Unwanted black areas when rendering smooth glossy mesh with sharp angles

[TomEPike]

I am trying to render a complext glossy object with HDRI background and reflections. It works fine except for the parts of the object which are 3D arrows consisting of a cylinder with a cone on top, set to “Smooth”. When set to “Flat” the balck areas dissapear, but then the reflections are flat off couse. The problem is easely reproducible with any simple Cone or Cylinder. This seems to be a common problem much descibed in various forums, however I have not yet found a good solution. I have tried the Modifiers “Edge Split” which did not make any difference. “Subdivision suface” distorts the object itself and is therefore useless for these objects. I also tiried “Mark Sharp” selcting the critical lines / vertecies. What has helped for black areas around the cone sharp point was to “cut off” the tip of the cone and then narrowing the top circle to such a small radius that it becomes invisible in the rendered picture.

Gratefull for any advice. I am a newbie and need pretty detailed advice…

[Secrop]

This is a known issue… The smooth normal is making the reflection ray to go inside the surface (since that normal vector is being blended with the normals of the back faces). This makes the reflection ray to hit the geometry and so the engine renders it black.

In your particular scene, turning the auto smooth option on (on the top of the mesh panel from the properties) with an angle above 60 or so, should fix it.

In other situations, adding loops around the sharp edges can also help (thought some darkening can still appear).