Have some shadow problem in cycles render (low poly mesh with normal map)

Question

sorry for my poor English first..

this is my low poly mesh without textures

and in marmoset toolbag 3 or blender eevee, the normal map works well

but in cycles, light causes strange bar shadow result(red arrow part), looks like the normal map comes into no effect.

Can someone help me how to fix this? thank you!

.blend file here!

https://drive.google.com/file/d/1QjGzhOqGrY05mmHspnv0MyzeH0I0pIOX/view?usp=sharing

Answer 1

To add to what @moonboots already said:

A normal map can basically be thought of as a high-performance way of telling the render engine "Hey, that light ray your shooting at this mesh? it'd look alot cooler if instead of bouncing away at this really sharp angle, it bounced away at a different softer angle"

How the mesh normally works while rendering.

What happens when you use a normal map:

Then, when you introduces smooth shading into the mix this is what happens:

Put a normal map on top of it and it gets even messier, You can see that there are parts of the mesh where the normal map has to do a ton of heavy lifting to get the mesh to look how it thinks it should. This is where those gradients are coming from.

So ultimately you have one of two options: Either split the normal with Auto Smooth and let the normal map do it's own thing entirely. This has it's drawbacks. Each split edge on the mesh has to be on it's own UV island or it will cause other normal map issues. From a technical standpoint it also turns each vertex on a split edge to two, which can increase poly count and slow performance (though this doesn't really matter much anymore these days).

Or you can bevel the edges slightly, and use weighted normals to find the happy medium between the two.

You want to pick one or the other though. Other wise you'll keep getting your render engine confused like this.

I would recommend reading all of EarthQuake's threads on normal map baking on polycount. I probably read them about 15 times apiece until I really "got" them.

Answer 2

Edit: So you have bad split normals, which gives this bad shading, and besides that the normal map is particular as the baking must have occured when you already had split normals edited, see the gradient, it creates the bad shading:

If I disable the Auto Smooth or if I Clear Custom Split Normals Data it won't give the best result, so maybe the best thing to do is to rebake from the high-poly onto the low-poly, but this time clear the custom split normals of the low-poly before baking.