fix: stl export normalization

Author: BCsabaEngine

src/lib/3d/stl.ts

@@ -2,13 +2,29 @@ import { writeFloatLE, writeInt16LE, writeInt32LE } from '$lib/buffer';
 
 const SOLID_NAME = 'THT-holder';
 
+const computeFaceNormal = (v: Float32Array, i: number): [number, number, number] => {
+	const [x0, y0, z0, x1, y1, z1, x2, y2, z2] = [
+		v[i]!, v[i + 1]!, v[i + 2]!,
+		v[i + 3]!, v[i + 4]!, v[i + 5]!,
+		v[i + 6]!, v[i + 7]!, v[i + 8]!,
+	];
+	const ex = x1 - x0, ey = y1 - y0, ez = z1 - z0;
+	const fx = x2 - x0, fy = y2 - y0, fz = z2 - z0;
+	const nx = ey * fz - ez * fy;
+	const ny = ez * fx - ex * fz;
+	const nz = ex * fy - ey * fx;
+	const len = Math.sqrt(nx * nx + ny * ny + nz * nz);
+	return len > 0 ? [nx / len, ny / len, nz / len] : [0, 0, 0];
+};
+
 export const generateStlFromVertices = (vertices: Float32Array): string[] => {
 	const lines: string[] = [`solid ${SOLID_NAME}`];
 
 	if (vertices.length % 9 === 0) {
 		let index = 0;
 		while (index < vertices.length) {
-			lines.push('facet normal 0 0 0', '    outer loop');
+			const [nx, ny, nz] = computeFaceNormal(vertices, index);
+			lines.push(`facet normal ${nx} ${ny} ${nz}`, '    outer loop');
 			for (let pointIndex = 0; pointIndex < 3; pointIndex++)
 				lines.push(`        vertex ${vertices[index++]} ${vertices[index++]} ${vertices[index++]}`);
 			lines.push('    endloop', 'endfacet');
@@ -28,7 +44,10 @@ export const generateBinaryStlFromVertices = (vertices: Float32Array): Uint8Arra
 	if (vertices.length % 9 === 0) {
 		let index = 0;
 		while (index < vertices.length) {
-			for (let pointIndex = 0; pointIndex < 3; pointIndex++) pos = writeFloatLE(buffer, 0, pos);
+			const [nx, ny, nz] = computeFaceNormal(vertices, index);
+			pos = writeFloatLE(buffer, nx, pos);
+			pos = writeFloatLE(buffer, ny, pos);
+			pos = writeFloatLE(buffer, nz, pos);
 			for (let pointIndex = 0; pointIndex < 9; pointIndex++)
 				pos = writeFloatLE(buffer, vertices[index++]!, pos);
 			pos = writeInt16LE(buffer, 0, pos);