Hello!
Currently I'm trying to implement a PBR shader. And now I want to know how you've implemented rougness. My idea is to have a cubemap with MIP levels and sample the cubemap on a certain level depending on the roughness value.
float3 reflectedColor = cubeMap.SampleLevel(samplerCube, R, nMips - roughness * nMips).rgb;
nMips is the number of MIP levels in the texture.
But this approach doesn't look so nice. You can see an example in the picture. So how did you guys do it?
Thanks :)
My idea is to have a cubemap with MIP levels..
There's a little missconception in your approach. Thought there is no real hard definition, PBR (physical based rendering) is more about applying physical laws, formula and approximation to rendering a surface depending on the incoming light and less about hacking until it looks good/right. The latter isn't evil, but it isnt really PBR either 
Using importance sampling, but it's very expensive, all is explained there :
http://blog.selfshadow.com/publications/s2013-shading-course/karis/s2013_pbs_epic_notes_v2.pdf
Roughness is not a factor of importance sampling..may be related, but not necessary. Importance sampling is more related to image base rendering. Roughness is usually a factor of the BRDF being used so one approach would be to pick a BRDF that allows for a roughness factor ex. the Cook-Torrance BRDF. There are plenty of papers/articles out there that covers microfacet BRDFs.
