An anisotropic map is a texture that controls the shape of the specular highlights from real-time lights, often used for hair shading. Anisotropic highlights are characterized by long directional strokes, instead of the typical circular shape.
The texture is usually called a Flow map which is very similar in concept to a normal map, except instead of controlling surface normals, it distorts the specular highlight in specific directions across the surface.
- Substance Painter: Flow map painting
- HAIR!? thread on Polycount by Paul 'Prolow' Lohman has some examples of an anisotropic hair shader in Maya.
- Making an anisotropic map for Marmoset Toolbag post on the Polycount forum from the Lead Engineer at 8monkey Labs.
- Deriving an anisotropic map from vectors post on the Polycount forum from a Character Artist at ArenaNet.
- Painting a "flow" normalmap? thread on the Tech-Artists.Org forum.
- HairTechnique on the wiki.