This document demonstrates the Cohen-Sutherland line clipping algorithm on a line segment that intersects the right and top boundaries of a clipping window. It shows the coding of the clipping codes at each endpoint and the adjustments made to the coordinates to clip the line segment within the window. The algorithm iterates, adjusting coordinates and flipping the codes until it determines the line segment is fully inside the window and can be drawn.

1. 1
Cohen Sutherland Line Clipping Algorithm Example
Author: Kasun Ranga Wijeweera
Email: krw19870829@gmail.com
Date: 20170821
Note
The implementation of the Cohen Sutherland Algorithm in C programming language is available
in following link.
https://www.academia.edu/2492014/Cohen_Sutherland_Line_Clipping_Algorithm_in_C
Problem
Demonstrate how Cohen Sutherland algorithm works for the line segment shown in the
following figure. Note that it intersects only with RIGHT and TOP boundaries.
Answer
done = 0, draw = 0;
done == 0  TRUE;
code1
code = 0000;
x < minx  FALSE;
x > maxx  FALSE;
y < miny  FALSE;
(x[0], y[0])
(x[1], y[1])
0
1

2. 2
y > maxy  TRUE;
code = code | TOP_EDGE = 0000 | 1000 = 1000;
code1 = 1000;
code2
code = 0000;
x < minx  FALSE;
x > maxx  TRUE;
code = code | RIGHT_EDGE = 0000 | 0010 = 0010;
y < miny  FALSE;
y > maxy  FALSE;
code2 = 0010;
ACCEPT (code1, code2) = ! (1000 | 0010) = ! 1010  FALSE;
REJECT (code1, code2) = 1000 & 0010 = 0000  FALSE;
INSIDE (code1) = ! 1000  FALSE;
x[0] != x[1]  TRUE;
m = (y[1] – y[0])/(x[1] – x[0]);
code1 & LEFT_EDGE = 1000 & 0001 = 0000  FALSE;
code1 & RIGHT_EDGE = 1000 & 0010 = 0000  FALSE;
code1 & BOTTOM_EDGE = 1000 & 0100 = 0000  FALSE;
code1 & TOP_EDGE = 1000 & 1000 = 1000  TRUE;
x[0] != x[1]  TRUE;
x[0] += (maxy – y[0])/m;
y[0] = maxy;
done == 0  TRUE;
code1
code = 0000;
x < minx  FALSE;
x > maxx  FALSE;
y < miny  FALSE;
y > maxy  FALSE;
code1 = 0000;
0
1

3. 3
code2
code = 0000;
x < minx  FALSE;
x > maxx  TRUE;
code = code | RIGHT_EDGE = 0000 | 0010 = 0010
y < miny  FALSE;
y > maxy  FALSE;
code2 = 0010;
ACCEPT (code1, code2) = ! (0000 | 0010) = ! 0010  FALSE;
REJECT (code1, code2) = 0000 & 0010 = 0000  FALSE;
INSIDE (code1) = ! 0000  TRUE;
swapPts;
swapCodes
code1 = 0010;
code2 = 0000;
x[0] != x[1]  TRUE;
m = (y[1] – y[0])/(x[1] – x[0]);
code1 & LEFT_EDGE = 0010 & 0001 = 0000  FALSE;
code1 & RIGHT_EDGE = 0010 & 0010 = 0010  TRUE;
y[0] += (maxx – x[0]) * m;
x[0] = maxx;
1
0
1
0

4. 4
done == 0  TRUE;
code1
code = 0000;
x < minx  FALSE;
x > maxx  FALSE;
y < miny  FALSE;
y > maxy  FALSE;
code1 = 0000;
code2
code = 0000;
x < minx  FALSE;
x > maxx  FALSE;
y < miny  FALSE;
y > maxy  FALSE;
code2 = 0000;
ACCEPT (code1, code2) = ! (0000 | 0000) = ! 0000  TRUE;
done = 1, draw = 1;
done == 0  FALSE;
draw == 1  TRUE;
Line segment is drawn.