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js/stlLoader.js

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+import { STLLoader } from 'three/addons/loaders/STLLoader.js';
+import * as THREE from 'three';
+
+const loader = new STLLoader();
+
+/**
+ * Load an STL from a File object.
+ * Returns { geometry, bounds } where bounds = { min, max, center, size } (THREE.Vector3).
+ * The geometry is translated so its bounding-box centre is at the world origin.
+ */
+export function loadSTLFile(file) {
+  return new Promise((resolve, reject) => {
+    const reader = new FileReader();
+    reader.onload = (e) => {
+      try {
+        const geometry = loader.parse(e.target.result);
+        setupGeometry(geometry);
+        const bounds = computeBounds(geometry);
+        resolve({ geometry, bounds });
+      } catch (err) {
+        reject(err);
+      }
+    };
+    reader.onerror = () => reject(new Error('Could not read file'));
+    reader.readAsArrayBuffer(file);
+  });
+}
+
+/**
+ * Ensure vertex normals exist, then centre the geometry on its bounding-box centroid.
+ */
+function setupGeometry(geometry) {
+  geometry.computeBoundingBox();
+  const box = geometry.boundingBox;
+  const centre = new THREE.Vector3();
+  box.getCenter(centre);
+  geometry.translate(-centre.x, -centre.y, -centre.z);
+  // Convert Z-up (3D-print convention) to Y-up (Three.js convention)
+  geometry.rotateX(-Math.PI / 2);
+  geometry.computeBoundingBox();
+  if (!geometry.attributes.normal) geometry.computeVertexNormals();
+}
+
+/**
+ * Compute the bounds object that all UV mapping functions depend on.
+ * Must be called after the geometry has been centred.
+ */
+export function computeBounds(geometry) {
+  geometry.computeBoundingBox();
+  const box = geometry.boundingBox;
+  const min  = box.min.clone();
+  const max  = box.max.clone();
+  const size = new THREE.Vector3();
+  box.getSize(size);
+  const center = new THREE.Vector3();
+  box.getCenter(center);
+  return { min, max, center, size };
+}
+
+/**
+ * Triangle count helper.
+ */
+export function getTriangleCount(geometry) {
+  const pos = geometry.attributes.position;
+  return geometry.index
+    ? geometry.index.count / 3
+    : pos.count / 3;
+}