Principle of Perspective Projection

PRINCIPLE OF PERSPECTIVE PROJECTION 

In perspective projection, the object is shown as it appears to the human eye, but the true shape and size cannot be seen. Picture plane is placed in between art of lellesq the object and observer.

Consider an object AB, standing in vertical position, is viewed by an observer from a fixed definite position as shown in Fig. 18.1. Let the image of the object is obtained on a flat and transparent plane (called picture plane). This image of the object ab, obtained on a picture plane as it would appear to the eye is called perspective projection. The rays of sight (ie. visual rays) from the object converge at the eye of the observer. So the perspective projection is a conical projection, the projectors being inclined to the plane of projection. Since the projectors are in conical form from object and converging at observer and the picture plane is placed in between the object and observer, the image of the object is reduced in size as shown in Fig. 18.1.

It is intersecting to note that the image becomes smaller as the distance of picture plane from the observer increases.

Consider the side view of a street with series of street lights as shown in Fig. 18.2.

Let AB, CD and EF are the street lights to be viewed by the observer from a fixed distance (known as station point). The transparent picture (Station plane on which the images of Point) street lights can be seen is placed at a fixed distance from the observer as shown in Fig.

ab, cd and ef are the corresponding images of street lights AB, CD and EF respectively. The observer sees the street lights through the transparent vertical plane. It is to be noted that the images ab, cd and ef are not of equal size though the street lights AB, CD and EF are of same height. The visual rays of the street light converge at a station point and hence the true shape and size of street light will not be seen on the picture plane.

When the distance of object is more from the position of picture plane the image of the object smaller. Therefore to draw an object in perspective projection the position of observer and picture plane should be well defined. To define these positions either the location of converging point (known as vanishing point) or the type of visual rays may be refered.