When dealing with anisotropic lighting models you often need to define the anisotropic direction. Usually it’s easiest to author your content to conform to some constant direction, like +U or -V in UV space. However, this is not always possible. This example shows how the anisotropic direction can be calculated radiating outwards from a specific point in UV space (the center, in this case).


Obviously this is a simple case, but it illustrates a way to define your anisotropic direction on a per-pixel basis. Another way to do this is by painting a gradient along the direction of flow and turning it into a normal map. One way to do this might be to use the puppet warp tool in Photoshop CS5+ to take a linear gradient and warp it along the path that you need. Then just generate a normal map and use the X and Y components as your anisotropic direction.