Difference Between Parallel and Perspective Projection
We humans look at everything with the help of perspective projection. In this case there is a horizon and the point from where everything looks small however generally grows up to be large when everything closes up. This perspective is commonly used in drawings and represents the real world on a piece of paper. Parallel projection is another method that is used to produce visual effects on paper. The projection brings in an effect of looking at a far away object through a telescope. In this projection the light rays enter the eyes are almost parallel and do not have the effect of depth.
Perspective projection is just a kind of drawing that tries to represent 3D objects on a 2D surface like paper. In these cases the person who is drawing tries to create a visual effect which will be quite same as the real object.
Parallel projection is an attempt to emulate of the real world as it ignores the extent of all points and is concentrates on the easiest way of getting the point on a 2D surface like paper. This is the key reason behind the ease of creation of parallel projections. They are a good proxy for prospective projection when creating them is not possible or where it would cause distortion.
The one factor that makes the two perspective and parallel projections stand apart is that prospective projections needs a distance between the person who will be looking and the point of target. It is often the small distances that help to produce tremendous effects of perspective unlike the large distance nullify these effects. In case of parallel projection the center of projection is located at infinity. However in case of prospective projection, the center of projection is located at a point.