Binormal

From Tech Artists Wiki

Example anisotropic highlights, from ATI's real time hair shader.

A binormal or bitangent is a vector perpendicular to the plane defined by the normal and tangent of a vertex. Alternatively, it can correspond to the U or V "grain" from one vert to a neighboring vertex.

A common use of binormals is with lighting, such as the transformation of a normal map into different coordinate systems.

The effect of a binormal can clearly be seen with anisotropic specular highlights. Anisotropic highlights use the tangent (or sometimes the binormal) to calculate specular, instead of the normal. The result is a specular that goes 'across' the mesh instead of having a circular spot highlight. If the binormal, instead of the tangent, were used to calculate highlights the highlight would be going vertically.