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Completely black Shading when roughness set to 0.003726621 or below, on EEVEE
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Description

System Information
Operating system: Windows 10 & Debian 10 KDE
Graphics card: Intel HD620 integrated graphics

Blender Version
Broken: 2.93; 2.93.1; 3.0.0 alpha
Worked: 2.92

Short description of error

  • This only happens on versions of blender from 2.93 onwards.
  • Any material where its final glossy roughness value is set between 0.0 and 0.003726621... is automatically shaded pure black, as if it had no shader at all.
  • Diffuse roughness works properly.
  • Viewport render view AND final render, on EEVEE, have the same problem

The following combinations cause a black material:


The problem even bypasses the mix node

In the following video I set roughness to 0 and shader goes full black. I switch to Cycles and the problem disappears:

The value "0.003726621" is rounded by Blender to 0.004.
The value "0.003726621" was found adding half an nth for every new digit:
0.00372663 yields a correct behaviour of the shader
0.00372662 results in a shadeless, black, material; add 0.000000005 and try again.
This number goes on and on.

Exact steps for others to reproduce the error

Default startup.
Use the default cube.
Apply any shader with a glossy roughness property.
Set glossy roughness to any value between 0 and 0.003726621.
This causes a shadeless material. Completely black.

The fact that I couldn't find any info about this bug at all on the internet tells me that my hardware is causing the problem. I did manage to find people with the same problem, but only a handful.
https://www.reddit.com/r/blender/comments/o7yeib/completely_black_shading_when_roughness_set_to_00/