CSGCylinder3D has incorrect normals compared to the CylinderMesh primitive

[Calinou]

See also #19143.

Godot version

4.0.dev (6d98f84), 3.3.2

System information

Fedora 34, GeForce GTX 1080 (NVIDIA 465.31)

Issue description

CSGCylinder3D has incorrect normals compared to the CylinderMesh primitive.

CSGTorus3D also has unexpected normals compared to a torus exported from Blender (due to a sharp line in the middle), but it's not as bad.

Top: CSG nodes (torus, box, sphere, cylinder) or meshes (Suzannes, sharp cylinder).
Bottom: MeshInstances with primitive meshes (box, sphere, smooth cyulinder) or OBJs exported from Blender (torus, Suzannes, sharp cylinder).

This also occurs on 3.3.2. CSGCylinder on top, MeshInstance with CylinderMesh at the bottom:

Steps to reproduce

• Create a CSGCylinder node and set the number of sides to 64 to match a CylinderMesh.
• Create a MeshInstance node, add a CylinderMesh to it.
• Place the MeshInstance above/below the CSGCylinder and compare the lighting on both.

Minimal reproduction project

• master: test_csg_normals.zip
• 3.x: test_csg_normals_3.x.zip

[devvoid]

CSGSphere has the same issue.

Taking a guess, this might be the CSG models not having merging vertices that share the same coordinates - you get similar errors in Blender if you use the spin modifier to make a circular object, but disable "Auto Merge". I've included an exported model in my test project to show what I mean.

From top to bottom: CSGSphere, MeshInstance sphere, UV sphere from Blender (made with the spin modifier, Auto Merge disabled).

CSGSphereNormals.zip

[unfa]

I have the same issue. Here's how the shading looks after subtracting a CSG cylinder from a block:

[unfa]

I suspect this is what's going on:
The CSG cylinder's curved shell's normals point away from the whole object's origin, rather than from the center of a ring of vertices they belong to (top or bottom).

Here's an illustration of what I suspect is going on:

Of course the normals are split on edges with the flat top and bottom faces, but I'm omitting these as they appear to be correct.

I have no idea how the code looks like, but maybe this is enough to help fix it?

[Calinou]

I looked briefly at fixing this, but it appears the CSG shape's normals are automatically generated with no way to manually override them:

godot/modules/csg/csg_shape.cpp

Lines 384 to 397 in 73ece5b

	Plane p(n->faces[i].vertices[0], n->faces[i].vertices[1], n->faces[i].vertices[2]);
	for (int j = 0; j < 3; j++) {
	Vector3 v = n->faces[i].vertices[j];
	Vector3 normal = p.normal;
	if (n->faces[i].smooth && vec_map.lookup(v, normal)) {
	normal.normalize();
	}
	if (n->faces[i].invert) {
	normal = -normal;
	}