function crossProduct(a, b) {
    return [
      a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2],
      a[0] * b[1] - a[1] * b[0]
    ];
  }
Compute cross-product between two 3-element vectors.
@param {Float32Array} a
@param {Float32Array} b