feature: mat3FromQuaternion

src/math/mat3/index.ts

@@ -1,5 +1,6 @@
 export { mat3FromMat4 } from './mat3FromMat4';
 export { mat3FromMat4Transpose } from './mat3FromMat4Transpose';
+export { mat3FromQuaternion } from './mat3FromQuaternion';
 export { mat3Inverse } from './mat3Inverse';
 export { mat3Multiply } from './mat3Multiply';
 export { mat3Transpose } from './mat3Transpose';

src/math/mat3/mat3FromQuaternion.ts

@@ -0,0 +1,22 @@
+import type { RawMatrix3 } from './RawMatrix3';
+import type { RawQuaternion } from '../quat/RawQuaternion';
+
+/**
+ * Convert a quaternion into a matrix3.
+ *
+ * Yoinked from Three.js.
+ *
+ * See: https://threejs.org/docs/#api/en/math/Matrix4.makeRotationFromQuaternion
+ */
+export function mat3FromQuaternion( quat: RawQuaternion ): RawMatrix3 {
+  const x = quat[ 0 ];
+  const y = quat[ 1 ];
+  const z = quat[ 2 ];
+  const w = quat[ 3 ];
+
+  return [
+    1.0 - 2.0 * y * y - 2.0 * z * z, 2.0 * x * y + 2.0 * z * w, 2.0 * x * z - 2.0 * y * w,
+    2.0 * x * y - 2.0 * z * w, 1.0 - 2.0 * x * x - 2.0 * z * z, 2.0 * y * z + 2.0 * x * w,
+    2.0 * x * z + 2.0 * y * w, 2.0 * y * z - 2.0 * x * w, 1.0 - 2.0 * x * x - 2.0 * y * y,
+  ];
+}

src/math/mat3/tests/mat3FromQuaternion.test.ts

@@ -0,0 +1,16 @@
+import '../../../tests/matchers/toBeCloseToArray';
+import { mat3FromQuaternion } from '../mat3FromQuaternion';
+import type { RawQuaternion } from '../../quat/RawQuaternion';
+
+describe( 'mat3FromQuaternion', () => {
+  it( 'returns a matrix3 made out of a quaternion', () => {
+    const quat: RawQuaternion = [ 0.189, 0.378, 0.567, 0.707 ];
+    const subject = mat3FromQuaternion( quat );
+
+    expect( subject ).toBeCloseToArray( [
+      0.071, 0.945, -0.320,
+      -0.659, 0.286, 0.696,
+      0.749, 0.161, 0.643,
+    ] );
+  } );
+} );