static void Circle2DGI( float x, float y, out float distance, out float angle ) {
  distance = ( float )Math.Sqrt( x * x + y * y );
  y *= 1.0f / distance;
  if( x >= 0 ) {
    angle = ( float )Math.Acos( y ) * 180.0f / ( float )Math.PI;
  } else {
    angle = 180.0f + ( float )Math.Acos( -y ) * 180.0f / ( float )Math.PI;
  }
}