If you're a Photoshop user, it's easy to create du/dv maps using nVidia's texture tools. (du/dv maps are for DX8 refractions.) But if you're using another program like PaintShopPro, you might have trouble getting the nVidia tools to work. Fortunately, a du/dv map can be easily created from a normal map in just two steps. You can also do this in Photoshop if you don't want to bother with the nVidia tools.
- Open the normal map. First go to Colors->Negative Image (PaintShopPro) or whatever inverses all the colors in your paint program.
- Now go to Colors->Adjust->Brightness/Contrast (or your paint program's equivalent). Leave brightness at 0, but raise contrast as high as it will go.
That's all! Just save and you're done..