A projection is a transformation in which the image figure is obtained from the original figure (plane or three-dimensional) by drawing from the point of the original figure straight lines, which are either parallel (parallel projection), or concurrent (central projection), and which intersect the image plane. The points of intersection of the straight lines (rays of projection) with the image plane are the image points, the totality of which yields the image figure. The figure is said to be projected onto the image plane.
A parallel projection is a projection in which all rays of projection are parallel. The direction of these rays is called the projection-direction. The projection-direction is given relative to the image-plane. We speak of oblique parallel projection if the rays of projection do not intersect the image-plane at right-angles. If the rays of projection do intersect the image-plane at right-angles, the projection is orthogonal. The general parallel projection is used for representing solid objects by plane images, e.g. in cavalier projection. Orthogonal parallel projection is used for horizontal projection and for two-plane projection.
See also projective geometry.