Page 137 - DCAP504_Computer Graphics

Computer Graphics

In this unit, you will learn the two most common types of clipping, namely point clipping and line
clipping. You will learn various clipping algorithms that are developed for extracting a part of a defined
scene for viewing, displaying multi-window environment, drawing, painting, and so on.
9.1 Overview of Clipping
Clipping, in general, refers to the removal of a part of a scene. Imagine a person seeing the world
through the window of a house. The part of the scene that cannot be viewed from the window is called
as cut off portion or left out portion. In computer graphics, the portion that is left outside the clip
window is called the clipped part and the process used to display the image within the window is
called clipping. In general, a clipping algorithm is used to remove the image components that lie
outside the viewing window.

According to Peter Comninos, Director of Bournemouth's National Centre for Computer Animation,
"Clipping is a process that subdivides each element of a picture to be displayed into its visible and
invisible parts, thus allowing us to discard the invisible part of the picture."
Clipping algorithms can be applied in world co-ordinates such that only the picture of the object inside
the window is mapped to the device co-ordinates. Alternatively, the entire world co-ordinate picture is
first mapped to the device co-ordinate picture and then clipped against the viewport boundaries.

World co-ordinate clipping does not consider pictures outside the clipping window. Thus, it eliminates
the processing required to transform those pictures to device space. On the other hand, viewport
clipping reduces calculations by linking the viewing and geometric transformation matrices. This unit
discusses three primitive types of clipping. They are:
1. Point clipping
2. Line Clipping
3. Polygon Clipping (Will be discussed in Unit-10)

World co-ordinate is the reference frame that is used to place the object in the
appropriate position. This co-ordinate system is linear along both axes.

Applications such as Word processors and Spreadsheets clip keyboard input to
Did you know?
prevent the input from appearing in the margins of a page. Computer-Aided Design
(CAD) programs clip graphics output to prevent the output from overwriting the
edges of a drawing.

9.2 Point Clipping
How does point clipping work? Point clipping removes points that are outside the clipping window.
Assume that the clip window is rectangular. The point p(x, y) is saved for display, if the following
inequalities are satisfied:

xw min ≤x ≤xw max
yw min ≤y ≤yw max
where, xw min and xw max are the edges of the clip window boundaries that are parallel to the y-axis and
yw min and yw max are the edges of the clip window boundaries that are parallel to the x-axis.

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