Subject: Computer Graphics

Topic: Two Dimensional Viewing and Clipping

Difficulty: Low

2D clipping: -

The procedure that identifies the portions of a picture that are either inside or outside of a specified region of space is referred to as clipping. The region against which an object is to be clipped is called a clip window or clipping window. It usually is in a rectangular shape, as shown in the figure (e).

The clipping algorithm determines which points, Lines or portions of lines lie within the clipping window. These points, lines or portions of lines are retained for display. All others are discarded.

Point clipping:-

The points are said to be interior to the clipping window if

The equal sign indicates that points on the window boundary are included within the window.

Line Clipping:-

The lines are said to be interior to the clipping window and hence visible if both end points are interior to the window, e.g., line P1 P2 in Fig. (e). However, if both end points of a line are exterior to the window, the line is not necessarily completely exterior to the window, e.g. line P7 P8 in Fig. (e). If both end points of a line are completely to the right of, completely to the left of, completely above, or completely below the window, then the line is completely exterior to the window and hence invisible. For example, line P3 P4 in Fig. (e).

The lines which across one or more clipping boundaries require calculation of multiple inter-section points to decide the visible portion of them.To minimize the intersection calculations and to increase the efficiency of the clipping algorithm, initially, completely visible and invisible lines are identified and then the intersection points are calculated for remaining lines.