Parallel vs Perspective Projection
Human beings see everything using perspective projection where there is always a horizon and the point where everything looks small at a distance, but large when close up. This type of projection is used in drawings and is really a cheap imitation of what the real world would look like if drawn on paper. Another method to produce realistic visual effects on paper is called parallel projection. This method closely resembles seeing a far away object with the help of a telescope. This projection makes the rays of light that enter the eyes to be nearly parallel thereby losing the effect of depth. This type of projection is mostly used by isometric game engines.
Perspective projection is a type of drawing that graphically approximates three dimensional objects on a two dimensional surface such as a paper. Here the main intention of the person who draws lines on paper is to produce a visual perception as close as possible to the real object.
As told before, parallel projection is a cheap imitation of the real world as it ignores the extent of all points and is concerned with the easiest way of getting the point on the screen or paper. For this very reason, parallel projections are very easy to achieve and a good substitute for prospective projection in circumstances where either perspective projection is not possible or where it would distort construction.
Difference between Parallel Projection and Perspective Projection
The major difference between perspective and parallel projections is that prospective projections require a distance between the viewer and the target point. Small distances produce great perspective effects while large distance reduce these effects and make them mild. In simplest of words, in parallel projection the center of projection is at infinity, while in prospective projection, the center of projection is at a point.