question: public static double angle ( Tuple3d a , Tuple3d b ) { double dx = b . x - a . x ; double dy = b . y - a . y ; double angle = 0.0d ; if ( dx == 0.0 ) { if ( dy == 0.0 ) angle = 0.0 ; else if ( dy > 0.0 ) angle = Math . PI / 2.0 ; else angle = ( Math . PI * 3.0 ) / 2.0 ; } else if ( dy == 0.0 ) { if ( dx > 0.0 ) angle = 0.0 ; else angle = Math . PI ; } else { if ( dx < 0.0 ) angle = Math . atan ( dy / dx ) + Math . PI ; else if ( dy < 0.0 ) angle = Math . atan ( dy / dx ) + ( 2 * Math . PI ) ; else angle = Math . atan ( dy / dx ) ; } return angle ; }
target:  Compute the angle between the first and second point.