From 1a23d7d7fa71dbbc18adfa327ff976a2938d8815 Mon Sep 17 00:00:00 2001
From: CNCKitchen <stefan.elitz@gmail.com>
Date: Tue, 17 Mar 2026 10:16:50 +0100
Subject: [PATCH] added new triangle decimation

---
 index.html        |   6 +-
 js/decimation.js  | 434 ++++++++++++++++++++++++++++++++++++++++++++++
 js/main.js        |  36 ++--
 js/subdivision.js |  37 ++--
 4 files changed, 480 insertions(+), 33 deletions(-)
 create mode 100644 js/decimation.js

diff --git a/index.html b/index.html
index 04fe463..4800c88 100644
--- a/index.html
+++ b/index.html
@@ -141,15 +141,15 @@
           <span class="val" id="refine-length-val">1.0 mm</span>
         </div>
         <div class="form-row slider-row">
-          <label for="max-triangles" title="Subdivision stops when this triangle count is reached">Max Triangles</label>
+          <label for="max-triangles" title="Mesh is fully subdivided first, then decimated down to this count">Output Triangles</label>
           <input type="range" id="max-triangles" min="100000" max="5000000" step="100000" value="1000000" />
           <span class="val" id="max-triangles-val">1.0 M</span>
         </div>
         <div id="tri-limit-warning" class="tri-limit-warning hidden">
-          ⚠ Triangle limit reached — some edges were not subdivided further.
+          ⚠ 5M-triangle safety cap hit during subdivision — result may still be coarser than requested edge length.
         </div>
         <p class="hint">
-          Smaller edge length = finer geometry. Does not affect preview.
+          Smaller edge length = finer displacement detail. Output is then decimated to the triangle limit.
         </p>
         <div id="export-progress" class="export-progress hidden">
           <div id="export-progress-bar" class="export-progress-bar"></div>
diff --git a/js/decimation.js b/js/decimation.js
new file mode 100644
index 0000000..391f4ca
--- /dev/null
+++ b/js/decimation.js
@@ -0,0 +1,434 @@
+/**
+ * QEM (Quadric Error Metric) mesh decimation.
+ *
+ * Algorithm: Garland & Heckbert 1997, with the three safety guards from
+ * PrusaSlicer's QuadricEdgeCollapse.cpp that eliminate holes, spikes and
+ * non-manifold edges:
+ *
+ *   Guard 1 – Boundary edge protection
+ *     Never collapse an edge shared by < 2 active faces.
+ *     The primary cause of holes in open STL meshes.
+ *
+ *   Guard 2 – Link-condition (non-manifold / pinch prevention)
+ *     Common neighbours of v1/v2 must equal exactly the apex vertices of
+ *     their shared triangles.  Extra common neighbours mean collapsing would
+ *     fuse disconnected surface regions → non-manifold edge.
+ *
+ *   Guard 3 – Normal-flip rejection
+ *     Recompute every affected face normal after the hypothetical collapse.
+ *     dot(original, new) < 0.2 (~78°) → reject.  Eliminates spikes / pits.
+ *
+ * @param {THREE.BufferGeometry} geometry        non-indexed input
+ * @param {number}               targetTriangles desired output face count
+ * @param {function}             [onProgress]    callback(0–1)
+ * @returns {THREE.BufferGeometry}
+ */
+
+import * as THREE from 'three';
+
+const QUANT    = 1e4;
+const FLIP_DOT = 0.2; // cos ~78° — reject collapse if new normal deviates more
+
+// ── Public API ───────────────────────────────────────────────────────────────
+
+export function decimate(geometry, targetTriangles, onProgress) {
+  const { positions, faces, vertCount, faceCount } = buildIndexed(geometry);
+
+  if (faceCount <= targetTriangles) return buildOutput(positions, faces, faceCount);
+
+  // Per-vertex error quadrics (10 doubles = upper triangle of symmetric 4×4)
+  const quadrics = new Float64Array(vertCount * 10);
