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Low poly mesh

I wonder what is the reason that causes the black patches in the result using cycles renderer with smooth shading? Is there any other way to fix them without making more subdivisions (The black blotches disappear when I use 2x subdivision surface)?

Blender file:

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  • $\begingroup$ It's probably because of the mesh topology. If you can please upload blend file to: blend-exchange.giantcowfilms.com and I'll take a look at it. $\endgroup$ – cgslav Apr 7 '17 at 18:41
  • $\begingroup$ Try selecting it and hiting "smooth" shading again. Also check if you have inverted face normals. If you can't afford more subdivs then consider subdividing it and the using the decimate modifier with "planar" setting. It should keep detail on the steep parts but dissolve verts on the flat parts. That way you can have more topology where you need it the most. $\endgroup$ – Cos Dim Apr 7 '17 at 18:50
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    $\begingroup$ Maybe could be related to the Terminator issue. Here's a related question: blender.stackexchange.com/questions/1998/… $\endgroup$ – Carlo Apr 7 '17 at 18:57
  • $\begingroup$ @CosDim I checked the normals. I think they are fine. More subdivision isn't a problem. I just want to know what causes these artifacts. $\endgroup$ – Davis Apr 7 '17 at 19:52
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    $\begingroup$ @Carlo Hmm, I think this is the case. Cycles renderer actually moves the ray's hit point when smooth shading is used, and it causes self shadowing when the displacement of the hit point exceeds the shadow bias. So, the treatment can be increasing the bias, increasing the poly resolution or using flat shading instead. $\endgroup$ – Davis Apr 7 '17 at 20:50
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There is an option to get rid of those shadows by smoothing the mesh. Reason of those shadows is hard angle between faces.

Main downside of it is loosing some details along the mesh, but you can try and smooth only needed "patches".

In my example I've smoothed whole mesh by Smooth Vertex 5 times.

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You can also Subdivide Smooth selected faces, but this will generate tris/ngons around selection.

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