Potentially entering and leaving points - P_{E} and P_{L}
The intersection point of the line and window might be classified either like potentially leaving or entering. Before get further details, we describe the real meaning of potentially leaving and entering points (PL and PE). Potentially leaving and entering points are dependent upon the edge orientation, that is: direction. So now understand how to determine PE and PL; here we move anticlockwise across the window boundary then area of left hand side encloses the window.
Figure: (a): Potentially Entering PE and Potentially Leaving PL points
So here, we are moving from point P to point Q, and we understand that whereas traversing the windows edges in anticlockwise method the area lying on the left hand side is considered to as the window region. Consequently, from the Figure (a) in above, you may see that at point PE that we are moving from P to Q we are entering the window region, hence this point of intersection should be considered to as the potentially entering point (PE). Now, consider to the other intersection demonstrated in the Figure (a) in above, this time, also the movement of points on the line are found by varying value of parameter t is from point P to Q and with respect to, the orientation of window boundary, we are leaving the window area. Therefore, this point of intersection is identified as potentially exiting point (PL).
PE that is potentially entering point while moving indirection of the window, the point of intersection among line PQ and the window edges faced are identified as potentially entering. PL that is potentially leaving point; these are the point of intersection encountered whereas we move away from window.