setFromTwoVectors method

Implementation

void setFromTwoVectors(Vector3 a, Vector3 b) {
  final Vector3 v1 = a.normalized();
  final Vector3 v2 = b.normalized();

  final double c = v1.dot(v2);
  double angle = math.acos(c);
  Vector3 axis = v1.cross(v2);

  if ((1.0 + c).abs() < 0.0005) {
    // c \approx -1 indicates 180 degree rotation
    angle = math.pi;

    // a and b are parallel in opposite directions. We need any
    // vector as our rotation axis that is perpendicular.
    // Find one by taking the cross product of v1 with an appropriate unit axis
    if (v1.x > v1.y && v1.x > v1.z) {
      // v1 points in a dominantly x direction, so don't cross with that axis
      axis = v1.cross(new Vector3(0.0, 1.0, 0.0));
    } else {
      // Predominantly points in some other direction, so x-axis should be safe
      axis = v1.cross(new Vector3(1.0, 0.0, 0.0));
    }
  } else if ((1.0 - c).abs() < 0.0005) {
    // c \approx 1 is 0-degree rotation, axis is arbitrary
    angle = 0.0;
    axis = new Vector3(1.0, 0.0, 0.0);
  }

  setAxisAngle(axis.normalized(), angle);
}