Categories of Parallel Projection

Parallel projection can be categorized as per to the angle which the direction of projection makes along with the projection plane. For illustration, in Isometric projection, the way of projection d=(d1,d2,d3) makes equal angle as α along  with all the 3-principal axes as in the figure 4.

Figure: Direction of projection

Rays from the object intersect the plane before passing by center of projection. In parallel projection, image points are determined as the intersection of view plane along with a projector as rays drawn from the object point and consisting of a fixed direction as in the figure 5.

Figure: Parallel projection

Parallel rays from the object might be perpendicular or not be to the projection plane. Whether the projection's direction d=(d1,d2,d3) of the rays is perpendicular to the projection plane or else d has the similar direction as the view plane normal N, we contain Orthographic projection if not then Oblique projection.