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The model is a modified cylinder by moving some of its faces upward along Z axis. It has a shading issue shown on the following screenshot. How to fix this please?

enter image description here

The following screenshots are what I see after checking the solution from KISKAart, who proposed to use knife tool to break the N-gon into quads, so that the shading issue could be fixed in a way.

However, the knife cuts and additional loop cuts cause new shading problems:

enter image description here

enter image description here

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  • $\begingroup$ You can use the knife tool and connect vertices to make quads. $\endgroup$
    – KISKAart
    Apr 13 at 1:36
  • $\begingroup$ But, do those quads fix the shading please? $\endgroup$ Apr 13 at 2:18
  • $\begingroup$ Yes, they do, shading problems are caused by n-gons and smooth shading, Blender doesn't know how to smooth them, that's why you get shading artefacts. $\endgroup$
    – KISKAart
    Apr 13 at 4:32
  • $\begingroup$ No, those quads do NOT fix this shading problem. If you believe I am wrong, then please provide your answer to prove I am wrong. Thank you! $\endgroup$ Apr 13 at 4:37
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    $\begingroup$ @BlenderLake unfortunately your model has bad topology. It lacks the edge flow to support subdivision modifier. You have to remodel it to have that proper edge flow. $\endgroup$ Apr 13 at 10:30

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The current edge flow of your model does not meet the requirements to effectively support a Subdivision modifier, leading to shading issues. You could try to remodel it like this then just adjust the supporting edges where you want more or less sharpness.

enter image description here

This is the result without shading issues:

enter image description here

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    $\begingroup$ I really should have said that the topology was really bad instead of trying to fix it. $\endgroup$
    – KISKAart
    Apr 13 at 13:32
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    $\begingroup$ Nice result! Concise and educational. $\endgroup$ Apr 14 at 6:31
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So what I did was to remove the incorrect support edge loops that form n-gons, add some new ones and crease the edges. Your question was "how do I make quads to fix this shading problem", you can do this with the knife tool if the mesh isn't too complex.
Here's how the fixed mesh looks with and without creases:
With creases Without creases

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  • $\begingroup$ Thank you for your answer! However, after checking your solution, I see new shading problems. My original question has been updated to show what the new shading problems are. $\endgroup$ Apr 13 at 9:18
  • $\begingroup$ @BlenderLake I have updated the file, you just have to add creases where you see these 'shading problems' if you ever need to. $\endgroup$
    – KISKAart
    Apr 13 at 9:29
  • $\begingroup$ The edge creases cause the edges to be overly sharp. Consequently, subd modifier can no longer be effective to bevel those sharp edges. Moreover, even with edge creases, the shading of this model still has issues. Thanks! $\endgroup$ Apr 13 at 10:28
  • $\begingroup$ @BlenderLake You can always change how sharp the edge is, if my answer was helpful in any way, you can accept it instead of thanking me. $\endgroup$
    – KISKAart
    Apr 13 at 10:50
  • $\begingroup$ If you let me change how sharp the edge is, then I will set the sharpness = 0, which means all those edge creases are deleted, because non-zero sharp edges cause subd to have hard time to bevel the edges. And, I can not accept your answer, because your answer did not solve the shading issue. I have already awarded you 10 reputation points, and that is the maximum I can give to you. $\endgroup$ Apr 13 at 11:50

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