Flat-shaded OBJ mesh is imported with more vertices in Godot 4

[Zylann]

Godot version

Godot 4 beta10

System information

Windows 10 64 bits NVIDIA GeForce GTX 1060

Issue description

In my game I process meshes to generate some data needed for some features. After porting it to Godot 4, some weird issues appeared and I realized Godot 4 is not sharing vertices of triangles in flat faces. Instead, they are ALL duplicated. At least, that's what I see when I inspect vertex data with surface_get_arrays. I do expect some vertices to be duplicated if they are on different faces (with different normals for example), but I don't expect it to be the case on every single triangle when the mesh is a quad for example.

This not only changes the behavior of my game since vertices supposed to be shared no longer are, but it also makes indexed vertices completely useless, because flat-shaded meshes end up with all triangles being individual. In Godot 3, triangles within a flat face were shared.

Steps to reproduce

Make a plane in Blender, export it as OBJ, import it in Godot, compare vertex arrays using a script to print the results of surface_get_arrays.

Result I get in Godot 3:

4 vertices, 6 indices
Vertices: 
(1, 0, 1)
(-1, 0, 1)
(1, 0, -1)
(-1, 0, -1)
Indices: 
0
1
2
2
1
3

In Godot 4:

6 vertices, 6 indices
Vertices: 
(1, 0, 1)
(-1, 0, 1)
(1, 0, -1)
(1, 0, -1)
(-1, 0, 1)
(-1, 0, -1)
Indices: 
0
1
2
3
4
5

Minimal reproduction project

Godot 3: PlaneImport_Godot3.zip
Godot 4: PlaneImport_Godot4.zip

Run each one's main scene, compare what is printed.

[Calinou]

Can you reproduce this with glTF or Collada import?

[Zylann]

It does not reproduce with gLTF.

[Calinou]

It may be worth checking if this occurs after disabling vertex compression in the Import dock.