Clipping - Types of Clipping in Computer Graphics

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Computer Graphics and Multimedia Application: Q1.Clipping? - Types of Clipping in Computer Graphics (Computer graphics and multimedia application All Notes)

Q1. What do you mean by Clipping? Various Types of Clipping? Types of Clipping in Computer Graphics

Ans. Clipping: Any procedure that identifies those portions of a picture that are either inside or outside of a specified region of space is referred to as a clipping algorithm or simply clipping. The region against which an object is to be clipped is called a clip window. Clipping is the process to identify the picture either inside or outside of the displaying area.

Application of clipping includes extracting part of a defined scene for viewing, identifying visible surface in three-dimensional views, object boundary, creating objects using solid modelling procedures, displaying a multi-window environment and drawing and painting operations, moving, cursing depending on the application, the clip window can be general polygon or it can have curved boundaries.

For the viewing transformation, we want to display only those picture parts that are within the window area. Everything outside the window is discarded. Clipping algorithm can be applied in world co-ordinates, so that only the contents of the window interior are mapped to device co-ordinates.

Types of Clipping :

These are the main types of clipping

1. Point clipping.
2. Line clipping (straight line segment).
3. Area clipping (polygons) or curve clipping.
4. Text clipping.

Line and polygon clipping routines are standard components of the graphics package, but many packages accommodate curved objects.

1. Point Clipping: Assuming that the clip window is a rectangle in the standard position we have a point P = (x, y) for display, if the following conditions are satisfied :

Xw min ≤ x ≤ Xw max  ,  Yw min ≤ y ≤ Yw max.

Where the edges of the clip window (Xw min , Xw max, Yw min, Yw max ) can be either the world co-ordinate window boundaries or viewport boundaries. If anyone of these four conditions is not satisfied the point is clipped.
For Example: Point clipping can be applied to the scene involving explosions that are modelled with particles (points) distributed in some region of the scene.

2. Line Clipping: A line clipping procedure involves several parts. First, we can test a given line segment to determine whether it lies completely inside the clipping window if it does not, we try to determine whether it lies completely outside the window.

Finally, if we cannot identify a line segment as completely inside or completely outside we must perform intersection calculations with one or more clipping boundaries.
A line with both endpoints insides all clipping boundaries, such as the line P1 to P2 is saved. A line with both endpoints outside any one of the clip boundaries (line P3 P4) is outside the window.

3. Area Clipping (Polygons) or Curve Clipping: Area with curved boundaries can be clipped with methods similar to point clipping and line clipping. Curve clipping procedures will involve non-linear equations and this requires more processing than for object with linear boundaries.
The bounding rectangle for a circle or other curved object can be used first to test the overlap with a rectangular clip window. If the bounding rectangle for the object completely inside the window, we save the object. If the rectangle is determined to be completely outside the window, we discard the object.

If the bounding rectangle test fail, we can look for other computation saving approaches. For a circle, we can use co-ordinate extent of individual quadrants and then octants for testing before calculating curve window intersections.

4. Text Clipping: There are several techniques that can be used to provide text clipping in a graphics package. The clipping technique used will depends on method used to generate characters and the requirements of a particular application.

The simplest method for processing character string relative to a window boundary is to use them all or one string clipping strategy. If all of the string is inside a clip window, we keep it. Otherwise, the string is discarded. This procedure is implemented by considering a bounding rectangle around the text pattern.

The boundary position of the rectangle are then compared to window boundaries and the string is rejected if there is any overlap.
An alternative to reject an entire character string that overlaps a window boundary is to use them all or none character clipping strategy. Here we discard only those characters that are not completely inside the window.

A final method for handling text clipping is to clip the components of individual characters. We now treat the character in the same way as the line.

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