Normal mapping is a technique for "faking" variations in a surface that doesn't really exist in the geometry of the surface. It is useful for producing surfaces that have bumps, dents, roughness, or wrinkles without actually providing enough position information (vertices) to fully define those deformations. The underlying surface is actually smooth, but is made to appear rough by varying the normal vectors using a texture (the normal map). The technique is closely related to bump mapping or displacement mapping. With normal maps, we modify the normal vectors based on information that is stored in a texture. This creates the appearance of a bumpy surface without actually providing the geometry of the bumps.
A normal map ...
