So let's say I am storing the diffuse lighting in my scene with lightmaps, which store the amount of light that enters a given fragment, but no directional information.
Using only this information, I'd really like to be able to implement a convincing "rough" shader, such as Oren-Nayar, for which the apparent luminosity depends on the viewing angle.
However, a cursory look at implementations of Oren-Nayar reflectance with a single point light shows that they all seem to have terms involving the light direction. Obviously I'd expect to see something like "dot(normal, lightdirection)" (which would be perfectly fine as the lightmap already stores the sum of all such terms); the problem is that I don't have access to the light direction (or light directions, as the case may be) on its own.
My question, then, is whether it is possible to implement Oren-Nayar (or, if not any such similar material like Minnaert, etc.) in a way that does not require this directional information. Can anyone provide any tips? My fallback solution is just to play with the shaders to see if I can get the term I don't like to go away, but it seems like this might be a common enough question that someone has already done this.