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Sometimes I encouter this problem. Anyone knows why this happens and how to fix it?

Here you can download the blend file: https://www.dropbox.com/sh/dkoikr6qwosq25f/AADJYnH1-uv2haNshs-nq7nWa?dl=0

Flat Shading. Top one is the problem Flat Shading. Top one is the problem Normals are all facing the same direction Normals are all facing the same direction Artifacts also happens in Smooth shading Artifacts also happens in Smooth shading With Subdivision modifier, the problem is gone. But why? With Subdivision modifier, the problem is gone. But why?

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  • $\begingroup$ As mentioned, a Subdivision modifier solves the problem. If applied and then un-Subdivide the model with Decimate Modifier and apply, the issue is completely gone. But Why and how? $\endgroup$
    – Daniel Hdz
    Jan 21 at 7:01
  • $\begingroup$ The subdivide modifier discards split normals by default; that's why it fixes it. $\endgroup$
    – scurest
    Jan 21 at 8:09
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With some imports, you may have problems with Split Normals, which are the normals that define the shading. They are modified when you shade smooth, shade flat, mark sharp, use normal modifiers, etc... The more they split the harder the transition between faces will look.

First thing, display them:

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As you can see the split normals are completely split:

enter image description here

What you need to do is AltN > Reset Vectors:

enter image description here

enter image description here

Or you can directly go into the Properties panel > Object Data > Geometry and Clear Custom Split Normals Data:

enter image description here

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  • $\begingroup$ This is amazing moonboots! Thank you. That is exactly what was happening. Today I learned something thanks to you. $\endgroup$
    – Daniel Hdz
    Jan 21 at 8:42

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