rotate (CWrp::File8WVR)

    # File ../lib/roller/CWRP.rb, line 779
      def rotate(x, y, by)

        # Convert to polar coordinates.
        angle = Math::atan2(y, x)
        magnitude = Math.sqrt(y ** 2 + x ** 2)

        # Rotate (degrees are clockwise and radians are anti-clockwise).
        angle -= by * Math::PI / 180

        # Back to cartesian.
        [Math::cos(angle) * magnitude, Math::sin(angle) * magnitude]
      end