Normal Bump Mapping refers to a new technique for simulating high-resolution surface detail on low-resolution polygonal models. Normal Bump Mapping is similar in some respects to regular bump mapping, but it conveys more complex surface detail than is possible in regular bump mapping. Normal bump maps store information on the direction normal of the surface in addition to the simple depth information used in regular bump mapping.

It is an application of the technique known as bump mapping. While bump mapping perturbs the existing normal (the way the surface is facing) of a model, normal mapping replaces the normal entirely. Like bump mapping, it is used to add details to shading without using more polygons. But where a bump map is usually calculated based on a single-channel (interpreted as grayscale) image, the source for the normals in normal mapping is usually a multichannel image (that is, channels for "red", "green" and "blue" as opposed to just a single color) derived from a set of more detailed versions of the objects.

Normal mapping is usually found in two varieties: object-space and tangent-space normal mapping. They differ in coordinate systems in which the normals are measured and stored.

One of the most interesting uses of this technique is to greatly enhance the appearance of a low poly model exploiting a normal map coming from a high resolution model. While this idea of taking geometric details from a high resolution model had been introduced in "Fitting Smooth Surfaces to Dense Polygon Meshes" by Krishnamurthy and Levoy, Proc. SIGGRAPH 1996, where this approach was used for creating displacement maps over nurbs, its application to more common triangle meshes came later. In 1998 two papers were presented with the idea of transferring details as normal maps from high to low poly meshes: "Appearance Preserving Simplification", by Cohen et al. SIGGRAPH 1998, and "A general method for recovering attribute values on simplified meshes" by Cignoni et al. IEEE Visualization '98. The former presented a particular constrained simplification algorithm that during the simplification process tracks how the lost details should be mapped over the simplified mesh. The latter presented a simpler approach that decouples the high and low polygonal mesh and allows the recreation of the lost details in a way that is not dependent on how the low model was created. This latter approach (with some minor variations) is still the one used by most of the currently available tools.

 

Example of normal mapping for recreating the details lost during a drastic simplification:
     
Original Mesh 4 Million Triangles
Simplified Mesh 500 Triangles
Simplified Mesh and Normal Mapping 500 Triangles
Normal Mapping Information Courtesy www.wikipedia.org & Paolo Cignoni. The Full article can be viewed here