+  initQuadrics(quadrics, positions, faces, faceCount);
+
+  // Vertex → set of incident face indices
+  const vertFaces  = buildAdjacency(faces, faceCount, vertCount);
+  const active     = new Uint8Array(vertCount).fill(1);
+  let   activeFaces = faceCount;
+
+  // Seed min-heap with one entry per unique edge
+  const heap     = new MinHeap();
+  const seedSeen = new Set();
+  for (let f = 0; f < faceCount; f++) {
+    if (faces[f * 3] < 0) continue;
+    for (let e = 0; e < 3; e++) {
+      const v1 = faces[f * 3 + e];
+      const v2 = faces[f * 3 + ((e + 1) % 3)];
+      const ek = v1 < v2 ? `${v1}:${v2}` : `${v2}:${v1}`;
+      if (!seedSeen.has(ek)) { seedSeen.add(ek); pushEdge(heap, quadrics, positions, v1, v2); }
+    }
+  }
+  seedSeen.clear();
+
+  const initFaces = activeFaces;
+  let   lastProg  = 0;
+
+  while (activeFaces > targetTriangles && heap.size() > 0) {
+    const { v1, v2, px, py, pz } = heap.pop();
+
+    // Stale-entry checks (lazy deletion)
+    if (!active[v1] || !active[v2]) continue;
+    if (!shareActiveFace(faces, vertFaces, v1, v2)) continue;
+
+    // ── Three safety guards ───────────────────────────────────────────────────
+    if (isBoundaryEdge(faces, vertFaces, v1, v2))   continue;  // Guard 1
+    if (hasLinkViolation(faces, vertFaces, v1, v2)) continue;  // Guard 2
+    const np = [px, py, pz];
+    if (checkFlipped(positions, vertFaces, faces, v1, v2, np)) continue; // Guard 3 v1-side
+    if (checkFlipped(positions, vertFaces, faces, v2, v1, np)) continue; // Guard 3 v2-side
+
+    // ── Collapse: keep v1 at new position, remove v2 ─────────────────────────
+    positions[v1 * 3]     = px;
+    positions[v1 * 3 + 1] = py;
+    positions[v1 * 3 + 2] = pz;
+    mergeQuadric(quadrics, v1, v2);
+
+    for (const f of vertFaces[v2]) {
+      if (faces[f * 3] < 0) continue;
+      for (let k = 0; k < 3; k++) {
+        if (faces[f * 3 + k] === v2) faces[f * 3 + k] = v1;
+      }
+      const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
+      if (fa === fb || fb === fc || fa === fc) {
+        vertFaces[fa]?.delete(f);
+        vertFaces[fb]?.delete(f);
+        vertFaces[fc]?.delete(f);
+        faces[f * 3] = faces[f * 3 + 1] = faces[f * 3 + 2] = -1;
+        activeFaces--;
+      } else {
+        vertFaces[v1].add(f);
+      }
+    }
+    vertFaces[v2].clear();
+    active[v2] = 0;
+
+    // Re-push edges for v1's updated neighbourhood
+    const neighbors = new Set();
+    for (const f of vertFaces[v1]) {
+      if (faces[f * 3] < 0) continue;
+      for (let k = 0; k < 3; k++) {
+        const nb = faces[f * 3 + k];
+        if (nb !== v1) neighbors.add(nb);
+      }
+    }
+    for (const nb of neighbors) {
+      if (active[nb]) pushEdge(heap, quadrics, positions, v1, nb);
+    }
+
+    if (onProgress) {
+      const p = Math.min(1, (initFaces - activeFaces) / (initFaces - targetTriangles));
+      if (p - lastProg > 0.02) { onProgress(p); lastProg = p; }
+    }
+  }
+
+  if (onProgress) onProgress(1);
+  return buildOutput(positions, faces, faceCount);
+}
+
+// ── Guard 1: Boundary edge protection ───────────────────────────────────────
+// An edge is a boundary if fewer than 2 active faces share it.
+// Collapsing boundary edges is the primary cause of holes in open meshes.
+
+function isBoundaryEdge(faces, vertFaces, v1, v2) {
+  let shared = 0;
+  for (const f of vertFaces[v1]) {
+    if (faces[f * 3] < 0) continue;
+    const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
+    if (fa === v2 || fb === v2 || fc === v2) { shared++; if (shared >= 2) return false; }
+  }
+  return shared < 2;
+}
+
