Surface normal
A surface normal, or just normal, is a three dimensional vector which is perpendicular to a given surface (such as a triangle). They are commonly used, in addition to other areas, in 3D computer graphics for lighting calculations—a surface's normal in comparison to a light source determines how the surface will be lit (dark, bright).
A polygon and its normal
For a polygon the surface normal can be calculated as the the vector cross product of two edges of the polygon.
In 3-D computer graphics, triangles are often used as the basic "building blocks" of a polygon since triangles are guaranteed to be planar. That is, given three points in space (x, y, z), the smallest surface connecting them all is guaranteed to be a flat plane. The same is not necessarily true for more complex polygons, such as quadrilaterals. A quad can be a flat plane, but given any four points, there is no guarantee that a flat plane connecting all four points can be generated. Since only one normal can exist for any given surface, bent surfaces (such as on a sphere) have to be broken up into component triangles.
See also: computer graphics
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