Clipping algorithms identify portions of an image that are inside or outside a specified clipping region. They are used to extract a defined scene for viewing, identify visible surfaces, and perform other drawing and display operations. Common types of clipping include point, line, polygon, and curve clipping. Algorithms like Cohen-Sutherland and mid-point subdivision use codes and binary subdivision to efficiently determine which image portions are visible and should be displayed.

1. Clipping Any Procedure that identifies those portions of a picture that are either inside or outside of a specified region of a space is referred to as a Clipping algorithm or simply Clipping . The region against which an object is to clipped is called a Clip Window.

2. Clipping * Clipping algorithms are 2D-3D. * Clipping algorithm can be implemented in hardware or software. * When implemented in software, clipping algorithms are slower than required for real time applications. * For this reason both 2D & 3D clipping algorithm have been implemented in hardware or firmware. * The purpose of a clipping algorithm is to determine which points, lines or portions of lines lie with in the clipping window. * These points, lines or portions of lines are relative for display. * All other are discarded.

3. * Application of clipping include extracting part of a defined scene for viewing. * Identifying visible surface in 3D views. * Antiliasing line segment or object boundaries. * Creating objects using solid-modeling procedures. * Displaying a multi window environment. * Drawing & painting operation that allow part of a picture to be selected for clipping, moving, erasing or duplicating.

8. Line Clipping * Lines are interior to the clipping window & hence visible if both end points are interior to the window. * Points are interior to the clipping window provided that XL <= X <= XR YB <= Y <= YT

9. I J G H E F A B C D K L Right Bottom Top Left Line Clipping

12. Cohen & Sutherland Algorithm The rightmost bit is the first bit. The bits are set to 1 based on the following scheme. 1) First bit Set- If the end point is to the left of the window. 2) Second bit Set- If the end point is to the right of the window. 3) Third bit Set- If the end point is to the below the window. 4) Fourth bit Set- If the end point is to the above the window. Otherwise the bit is set to Zero. It is obvious that, if both end point codes are zero, then both ends of the line lie inside the window & the line is visible.

23. Mid Point Subdivision Algorithm P1 P2 Pm1 Pm2 Pm2 P1 P2 Pm1 Pm2