+// ── Guard 2: Link condition (non-manifold / pinch prevention) ────────────────
+// The set of common neighbours of v1 and v2 must equal exactly the apex
+// vertices of their shared faces.  Extra common neighbours mean collapsing
+// would fuse disconnected regions → non-manifold edge.
+
+function hasLinkViolation(faces, vertFaces, v1, v2) {
+  // Collect apex vertices of the shared faces
+  const apices = new Set();
+  for (const f of vertFaces[v1]) {
+    if (faces[f * 3] < 0) continue;
+    const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
+    if (fa === v2 || fb === v2 || fc === v2) {
+      if (fa !== v1 && fa !== v2) apices.add(fa);
+      if (fb !== v1 && fb !== v2) apices.add(fb);
+      if (fc !== v1 && fc !== v2) apices.add(fc);
+    }
+  }
+
+  // Build neighbour sets for v1 and v2 from active faces
+  const nb1 = new Set(), nb2 = new Set();
+  for (const f of vertFaces[v1]) {
+    if (faces[f * 3] < 0) continue;
+    for (let k = 0; k < 3; k++) { const nb = faces[f * 3 + k]; if (nb !== v1) nb1.add(nb); }
+  }
+  for (const f of vertFaces[v2]) {
+    if (faces[f * 3] < 0) continue;
+    for (let k = 0; k < 3; k++) { const nb = faces[f * 3 + k]; if (nb !== v2) nb2.add(nb); }
+  }
+
+  // Check common neighbours match apices exactly
+  for (const nb of nb1) {
+    if (nb !== v2 && nb2.has(nb) && !apices.has(nb)) return true;
+  }
+  return false;
+}
+
+// ── Guard 3: Normal-flip rejection ──────────────────────────────────────────
+// After hypothetical collapse of v_collapse → newPos, recompute normals of
+// all affected faces.  If any flip by more than ~78° (dot < FLIP_DOT) reject.
+
+function checkFlipped(positions, vertFaces, faces, v_collapse, v_other, newPos) {
+  for (const f of vertFaces[v_collapse]) {
+    if (faces[f * 3] < 0) continue;
+    const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
+    // Skip faces shared with v_other; they will be deleted on collapse
+    if (fa === v_other || fb === v_other || fc === v_other) continue;
+
+    // Original normal
+    const [onx, ony, onz] = faceNormal(
+      positions[fa*3], positions[fa*3+1], positions[fa*3+2],
+      positions[fb*3], positions[fb*3+1], positions[fb*3+2],
+      positions[fc*3], positions[fc*3+1], positions[fc*3+2]
+    );
+
+    // New normal with v_collapse replaced by newPos
+    const ax = fa === v_collapse ? newPos[0] : positions[fa*3];
+    const ay = fa === v_collapse ? newPos[1] : positions[fa*3+1];
+    const az = fa === v_collapse ? newPos[2] : positions[fa*3+2];
+    const bx = fb === v_collapse ? newPos[0] : positions[fb*3];
+    const by = fb === v_collapse ? newPos[1] : positions[fb*3+1];
+    const bz = fb === v_collapse ? newPos[2] : positions[fb*3+2];
+    const cx = fc === v_collapse ? newPos[0] : positions[fc*3];
+    const cy = fc === v_collapse ? newPos[1] : positions[fc*3+1];
+    const cz = fc === v_collapse ? newPos[2] : positions[fc*3+2];
+    const [nnx, nny, nnz] = faceNormal(ax, ay, az, bx, by, bz, cx, cy, cz);
+
+    const dot = onx * nnx + ony * nny + onz * nnz;
+    if (dot < FLIP_DOT) return true;
+  }
+  return false;
+}
+
+function faceNormal(ax, ay, az, bx, by, bz, cx, cy, cz) {
+  const ux = bx - ax, uy = by - ay, uz = bz - az;
+  const vx = cx - ax, vy = cy - ay, vz = cz - az;
+  const nx = uy * vz - uz * vy;
+  const ny = uz * vx - ux * vz;
+  const nz = ux * vy - uy * vx;
+  const len = Math.sqrt(nx * nx + ny * ny + nz * nz) || 1;
+  return [nx / len, ny / len, nz / len];
+}
+
+// ── Quadric helpers ──────────────────────────────────────────────────────────
+// Symmetric 4×4 quadric stored as 10 upper-triangle values per vertex.
+
