Page 155 - DCAP504_Computer Graphics

Computer Graphics

There are two commonly used methods of projection. They are:
1. Perspective Projection
2. Parallel Projection
Perspective Projection

Basic Principle: The techniques of perspective projection are an overview of the principles used by
artists in arranging the perspective drawings of three–dimensional objects and scenes. The eye of the
artist is positioned at the center of projection and the plane holding the canvas becomes the view plane
as shown in figure 10.7. With the help of a projector, an image point that goes from an object point to
the center of projection is determined.

Figure 10.7 Perspective Projection

Perspective drawings are distinguished by perspective foreshortening and vanishing points.
Perspective foreshortening is the illusion of the objects and lengths that appear to be small as their
distance from the center of projection increases. The illusion is nothing but sets of parallel lines that
appear to meet at some point. This point is called as vanishing point. The major vanishing points are
formed by intersecting lines parallel to x, y, or z axes. The number of principal axes that are intersected
by the view plane determines the number of principal vanishing points.
Perspective Anomalies

The process of constructing a perspective view introduces certain inconsistencies that improve
practicality in terms of depth cues but alter the actual sizes and shapes.
The following are the perspective anomalies:

1. Perspective Foreshortening: The farther the object is from the center of projection, the smaller it
appears (its projected size becomes smaller). As shown in figure 10.8, object B is twice the size of
object A. However, both the objects appear to be of same size when projected on the view plane.

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