OBJ import has broken smooth normals on certain files since 4.0.beta16

[Calinou]

• Related to Normals in imported 3D model look different when importing obj vs other formats #38854.

Godot version

4.0.beta16, 4.1.2.stable, 4.2.beta 6afd320

System information

Fedora 38, GeForce RTX 4090 (NVIDIA 535.113.01)

Issue description

OBJ import has broken normals on certain files in 4.0.beta16 and any later version, but 4.0.beta15 and earlier are not affected by this issue. Notice how certain parts of the scene are overly smoothed out:

OBJ 4.0.beta15	OBJ 4.0.beta16 and later	glTF import	Blender OBJ import

In comparison, glTF import in Godot or OBJ import in Blender restitutes normals correctly (the source file is 100% flat-shaded).

This occurs regardless of the import mode chosen (Mesh or OBJ as Scene), and regardless of whether scaling options are used at import.

PS: Why is the OBJ import being smoothed out automatically anyway (even in 4.0.beta15 where it usually looks good enough)? I'd expect this to be something you have to explicitly opt into (along with a max angle setting). Disabling Generate Tangents and Optimize Mesh doesn't affect this.

Steps to reproduce

• Import a 3D OBJ file as a mesh or scene.
• For comparison purposes, import this OBJ file in Blender (using either the standard or legacy importer¹) then export it as glTF. Import that glTF scene in Godot.

Minimal reproduction project

test_obj_gltf_import_normals.zip
Open octavus_obj.tscn for testing. Make sure to remove .godot/ after switching Godot versions.

Footnotes

1. I used the legacy importer in my tests, but the results should generally be identical with the standard importer. ↩