comput slope radian line point param point param second point slope radian rang slope pi slope point point angl theta arctan opposit adjac increas go screen posit angl clockwis counterclockwis opposit adjac theta adjac shouldn happen line segment size check opposit go vertic goos gun opposit theta math pi theta math pi arctan return pi pi handl quadrant normal pi theta math atan opposit adjac quadrant ii iii adjac theta math pi quadrant iv theta theta math pi theta 