+function initQuadrics(quadrics, positions, faces, faceCount) {
+  for (let f = 0; f < faceCount; f++) {
+    if (faces[f * 3] < 0) continue;
+    const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
+    const [nx, ny, nz] = faceNormal(
+      positions[fa*3], positions[fa*3+1], positions[fa*3+2],
+      positions[fb*3], positions[fb*3+1], positions[fb*3+2],
+      positions[fc*3], positions[fc*3+1], positions[fc*3+2]
+    );
+    const d = -(nx * positions[fa*3] + ny * positions[fa*3+1] + nz * positions[fa*3+2]);
+    addPlaneQ(quadrics, fa, nx, ny, nz, d);
+    addPlaneQ(quadrics, fb, nx, ny, nz, d);
+    addPlaneQ(quadrics, fc, nx, ny, nz, d);
+  }
+}
+
+function addPlaneQ(q, v, a, b, c, d) {
+  const o = v * 10;
+  q[o]   += a*a; q[o+1] += a*b; q[o+2] += a*c; q[o+3] += a*d;
+                 q[o+4] += b*b; q[o+5] += b*c; q[o+6] += b*d;
+                                q[o+7] += c*c; q[o+8] += c*d;
+                                               q[o+9] += d*d;
+}
+
+function mergeQuadric(q, v1, v2) {
+  const o1 = v1 * 10, o2 = v2 * 10;
+  for (let i = 0; i < 10; i++) q[o1 + i] += q[o2 + i];
+}
+
+function evalQ(q, v, x, y, z) {
+  const o = v * 10;
+  return q[o]   * x*x + 2*q[o+1]*x*y + 2*q[o+2]*x*z + 2*q[o+3]*x
+       + q[o+4] * y*y + 2*q[o+5]*y*z + 2*q[o+6]*y
+       + q[o+7] * z*z + 2*q[o+8]*z
+       + q[o+9];
+}
+
+function evalQSum(q, v1, v2, x, y, z) {
+  return evalQ(q, v1, x, y, z) + evalQ(q, v2, x, y, z);
+}
+
+const _s = new Float64Array(3);
+
+function solveQ(q, v1, v2) {
+  const o1 = v1 * 10, o2 = v2 * 10;
+  const a00 = q[o1]   + q[o2];
+  const a01 = q[o1+1] + q[o2+1];
+  const a02 = q[o1+2] + q[o2+2];
+  const a11 = q[o1+4] + q[o2+4];
+  const a12 = q[o1+5] + q[o2+5];
+  const a22 = q[o1+7] + q[o2+7];
+  const b0  = -(q[o1+3] + q[o2+3]);
+  const b1  = -(q[o1+6] + q[o2+6]);
+  const b2  = -(q[o1+8] + q[o2+8]);
+
+  const det = a00*(a11*a22 - a12*a12) - a01*(a01*a22 - a12*a02) + a02*(a01*a12 - a11*a02);
+  if (Math.abs(det) < 1e-10) return false;
+
+  const inv = 1 / det;
+  _s[0] = inv * (b0*(a11*a22 - a12*a12) - a01*(b1*a22 - a12*b2) + a02*(b1*a12 - a11*b2));
+  _s[1] = inv * (a00*(b1*a22 - a12*b2) - b0*(a01*a22 - a12*a02) + a02*(a01*b2 - b1*a02));
+  _s[2] = inv * (a00*(a11*b2 - b1*a12) - a01*(a01*b2 - b1*a02) + b0*(a01*a12 - a11*a02));
+  return true;
+}
+
+function pushEdge(heap, quadrics, positions, v1, v2) {
+  let px, py, pz;
+
+  if (solveQ(quadrics, v1, v2)) {
+    px = _s[0]; py = _s[1]; pz = _s[2];
+  } else {
+    const mx = (positions[v1*3]   + positions[v2*3])   / 2;
+    const my = (positions[v1*3+1] + positions[v2*3+1]) / 2;
+    const mz = (positions[v1*3+2] + positions[v2*3+2]) / 2;
+    const e1 = evalQSum(quadrics, v1, v2, positions[v1*3],   positions[v1*3+1], positions[v1*3+2]);
+    const e2 = evalQSum(quadrics, v1, v2, positions[v2*3],   positions[v2*3+1], positions[v2*3+2]);
+    const em = evalQSum(quadrics, v1, v2, mx, my, mz);
+    if      (e1 <= e2 && e1 <= em) { px = positions[v1*3]; py = positions[v1*3+1]; pz = positions[v1*3+2]; }
+    else if (e2 <= em)             { px = positions[v2*3]; py = positions[v2*3+1]; pz = positions[v2*3+2]; }
+    else                           { px = mx; py = my; pz = mz; }
+  }
+
+  const cost = evalQSum(quadrics, v1, v2, px, py, pz);
+  heap.push({ cost, v1, v2, px, py, pz });
+}
+
+// ── Indexed <-> Non-indexed conversion ──────────────────────────────────────
+
+function buildIndexed(geometry) {
+  const posAttr = geometry.attributes.position;
+  const n = posAttr.count;
+
+  const positions  = [];
+  const vertMap    = new Map();
+  const indexRemap = new Int32Array(n);
+
+  for (let i = 0; i < n; i++) {
