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When I walk on an asphalt road on a summer day and I look below me ( where I set my feet ) the area around my feet appear to be lighter than the area a few meters away. I think I notice the same with brick walls. They appear to be darker to me at grazing angles. But all theory I can find and what I experience in blender is that when you increase the roughness of a material, and/or you play with Fresnel the materials created in blender show a lighter color at grazing angles. See also the youtube videos about physically based rendering from CynicatPro at https://www.youtube.com/playlist?list=PLlH00768JwqFtFtsTKBxnLwadWsNo36R-

I started to think that glossy materials might appear lighter at grazing angles and rough materials darker, but I am sure my theory is far from correct. I hope anyone can tell me more about it. Is it maybe that the bumpiness caused self shadowing when it comes to materials like roads and walls, and so they appear to be darker at grazing angles ?

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It's not about being lighter or darker. It's that the material is more reflective at grazing angles. So if the surrounding environment is lighter than the object, the grazing angles will appear lighter. If the surrounding environment is darker it will appear darker. The material's lowest point of reflectivity is also a factor.

To understand why the material reflects more at grazing angles, take this example of a sphere with lots of tiny particles being emitted from it. These little shards reflect light more, whereas the sphere itself absorbs it more (as perceived from the viewing angle). This is a simulation of how roughness works. The "roughness" value of a shader simulates the effect of microscopic bumps on the surface of materials.

Particles being used to simulate roughness and the Fresnel effect

In the image below, the sky is a black and white gradient. Here we can see that the Fresnel effect is not lightening or darkening, but rather reflecting more or less. So areas of the sky that are more white are causing lighter edges and areas that are more black are causing darker ones.

Grazing angles are more reflective and can be lighter or darker depending on the environment

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  • $\begingroup$ Sorry for the late reply. I Did a lot of experiments, after reading your answer, and indeed this seems to be correct to me. I checked in real life; most paper materials appear almost like a mirror when viewing at a very grazing angle. $\endgroup$ – robwesseling Sep 11 '15 at 8:57
  • $\begingroup$ @robwesseling Cool :-) would you mind accepting the answer if it sufficiently addressed your question? $\endgroup$ – Mentalist Sep 15 '15 at 12:54

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