Taxonomy of Projection - viewing transformation
There are different types of projections as per to the view that is essential. The subsequent figure 3 demonstrates taxonomy of the families of Parallel and Perspective Projections. This categorization is based upon where the rays from the object converge at COP that is center of projection or not and where the rays intersect the projection plane perpendicularly or not. The former condition divides the perspective projection from the parallel projection and in the last condition separates the Orthographic projection from the Oblique projection.
Figure: Taxonomy of projection
The direction of rays is very vital only in the condition of Parallel projection. Conversely, for Perspective projection, the rays converging at the center of projection, they do not have a fixed direction, which is, each ray intersects the projection plane along with a diverse angle. For Perspective projection the direction of viewing is significant as this only finds out the occurrence of a vanishing point.