This document discusses projection theory and different types of projections. It defines a projection as mapping a 3D space onto a 2D subspace, with the 2D space called the projection plane. Projections are classified as either perspective or parallel projections, depending on whether they use a single center of projection or parallel lines of projection. Common elements of any projection system include the center of projection, projection plane, objects to be projected, and projectors (sightlines).