+    const x = posAttr.getX(i), y = posAttr.getY(i), z = posAttr.getZ(i);
+    const key = `${Math.round(x * QUANT)}_${Math.round(y * QUANT)}_${Math.round(z * QUANT)}`;
+    let idx = vertMap.get(key);
+    if (idx === undefined) {
+      idx = positions.length / 3;
+      positions.push(x, y, z);
+      vertMap.set(key, idx);
+    }
+    indexRemap[i] = idx;
+  }
+
+  const faceCount = n / 3;
+  const faces = new Int32Array(faceCount * 3);
+  for (let f = 0; f < faceCount; f++) {
+    faces[f * 3]     = indexRemap[f * 3];
+    faces[f * 3 + 1] = indexRemap[f * 3 + 1];
+    faces[f * 3 + 2] = indexRemap[f * 3 + 2];
+  }
+
+  return { positions: new Float64Array(positions), faces, vertCount: positions.length / 3, faceCount };
+}
+
+// ── Adjacency helpers ────────────────────────────────────────────────────────
+
+function buildAdjacency(faces, faceCount, vertCount) {
+  const adj = new Array(vertCount);
+  for (let v = 0; v < vertCount; v++) adj[v] = new Set();
+  for (let f = 0; f < faceCount; f++) {
+    if (faces[f * 3] < 0) continue;
+    adj[faces[f * 3]].add(f);
+    adj[faces[f * 3 + 1]].add(f);
+    adj[faces[f * 3 + 2]].add(f);
+  }
+  return adj;
+}
+
+function shareActiveFace(faces, vertFaces, v1, v2) {
+  for (const f of vertFaces[v1]) {
+    if (faces[f * 3] < 0) continue;
+    const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
+    if (fa === v2 || fb === v2 || fc === v2) return true;
+  }
+  return false;
+}
+
+function buildOutput(positions, faces, faceCount) {
+  let activeFaces = 0;
+  for (let f = 0; f < faceCount; f++) {
+    if (faces[f * 3] >= 0) activeFaces++;
+  }
+
+  const posArray = new Float32Array(activeFaces * 9);
+  let out = 0;
+  for (let f = 0; f < faceCount; f++) {
+    if (faces[f * 3] < 0) continue;
+    for (let v = 0; v < 3; v++) {
+      const vi = faces[f * 3 + v];
+      posArray[out++] = positions[vi * 3];
+      posArray[out++] = positions[vi * 3 + 1];
+      posArray[out++] = positions[vi * 3 + 2];
+    }
+  }
+
+  const geo = new THREE.BufferGeometry();
+  geo.setAttribute('position', new THREE.BufferAttribute(posArray, 3));
+  geo.computeVertexNormals();
+  return geo;
+}
+
+// ── Binary Min-Heap ──────────────────────────────────────────────────────────
+
+class MinHeap {
+  constructor() { this._data = []; }
+
+  size() { return this._data.length; }
+
+  push(item) {
+    this._data.push(item);
+    this._bubbleUp(this._data.length - 1);
+  }
+
+  pop() {
+    const top = this._data[0];
+    const last = this._data.pop();
+    if (this._data.length > 0) { this._data[0] = last; this._sinkDown(0); }
+    return top;
+  }
+
+  _bubbleUp(i) {
+    const d = this._data;
+    while (i > 0) {
+      const p = (i - 1) >> 1;
+      if (d[p].cost <= d[i].cost) break;
+      [d[p], d[i]] = [d[i], d[p]];
+      i = p;
+    }
+  }
+
+  _sinkDown(i) {
+    const d = this._data;
+    const n = d.length;
+    for (;;) {
+      let s = i;
+      const l = 2 * i + 1, r = 2 * i + 2;
+      if (l < n && d[l].cost < d[s].cost) s = l;
+      if (r < n && d[r].cost < d[s].cost) s = r;
+      if (s === i) break;
+      [d[s], d[i]] = [d[i], d[s]];
+      i = s;
+    }
+  }
+}
diff --git a/js/main.js b/js/main.js
index 69c5513..561742e 100644
--- a/js/main.js
+++ b/js/main.js
@@ -4,6 +4,7 @@ import { PRESETS, loadCustomTexture }  from './presetTextures.js';
 import { createPreviewMaterial, updateMaterial } from './previewMaterial.js';
 import { subdivide }          from './subdivision.js';
 import { applyDisplacement }  from './displacement.js';
+import { decimate }           from './decimation.js';
 import { exportSTL }          from './exporter.js';
 
