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If I create a symmetric object without the mirror modifier, the triangularizations of the mesh aren't symmetric during rendering.

Is this a feature or bug ?

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    Could you include some screenshots of such model? And some description about how did you create that symmetric object? – Mr Zak Aug 31 '17 at 21:46

3 Answers3

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I recognize this is an older question, but your explicit question is answerable: this is a feature.

The default triangulation method of Blender is "fixed". This triangulates quads based on their vertices' indices, which means that some symmetrical meshes will not have symmetrical triangulation (it depends on how you made the mesh.)

There is a good reason to prefer "fixed" triangulation, in that you never get any quads that flip their triangulation when you deform them, like with an armature modifier. Flipping triangulation looks much worse than asymmetrical triangulation. Note that triangulation happens after the entire modifier stack, immediately prior to rendering, so our meshes at that point are much less likely to be symmetrical anyways (they've probably been deformed by modifiers.)

You can use a triangulate modifier on Shortest Diagonal mode to get symmetrical triangulation on a symmetrical mesh. You'll usually want to enable Keep Normals and use custom normals (from a quad-only copy). You'll probably want this near the top of the modifier stack if you want symmetrical triangulation in the presence of asymmetrical modifiers, and it will probably not make you happy with your subdivision surface modifiers that come afterwards.

Of course, you can also use mesh generation methods, like an applied or live mirror modifier, that create vertex indices that give you symmetrical fixed-mode triangulation. This is probably the best.

Nathan
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You can use a Triangulate Modifier before the Mirror Modifier and the Subsurface could go before the Triangulation Modifier. But normally if you just use high enough subdivision level it shouldn't matter either

HenrikD
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  • Maybe the tool Poke Faces is the solution. Apply Mirror, then Subdivision, then Poke Faces. –  Sep 01 '17 at 06:32
  • but sadly there is no Modifier for it so it can't be used on animated meshes and other deforming things that easily – HenrikD Sep 01 '17 at 06:52
  • This answer implicitly advises to use Subdivision Modifier before the Mirror Modifier, which unfortunately creates poor results - it creases on the mirror plane. – Markus von Broady Nov 10 '22 at 13:16
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First of all, why do you care? It shouldn't affect the shading at all, since the triangles will still be coplanar.

And I would say it's neither a feature nor a bug, since it probably wasn't planned but doesn't really matter one way or the other. (Of course, if it must be triangulated a certain way, you can always triangulate it yourself.)

SilverWolf
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  • The shading is affected really. But if I manually triangulate I can check that the tris were symmetric. I didn't think this idea. –  Aug 31 '17 at 23:08
  • Yes, but there is a big problem with subdivision surfaces –  Aug 31 '17 at 23:27
  • Hm. How can you tell it's triangulated wrong during rendering? Are you using the Wireframe node in Cycles? – SilverWolf Sep 01 '17 at 12:17
  • I see this using flat shading. You can see triangles of slightly different colors in non planar ngons. –  Sep 07 '17 at 21:42
  • Non-planar ngons? I would definitely convert those to tris or quads manually—they can mess you up in all sorts of ways. – SilverWolf Sep 08 '17 at 14:10
  • if by ngon you mean a polygon of at least 5 edges, then it's worth to note quads also aren't necessarily coplanar, in fact in usual workflow a lot of quads aren't. Even Suzanne has quads that aren't coplanar. – Markus von Broady Nov 10 '22 at 13:13