Shading errors on gltf imports

[Faradax]

Godot version

v4.2.beta1.mono.official [b137180]

System information

Godot v4.2.beta1.mono - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2070 SUPER (NVIDIA; 31.0.15.3699) - Intel(R) Core(TM) i9-9900 CPU @ 3.10GHz (16 Threads)

Issue description

I'm using blender to export assets as gltf. When importing gltfs into Godot 4.2, I observe shading issues where parts of surfaces are darker or lighter in odd spots.
As a minimal example, I can even reproduce this with blenders "default cube" as it appears at the startup of the application without any modifications to either mesh or material. Checking "Data>Mesh>Tangets" on export does not affect the outcome.

In the following two pictures, the left cube is the imported blender default cube, the right one is a MeshInstance3D with a 2x2x2 BoxMesh for comparison.

v4.2.beta1.mono.official [b137180]

When I import the same gltf in v4.1.1.stable, the result looks correct, so I assume this to be a regression:
v4.1.1.stable.mono.official [bd6af8e]

Other simple examples of some subdivided/beveled cubes, as seen in the "Advanced Import Settings" dialog in v4.2.beta1.mono.official [b137180]:

Steps to reproduce

• Export blender default cube as gltf
• Import in v4.2.beta1.mono.official [b137180]

Minimal reproduction project

N/A

[clayjohn]

Can you try reimporting with force disable compression enabled in the import settings?

[Faradax]

Enabling the "Force disable compression" option gives this difference (left with option disabled, right with option enabled):

v4.2.beta1.mono.official [b137180]

So it does indeed seem to prevent this issue.

[stevenctl]

Force disable makes a difference, things still look odd.

Edit: it must be due to some weird geometry (procgen in blender -> gltf -> godot)

Screenshots

Force disable off:

Force disable on:

Here's 4.1.1

The wireframe of one of the messed up meshes:

[clayjohn]

@stevenctl are you able to share that mesh?

[stevenctl]

bugmesh.zip

Looks like verts are duplicated on export from Blender. Trying to merge vertices and re-export/import yields the same separation of verts. Importing to 4.1.1 looks ok though.

[Faradax]

For the mesh @stevenctl provided, I get a correct-looking import if I disable "Ensure Tangets". "Force Disable Compression" doesn't affect the result after that.

[lyuma]

Is this the same issue as #84270 ?

Is this fixable in cases of incorrect normals or are we hitting some limit of mesh compression?

Also, is there a way to detect that we have significantly distorted normals with mesh compression and warn the user, disable compression or fail the import in such cases?