OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Bump Mapping

We have already seen procedural shader examples that modified color (brick and stripes) and opacity (lattice). Another class of interesting effects can be applied to a surface with a technique called bump mapping. Bump mapping involves modulating the surface normal before lighting is applied. We can perform the modulation algorithmically to apply a regular pattern, we can add noise to the components of a normal, or we can look up a perturbation value in a texture map. Bump mapping has proved to be an effective way of increasing the apparent realism of an object without increasing the geometric complexity. It can be used to simulate surface detail or surface irregularities.

The technique does not truly alter the shape of the surface ...