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I have a mesh in Blender, imported from elsewhere. You can see individual triangles on it in a funny checkered pattern, like this:

Blender screenshot - half the triangles are darker

This is consistent with a popular bug where every other triangle is inverted, and you see half of them from the inside and half from the outside. But if you enable normal display, all the normals are facing outwards, as expected.

The mesh is available at http://www.jishop.com/temp/4osh.blend . Can someone please take a look at it explain the effect?

Seva Alekseyev
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    Seva, might you be willing to upload a copy of the ~.blend file you linked to in your question to Blend-exchange, and change the link? The reason for this request is that your question and the screenshot will persist for some time, but I fear that a file located on a page whose URL contains "/temp/" may not last as long as the question. The question is more valuable to future Blenderheads with your ~.blend file. – brasshat Aug 25 '15 at 04:18
  • I entered Edit mode, pressed the Spacebar and typed the tool name "Tris To Quads". The model looks fine after that. It's usually a good idea to avoid triangulation unless you are exporting a finished model to a game engine. – MarcClintDion Aug 25 '15 at 05:36
  • @MarcClintDion I didn't know there's a tris to quads function! I have to ask though, why are you posting this as a comment? – TARDIS Maker Aug 25 '15 at 12:52
  • @brasshat: that's my site :) Temp or not, it'll stay for a while. – Seva Alekseyev Aug 25 '15 at 14:07
  • It's a nice tool because it has an undocumented alternate function. You can import a triangulated model and if Tris To Quads fails then you know that area needs some repair. If this tools fails then that area of the model will likely cause animation and rendering errors. It basically finds mistakes in loop flow that a person might not easily see ahead of time. :) – MarcClintDion Aug 26 '15 at 08:34

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The triangles are all being displayed in 'Shade Flat' mode, which is the default. To change this, go to edit mode and select all faces (hit 'A' once or twice, until all triangles are selected). Then Hit 'W' then select 'Shade Smooth'

[Shade Flat] mode is appropriate for modeling a faceted gemstone, or cut crystal glass, for example. Since blender cannot know ahead of time what type of shading you need, it defaults to 'Shade Flat'.

Mike Metcalf
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  • Shade smooth is for hard edges with low topology, and us good for modeling at an earlier stage (most of the time it's easier to see what's going on with sharp edges). Smooth shading is best for higher topology objects where you can see the edges, or smooth parts of low poly models. I'm pointing this out, because I don't think shade smooth is good with the examples you provided. I find that it's usually best to set the shading in object mode (because it does it globally). I've had issues with having some faces set to one type of shading, but something else in object mode. This could just be me. – TARDIS Maker Aug 25 '15 at 00:32
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    I rearranged my answer after posting, not realizing that the clause 'This mode...' was now seemingly referring to 'Shade Flat' instead of the intended 'Shade Smooth'. Thanks for making this clear. (edited my answer to fix this) – Mike Metcalf Aug 25 '15 at 03:03
  • This helped; the mesh looks, well, smooth :) What's the theory behind this, please? Is it a property of the mesh or a rendering setting? – Seva Alekseyev Aug 25 '15 at 14:05
  • @brasshat: that' – Seva Alekseyev Aug 25 '15 at 14:07
  • The difference between flat and smooth shading is the calculation of surface normals. In flat shading, all three (or four, or x) vertices in a face share the same surface normal. In smooth shading, a face will have (or may have) different surface normals for each vertex. As surface normals are computed from the underlying mesh, it might be said that it is a property of the mesh. But it is a render-setting that determines whether blender uses face normals (flat shading) or vertex normals (smooth shading). – Mike Metcalf Aug 25 '15 at 15:27