Lighting of GLTF models with negative scale is inverted

[adamwych]

Update https://github.com/bevyengine/bevy/issues/7901#issuecomment-1454984501

Bevy version

0.9.1 / main branch at f87de368436a1a65aecb8e0088d93244dd5a467c

Relevant system information

AdapterInfo { name: "NVIDIA GeForce GTX 1080 Ti", vendor: 4318, device: 6918, device_type: DiscreteGpu, driver: "NVIDIA", driver_info: "528.49", backend: Vulkan }

What you did

I made a model in Blender, exported it to GLTF and imported it inside my Bevy application using SceneBundle:

let box_dude = assets.load("box_dude.glb#Scene0");

commands.spawn(SceneBundle {
    scene: box_dude,
    ..default()
});

commands.spawn(Camera3dBundle {
    transform: Transform::from_xyz(12.0, 36.0, 12.0).looking_at(Vec3::ZERO, Vec3::Y),
    ..default()
});

commands.spawn(PointLightBundle {
    transform: Transform::from_xyz(4.0, 2.0, 4.0),
    point_light: PointLight {
        color: Color::WHITE,
        shadows_enabled: true,
        ..Default::default()
    },
    ..Default::default()
});

What went wrong

Outside faces of box_dude are not lit up:

If you zoom in, you can see that the edge of the face is kinda lit up, but I guess this is just some bleeding due to the camera being far away and it's actually the inside that's lit up.

If I move the box_dude to be inside the point light, you can see that his head's outside faces are lit up, which is incorrect.

I tried flipping the normals and re-exporting, but it does not change anything.

The same file looks correct in online GLTF viewers:

Additional information

Here's the .glb file I used: https://www80.zippyshare.com/v/4UrsUvvL/file.html

[james7132]

Just to be sure, can you reproduce this on main? 0.10 is just about to release.

[adamwych]

Update (another update below!): I fixed it using https://www.gltfeditor.com/ to flip the normals instead of Blender:

I guess Blender is not flipping the normals correctly for some reason, but the issue still stands, since now the normals are actually inverted (going towards inside), while they were correct in the original file and now it looks wrong in other viewers.

@james7132 I used 0.9.1, I'll have to build 0.10 first. I'll try that in a moment.

[adamwych]

@james7132 Also happens on main at f87de368436a1a65aecb8e0088d93244dd5a467c:

[adamwych]

Update 2: Using "Apply Transform" on the entire object in Blender fixes the issue.

I did some experiments and figured out that while modeling, I must have mirrored everything at some point. Blender implements mirroring by simply setting the scale to the negative of whatever it currently is and that's where the issue was - the scale of my model was negative so my lighting was "negative" too.

I understand the issue now, but maybe there should be some mechanism in place in Bevy to fix those kinds of models during import? Other apps didn't have such problem with this file and it might be very confusing to someone in the future.

[janhohenheim]

I can confirm that this still happens on 0.10, since I just spent a while debugging it :/

[tim-blackbird]

Perhaps setting double_sided to true on the material would fix it. Not really sure why we don't enable it by default.