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In What does the Roughness setting on a diffuse shader do? Ibalaszs states that he divides the glossy roughness by 3 to get the diffuse roughness. He then states that this is also just a thumb rule he somehow eyeballed, but also makes sense as most materials have a diffuse roughness of up to 0.5 while most are topping of at 0.3. So apart from that does anyone know how to actually use the diffuse roughness value and do you even use it at all? I did some render test with suzanne at different diffuse roughness level:

Diffuse roughness=0 enter image description here

Diffuse roughness=0.5 enter image description here Diffuse roughness=1 enter image description here

One thing I noticed that with high diffuse roughness values the shader almost starts to look like velvet.

tempdev nova
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It says so right in the docs - blender uses Lambertian Reflection Shading (equally bright from all viewing angles) by default, but increasing the Diffuse Roughness value (in cycles only), shifts it to an Oren-Nayar Shading system (basically Lambertian with extra features to handle surface roughness more accurately - viewing angle can affect surface appearance):

Reflect

An additional note on shading systems - Though there are exceptions, a general rule of thumb is to use (if available):

Blinn/Phong      - Plastics
Cook-Torrence    - Metallic surfaces
Lambert          - Smooth non-reflective surfaces
Oren-Nayer       - Rough non-reflective surfaces
Ward-Anisotropic - Brushed/Warped metals
Minnaert         - Large "Planetary" objects
Christopher Bennett
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