Steps for clip a line segment-PQ
- Initially, find all the points of intersections of the line segment PQ along with the edges of the polygonal window and describe them either as PE and PL points. Also find out the value of parameter t, by using equation (2) for respective PE's and PL's.
If value of the normal to particular edges is not specified then, determine the value of normal to every edge of the window, then find out the value of parameter t, by using equation (2) for particular point of intersection among line segment and window edge then on the basis of the value of parameter t mark them like PE and PL given the value of t lies from 0 to 1 in magnitude.
- Secondly, out of the diverse values of t for PE's find out the maximum value of t say it is tmax. Likewise, out of the diverse values of t for PL's find out the minimum value of t say it be tmin . Remember here that for clipping to be possible tmin > tmax.
- at last, vary the parameter t from tmax to tmin and find out the clipped line as outcome.
For well understanding of steps here figure is shown below:
Figure: Steps of Cyrus Beck Clipping
During in case of Line 1: (PQ)
1) Point 1 is potentially entering (PE1) since as we move along PQ, the LHS of e1 is window and hence, it seems that we are entering the window.
2) Point 2 is again PE2 same as point 1.
3) Point 3 is potentially leaving (P4) ? as we move along PQ, the LHS of e2 is window and hence, it seems that we are leaving the window.
4) As the same, point 4 is also PL2.
Line 2 and 3 (PQ):
By using similar logic as for line 1 we determine p‾L and PE. Here, it is to be noted that for all points of intersection we have several value of t.
Say t1 is value of t for PE1
Say t2 is value of t for PE2
Say t3 is value of t for PL1
Say t4 is value of t for PL2