * 0.94; }
       else if (wy < surfY[wx] && ph !== PH_SOLID) { const v = skyK * 0.9; r = v; g = v * 0.98; b = v * 0.95; }

test/t_light.js

@@ -60,4 +60,48 @@ run(`SET.daylight = 0; temp[140 * W + 80] = 1500; computeLight();`);
 check('hot material inside the base lights its surroundings', run('lightAt(R.lr, 88, 140) > 0.2'));
 run(`temp[140 * W + 80] = AMB[140]; bot.x = 72; bot.y = 130; bot.lamp = true; computeLight();`);
 check('the bot lamp still lights the base', run('lightAt(R.lr, 80, 140) > 0.2'));
+
+flatWorld(run, 320, 200, 160);
+run(`
+  SET.daylight = 1; bot.lamp = false;
+  for (let bx = 8; bx < 30; bx++) placeBlock(BP_BELT, 1, bx, 20, true);
+  for (let x = 32; x < 120; x++) for (let y = 77; y < 80; y++) setCell(y * W + x, SAND, AMB[y], 0);
+  for (let x = 180; x < 280; x++) {
+    setCell(80 * W + x, STONE, AMB[80], F_NAT);
+    for (let y = 77; y < 80; y++) setCell(y * W + x, SAND, AMB[y], 0);
+  }
+  updateRoof();
+`);
+for (const mode of ['full', 'fast']) {
+  run(`SET.lighting = '${mode}'; computeLight();`);
+  for (const channel of ['lr', 'lg', 'lb']) {
+    const [belt, open, stone] = run(`[lightAt(R.${channel}, 80, 84), lightAt(R.${channel}, 152, 84), lightAt(R.${channel}, 232, 84)]`);
+    check(`${mode} ${channel}: a loaded belt leaves daylight below it`, Math.abs(belt - open) < 1e-6, `${belt} / ${open}`);
+    check(`${mode} ${channel}: sand on natural stone still shades below`, stone < open * 0.95, `${stone} / ${open}`);
+  }
+  run('setCell(76 * W + 80, WATER, AMB[76], 0); updateRoof(); computeLight();');
+  const a = run('lightAt(R.latt, 80, 76)');
+  check(`${mode}: a wet belt load transmits like air`, Math.abs(a - Math.pow(0.978, mode === 'fast' ? 4 : 2)) < 1e-6, a);
+  run('setCell(76 * W + 80, EMPTY, AMB[76], 0); updateRoof();');
+}
+for (const [name, support, depth, gap, expected] of [
+  ['left belt', 'mat[i] = BELTL', 12, '', true],
+  ['right belt', 'mat[i] = BELTR', 12, '', true],
+  ['filter', 'part[i] = P_FILTER', 12, '', true],
+  ['deep heap', 'mat[i] = BELTR', 13, '', false],
+  ['air gap', 'mat[i] = BELTR', 3, 'EMPTY', false],
+  ['gas gap', 'mat[i] = BELTR', 3, 'STEAM', false],
+  ['natural ledge', 'mat[i] = STONE; flags[i] = F_NAT', 3, '', false],
+  ['wall floor', 'mat[i] = STONE', 3, '', false],
+  ['tube', 'part[i] = P_TR', 3, '', false],
+]) {
+  run(`
+    for (let y = 40; y <= 53; y++) { const i = y * W + 300; mat[i] = SAND; part[i] = 0; flags[i] = 0; }
+    { const i = (40 + ${depth}) * W + 300; mat[i] = EMPTY; mat[i + W] = BELTR; ${support}; }
+    ${gap ? `mat[41 * W + 300] = ${gap};` : ''}
+  `);
+  check(`${name}: belt load detection`, run('onBeltForLight(40 * W + 300)') === expected);
+}
+run('mat[(H - 1) * W + 300] = SAND;');
+check('belt load detection stops at the world bottom', run('!onBeltForLight((H - 1) * W + 300)'));
 done();