How to fix hair particle shading/topology issue? There is no way to describe the issue, that's why I'm just going to show the rendered image:

Those are 4 planes with no modifiers, the same dimensions, the same particles, and the same material. The only difference is the topology. All planes are hidden - only the hair is visible.

Number 1 is a simple plane

Number 2 is a plane with loop cuts (Ctrl + R)

Number 3 is extruded (I started with a small plane and made it bigger with extrusions)

Number 4 is the same as Num.3 but the cuts are made with &quot;Knife projection&quot;




Numbers 1 and 2 are good, the issues are in 3 and 4
All faces have the same normals. I tied recalculating the normals, I also tried flipping them - nothing worked.
Just in case anyone asks - I'm making a big project and the model with the hair is very complex (I used extrusions, knife projections, and more) that's why I need to fix it without retopologizing the whole mesh. Because the project file is big I made this .blend file with the planes where I recreated the same issue.
Here is the .blend file:
I'm not sure I entirely understand it, but what I think is going on is that the hairs are set to have &quot;children&quot; and blender is applying a slight rotation to the kink direction of the child hairs via the &quot;Amplitude&quot; value of the kink curl. Setting the value to 0 removes the strange patterning, as does changing the kink type to &quot;nothing&quot;, &quot;radial&quot; and &quot;spiral&quot;. The other options still show artifacts. I still can't fully explain it, but at least this should make it &quot;go away&quot;, if that's what you're after.