Basic Engineering Graphics

An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. Examples of an entity in graphics are a single point, single line, or single polygon.

Projection is a correspondence between the points of a figure and a surface (or line). In-plane projections, a series of points on one plane may be projected onto a second plane by choosing any focal point, or origin, and constructing lines from that origin that pass through the points on the first plane and impinge upon the second. This type of mapping is called a central projection. The figures made to correspond by the projection are said to be in perspective, and the image is called a projection of the original figure. If the rays are parallel instead, the projection is likewise called “parallel”; if, in addition, the rays are perpendicular to the plane upon which the original figure is projected, the projection is called “orthogonal.”