Compute flat normals in glTF-Transform

flat-normals.ts

import { vec3 } from 'gl-matrix';
import { unweld } from '@gltf-transform/functions';

await document.transform(unweld());

for (const mesh of document.getRoot().listMeshes()) {
	for (const prim of mesh.listPrimitives()) {
		const position = prim.getAttribute('POSITION');
		const normal = document.createAccessor()
		.setArray(new Float32Array(position.getCount() * 3))
		.setType('VEC3');
		
		const a = [0, 0, 0] as vec3; 
		const b = [0, 0, 0] as vec3; 
		const c = [0, 0, 0] as vec3;
		
		for (let i = 0; i < position.getCount(); i += 3) {
			position.getElement(i + 0, a);
			position.getElement(i + 1, b);
			position.getElement(i + 2, c);
			const triangleNormal = computeNormal(a, b, c);
			normal.setElement(i + 0, triangleNormal);
			normal.setElement(i + 1, triangleNormal);
			normal.setElement(i + 2, triangleNormal);
		}
		
		prim.setAttribute('NORMAL', normal);
	}
}

// https://stackoverflow.com/a/23709352/1314762
function computeNormal(a: vec3, b: vec3, c: vec3): number[] {
	const A = [b[0] - a[0], b[1] - a[1], b[2] - a[2]];
	const B = [c[0] - a[0], c[1] - a[1], c[2] - a[2]];
	const n = [
		A[1] * B[2] - A[2] * B[1],
		A[2] * B[0] - A[0] * B[2],
		A[0] * B[1] - A[1] * B[0],
	] as vec3;
	return vec3.normalize([0, 0, 0], n) as number[];
}