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js/exporter.js

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+import * as THREE from 'three';
+import { STLExporter } from 'three/addons/exporters/STLExporter.js';
+
+const exporter = new STLExporter();
+
+/**
+ * Export a BufferGeometry as a binary STL file download.
+ *
+ * @param {THREE.BufferGeometry} geometry
+ * @param {string} [filename]
+ */
+export function exportSTL(geometry, filename = 'textured.stl') {
+  // The geometry was rotated -90° around X on load to convert Z-up → Y-up for the viewer.
+  // Undo that rotation before export so the STL lands back in the original Z-up orientation
+  // that 3D-print slicers expect.
+  const exportGeom = geometry.clone();
+  exportGeom.applyMatrix4(new THREE.Matrix4().makeRotationX(Math.PI / 2));
+
+  const mesh = new THREE.Mesh(exportGeom, new THREE.MeshBasicMaterial());
+  const result = exporter.parse(mesh, { binary: true });
+  exportGeom.dispose();
+
+  // result is an ArrayBuffer in binary mode
+  const blob = new Blob([result], { type: 'application/octet-stream' });
+  const url  = URL.createObjectURL(blob);
+
+  const a = document.createElement('a');
+  a.href     = url;
+  a.download = filename;
+  a.style.display = 'none';
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+
+  // Revoke after a short delay so the download has time to start
+  setTimeout(() => URL.revokeObjectURL(url), 10000);
+}