Incorrect winding

[agg23]

It seems like at least some generated models have incorrect winding, which conflicts with the intended counter-clockwise winding order. This results in broken rendering in those scenarios.

See also warrenm/GLTFKit2#59

[djeedai]

Hi @agg23, weldr doesn't do any rendering or even any primitive manipulation, only parsing. And I think you found already that the issue was in your Blender file anyway? Closing this, please feel free to reopen if you think this is incorrect. Thanks!

[agg23]

I don't understand. weldr-bin converts and places LDRAW primitives into a 3D environment in GLTF format. Thus while the winding problems probably come from the actual LDRAW models, the parser isn't properly handling it in a GLTF spec compliant manner, causing problems in many different 3D softwares.

[djeedai]

Oh sorry I was thinking about the library. Let me reopen this. It's been a while since I looked at this project 🥲

[agg23]

Thank you.

I am very naive about doing 3D stuff; I briefly looked through the GLTF generation code and didn't see anything obvious that would make fixing the winding easy. I'm guessing that after the polygons of a model are extracted, there would need to be a pass to rearrange their vertices, but again I don't really know what I'm doing.

Not having looked at it, I wonder if the gltf crate (which you mentioned looking at moving to) would fix the winding for us, given it's part of the spec, but I doubt it.

[djeedai]

Yes it's very possible my old and unexperienced self writing his first Rust crate did a mistake in windings. I'm not sure the gltf crate would help much, my guess is that the LDraw file doesn't care about winding and the weldr-bin conversion code just outputs triangles with a random winding which happens to work most of the time by luck maybe. I'd have to dig into the bug.

[djeedai]

@agg23 do you have a sample LDraw file exhibiting the issue?

[agg23]

A single 2x4 will show the issue: Brick.ldr.zip

[djeedai]

@agg23 I can't repro on main, I however found another bug which I think is not present in the version you're using because it was skipping some faces of the 2x4 brick. Could you please build from sources and try again? There were quite a few changes with the merging of MPD support so it's possible this is already fixed.