Clipping is the process of extracting a portion of a data base or identifying elements of a scene or picture inside or outside a specified clipping region. The Cohen-Sutherland line clipping algorithm uses 4-bit codes to indicate which of nine regions each endpoint of a line segment lies in relative to the clipping window boundaries. If both endpoints have a code of zero, the line is fully visible; if the codes are nonzero but their bitwise AND is zero, the line is partially visible; otherwise if the bitwise AND is nonzero, the line is fully invisible. The algorithm subdivides the line segment at the clipping window boundary to clip away the invisible portions.

1. Clipping

2. Clipping
• Clipping is the process of extracting a portion of a data base or identifying
the elements of a scene or picture inside or outside a specified region, called
the clipping region or clipping window.
• Two dimensional clipping window: defined by left (L), right (R), top (T)
and Bottom (B) edges.

3. A Simple Visibility Algorithm
(X,Y)
XL
YT
XR
YB

4. A Simple Visibility Algorithm

5. A Simple Visibility Algorithm

6. Dan Cohen and Ivan Sutherland
End Point Code Algorithm
• Uses 4 bit code to indicate which of the nine regions contains the end point of a line.
• Rightmost bit is the first bit
• Bits are set to 1, based on the following schemes:
• Otherwise , the bit is set to zero.

7. Dan Cohen and Ivan Sutherland
End Point Code Algorithm
• if both the endpoint codes are zero, then both ends of the line lie inside the window
and the line is visible.
• if the end point codes are non zero the line is totally invisible or partially visible.
• if the bit-by-bit logical AND of the two endpoint codes is not zero, then the line is totally
invisible.
• However, if the endpoint codes are nonzero but the bit-by-bit logical AND of the two
endpoint codes is zero then the line is partially visible.

8. Dan Cohen and Ivan Sutherland
End Point Code Algorithm
XL
YT
XR
YB

9. Dan Cohen and Ivan Sutherland
End Point Code Algorithm
YT

10. Dan Cohen and Ivan Sutherland
End Point Code Algorithm

11. Cohen-Sutherland
Subdivision
Line Clipping Algorithm

12. Cohen-Sutherland
Subdivision
Line Clipping Algorithm

13. Cohen-Sutherland
Subdivision
Line Clipping Algorithm

14. Cohen-Sutherland
Subdivision
Line Clipping Algorithm

15. Cohen-Sutherland
Subdivision
Line Clipping Algorithm