Convex curve has at least four vertices, points that are local minima or local maxima of curvature. A curve "C" is convex if and only if there are no three different points in "C" such that the tangents in these points are parallel. Proof: ⇒ If there are three parallel tangents, then one of them, say "L", must be between the other two. This means that "C" lies on both sides of "L", so it cannot be convex. ⇐ If "C" is not convex, then by definition there is point "p" on "C" such that the tangent line at "p" (call it
Parallel projection Parallel projection A parallel projection is a projection of an object in three-dimensional space onto a fixed plane, known as the "projection plane" or "image plane", where the "rays", known as "lines of sight" or "projection lines", are parallel to each other. It is a basic tool in descriptive geometry. The projection is called "orthographic" if the rays are perpendicular (orthogonal) to the image plane, and "oblique" or "skew" if they are not. A parallel projection is a particular case of "projection" in mathematics and "graphical projection" in technical drawing. Parallel projections can be seen as the limit of a