Normal

A flat polygon situated in 3-D Coordinate Space necessarily has an orientation. It faces some unique direction. An imaginary ray pointing out from the surface of the polygon, and perpendicular to that surface, is called the normal of the polygon.

As there will always be two normals, one on each side of the surface, and pointing in opposing directions, the choice of the side from which the normal projects defines the front or "face" of the polygon. In 3-D computer graphics, as opposed to the physical world, it is usual for a polygon to have only one face or side, and therefore only one normal. This is because polygons are typically used to create a closed mesh representing the surface of a 3-D object and the back side of the polygon is therefore hidden inside the object. To save render time, polygons are kept single-sided and the normal projects from only the exposed face. However, occasionally it is necessary to create double-sided polygons that have normals pointing from both sides, and which therefore can be rendered from both sides as the different sides come into view during the course of an animation. The face side of a polygon is typically established in a Model file by the order in which the vertices of the polygon are listed, clockwise or counterclockwise around the facing (normal) side.

Normals can be associated not only with the flat surfaces of the polygons, but also with the individual points that make up the vertices where polygons meet on the surface of a model. This technique is used in rendering to create the appearance of curved surfaces rather that flat, faceted sides. Such vertex normals can be directly assigned in the model file, but are usually computed during rendering by averaging the normals of the adjacent polygons. This rather subtle idea is described and illustrated in the Lesson 3 tutorial.

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