# projection

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.

## Parallel projection

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.