 // ── State ─────────────────────────────────────────────────────────────────────
@@ -311,17 +312,13 @@ async function handleExport() {
   try {
     setProgress(0.02, 'Subdividing mesh…');
 
-    // Run subdivision synchronously (may take a few seconds on large meshes)
-    // Wrap in a small yielding loop to allow the browser to repaint the progress bar.
-    const { geometry: subdivided, limitReached } = await runAsync(() =>
-      subdivide(currentGeometry, settings.refineLength, settings.maxTriangles,
-                (p) => setProgress(p * 0.6, 'Subdividing mesh…'))
+    const { geometry: subdivided, safetyCapHit } = await runAsync(() =>
+      subdivide(currentGeometry, settings.refineLength,
+                (p) => setProgress(0.02 + p * 0.35, 'Subdividing mesh…'))
     );
 
-    triLimitWarning.classList.toggle('hidden', !limitReached);
-
     const subTriCount = subdivided.attributes.position.count / 3;
-    setProgress(0.62, `Applying displacement to ${subTriCount.toLocaleString()} triangles…`);
+    setProgress(0.38, `Applying displacement to ${subTriCount.toLocaleString()} triangles…`);
 
     const displaced = await runAsync(() =>
       applyDisplacement(
@@ -331,15 +328,30 @@ async function handleExport() {
         activeMapEntry.height,
         settings,
         currentBounds,
-        (p) => setProgress(0.62 + p * 0.35, `Displacing vertices…`)
+        (p) => setProgress(0.38 + p * 0.32, `Displacing vertices…`)
       )
     );
 
-    setProgress(0.98, 'Writing STL…');
+    const dispTriCount = displaced.attributes.position.count / 3;
+    const needsDecimation = dispTriCount > settings.maxTriangles;
+    triLimitWarning.classList.toggle('hidden', !safetyCapHit);
+
+    let finalGeometry = displaced;
+    if (needsDecimation) {
+      setProgress(0.71, `Decimating ${dispTriCount.toLocaleString()} → ${settings.maxTriangles.toLocaleString()} triangles…`);
+      finalGeometry = await runAsync(() =>
+        decimate(
+          displaced,
+          settings.maxTriangles,
+          (p) => setProgress(0.71 + p * 0.25, `Decimating mesh…`)
+        )
+      );
+    }
+
+    setProgress(0.97, 'Writing STL…');
     await yieldFrame();
 
