1. The main classification

Here the only things that matter are the viewplane and the viewpoint. Forget about the object.

(I) Orthographic--a projection with parallel rays all perpendicular to the viewplane. The viewpoint is at infinity on the $z$-axis.

(II) Oblique--a projection with parallel rays slanted with respect to the viewplane. The viewpoint is at infinity but not on the $z$-axis.

(III) Perspective--a projection from a finite viewpoint.

Here (I) and (II) can be grouped together under the heading ``parallel projections.''

For pictures produced by each, see Carlbom and Paciorek [1] and Figures and below. In Figures and , the rays shown, if extended, actually go through the vertices of the cube and then to their images.

Figure: Orthographic and oblique projections

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Figure: A perspective projection