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The images produced by central projection convey the sensation of depth, or, in other words,
of the actual space. This is the only projection that produces the real perspective. These matters are
taught in courses on Perspective, such as given in the University of Architecture.
Descriptive Geometry is based on another type of projection, specifically parallel projection,
in most cases parallel, orthogonal projection. The result is realistic for small objects placed at long
distances from the viewer. Even if the result is not realistic for other objects, the processes of
projection and of measuring in the resulting drawing are simplified to such an extent that the
method imposed itself in Engineering.
We make now a new, important assumption: the projection center is sent to infinity.
Obviously, all projections rays become parallel. What we obtain is called parallel projection (Fig.
1.3).

Figure 1. 3
The object of projection is point A.
The projection direction is the vector n.
The image (projection) plane is plane .
The ray passing in parallel with the projection direction (the vector n) and the object of
projection (point A) is called ray of projection (t).
The image (point A ) is the point of intersection of the ray of projection (t) and the image
plane () (Fig.1.4).

Figure 1.4

1.3 PROPERTIES OF PARALLEL PROJECTION

- The image of point is point.
- The image of straight line is straight line (in general).
- The simple ratio of three collinear points is an invariant of parallel projection.
- Parallel lines are projected as parallel.
- When the object plane is parallel to the image plane, the lengths of segments and the sizes of
angles are invariant.
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