-    const baseName = 'textured';
-    exportSTL(displaced, `${baseName}.stl`);
+    exportSTL(finalGeometry, 'textured.stl');
 
     setProgress(1.0, 'Done!');
     setTimeout(() => {
diff --git a/js/subdivision.js b/js/subdivision.js
index 199b784..f5fe435 100644
--- a/js/subdivision.js
+++ b/js/subdivision.js
@@ -1,62 +1,63 @@
 /**
  * Edge-based adaptive mesh subdivision.
  *
+ * Subdivides until every edge is ≤ maxEdgeLength.  A hard safety cap of
+ * SAFETY_CAP triangles prevents OOM on very fine settings; the caller
+ * (export pipeline) hands the result to the QEM decimator which then trims
+ * it to the user-requested budget.
+ *
  * @param {THREE.BufferGeometry} geometry   – non-indexed input from STLLoader
  * @param {number} maxEdgeLength            – maximum allowed edge length (same unit as STL)
- * @param {number} maxTriangles             – hard cap on output triangle count
  * @param {function} [onProgress]           – optional callback(fraction 0–1)
- * @returns {{ geometry: THREE.BufferGeometry, limitReached: boolean }}
+ * @returns {{ geometry: THREE.BufferGeometry, safetyCapHit: boolean }}
  */
 
 import * as THREE from 'three';
 
-const QUANTISE = 1e4;
+const QUANTISE   = 1e4;
+const SAFETY_CAP = 5_000_000; // absolute OOM guard
 
 // ── Public entry point ───────────────────────────────────────────────────────
 
-export function subdivide(geometry, maxEdgeLength, maxTriangles, onProgress) {
+export function subdivide(geometry, maxEdgeLength, onProgress) {
   const { positions, normals, indices } = toIndexed(geometry);
 
   const maxIterations = 12;
   let currentIndices = indices;
-  let limitReached = false;
+  let safetyCapHit = false;
 
   for (let iter = 0; iter < maxIterations; iter++) {
     const triCount = currentIndices.length / 3;
-    if (triCount >= maxTriangles) {
-      limitReached = true;
+    if (triCount >= SAFETY_CAP) {
+      safetyCapHit = true;
       break;
     }
 
     const { newIndices, changed } = subdividePass(
-      positions, normals, currentIndices, maxEdgeLength, maxTriangles
+      positions, normals, currentIndices, maxEdgeLength, SAFETY_CAP
     );
     currentIndices = newIndices;
 
-    // Check if the pass itself hit the limit
-    if (newIndices.length / 3 >= maxTriangles) {
-      limitReached = true;
-    }
+    if (newIndices.length / 3 >= SAFETY_CAP) safetyCapHit = true;
 
     if (onProgress) onProgress(Math.min(0.95, (iter + 1) / maxIterations));
-    if (!changed || limitReached) break;
+    if (!changed || safetyCapHit) break;
   }
 
-  return { geometry: toNonIndexed(positions, normals, currentIndices), limitReached };
+  return { geometry: toNonIndexed(positions, normals, currentIndices), safetyCapHit };
 }
 
 // ── One subdivision pass ──────────────────────────────────────────────────────
 
-function subdividePass(positions, normals, indices, maxEdgeLength, maxTriangles) {
+function subdividePass(positions, normals, indices, maxEdgeLength, safetyCap) {
   const maxSq = maxEdgeLength * maxEdgeLength;
   const midCache = new Map();
   const nextIndices = [];
   let changed = false;
 
   for (let t = 0; t < indices.length; t += 3) {
-    // Hard stop: don't add more triangles once the cap is reached
-    if (nextIndices.length / 3 >= maxTriangles) {
-      // Push remaining unsplit triangles as-is
+    // Safety cap: stop splitting, carry remaining triangles as-is
+    if (nextIndices.length / 3 >= safetyCap) {
       for (let r = t; r < indices.length; r++) nextIndices.push(indices[r]);
       break;
     }