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Are 'Not Planar Polygons' an issue?


LeChuck
Go to solution Solved by Cerbera,

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Hi,

 

I am curious to know whether the 'Not Planar Polygons' are an issue when modelling or if it's just a best practice thing? I notice when I set the Planarity Threshold to 0 I get some pink polygons which I guess means they are slightly skewed. When I set this to say 10 degrees they go away. 

 

They have never affected my modelling in any detrimental way so I have largely ignored them but thought I'd ask as I'm quite interested to know if there is a theoretical reason polygons must all be planar.

 

Thanks,

LeChuck

 

246191034_ScreenShot2021-09-11at17_52_10.png.b61f9761e5af8aadaa784bd182343057.png

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  • Solution

No, in a subdivision surface context these are fine, and nothing to worry about. indeed it is almost impossible to keep low poly meshes fully planar, especially if the surfaces they are making are curved, and in more than one direction.

 

That function is aiming to prevent 'polygon twist', which can lead to surface problems.

And in hard surface situations, where subdivision is not involved, then polygon twisting is more serious, and can lead to phong shading and render artefact type errors, which is why we sometimes need to find the ones that do it, so that we can try and lessen it to the point where it is no longer problematic.

 

The reason Mesh checker flags them is more for 'completeness' than anything else these days, but there are situations in which it is useful to see which polys are planar, and which are not. In models where there are hundreds of thousands of polys it is sometimes difficult to spot any that have become in advertently 'twisted', and therefore might produce render artefacts.

 

But note there is also a threshold provided, and to usefully check the non-planars that must be set to an angle greater than the maximum amount of twist per polygon you will allow because of the curvature of the object. Typically I will allow up to 10-20 degrees of twist in a low poly base mesh, knowing that by the time that poly is later subdivided, the degree of twist will be orders of magnitude less, to the point where it is largely negligible, because of the increase in poly resolution and the interpolating / smoothing effect of the SDS.

 

In most diagnostic circumstances, it is OK to simply turn off the search for non planar faces, and instead focus on any polys flagged bad, non manifold, and any orphaned points, or unintentional borders that aren't on edges.

 

CBR

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Thanks CERBERA for your detailed answer. Interesting to know, I suspected in SDS context non-planar polygons were not really an issue as from past experience the polys often subdivide without any artefacts. It seems it is more a feature  for hard surface modelling as you say. 

 

Thanks for again for clarifying 👍 

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