Concave polygon a convex polygon. As with any simple polygon, the sum of the internal angles of a concave polygon is ("n" − 2) radians, equivalently 180°×("n" − 2), where "n" is the number of sides. It is always possible to partition a concave polygon into a set of convex polygons. A polynomial-time algorithm for finding a decomposition into as few convex polygons as possible is described by . A triangle can never be concave, but there exist concave polygons with "n" sides for any "n" > 3. An example of a concave quadrilateral is the dart. At least one interior angle
Parallel projection has the following properties. Orthographic projection is derived from the principles of descriptive geometry, and is a type of parallel projection where the projection rays are perpendicular to the projection plane. It is the projection type of choice for working drawings. The term "orthographic" is sometimes reserved specifically for depictions of objects where the principal axes or planes of the object are parallel with the projection plane (or the paper on which the orthographic or parallel projection is drawn). However, the term multiview projection is also used. In "multiview projections", up to six pictures of an object are produced, with