Function toGroupedBufferGeometryData

Convert a ShapeMesh into grouped BufferGeometry data with face material groups.

Each face becomes a separate group, allowing per-face materials in Three.js:

const data = toGroupedBufferGeometryData(mesh);
const geo = new THREE.BufferGeometry();
geo.setAttribute('position', new THREE.BufferAttribute(data.position, 3));
geo.setAttribute('normal', new THREE.BufferAttribute(data.normal, 3));
geo.setIndex(new THREE.BufferAttribute(data.index, 1));
for (const g of data.groups) {
  geo.addGroup(g.start, g.count, g.materialIndex);
}
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