Vulkan: Automatic mesh LOD always picks lowest level of detail in orthogonal camera

[TokageItLab]

Godot version

4.0.dev (2f57a11)

System information

MacOS 11.6

Issue description

Meshes are broken in Orhtogonal view. Is it because of Auto LOD?

Steps to reproduce

Put on glb model in 3D scene, use orthogonal view.

Minimal reproduction project

broken_mesh.zip

[Calinou]

I can confirm this on master fc09d78.

This is caused by automatic LOD, as disabling it in the advanced debug draw mode options using a viewport's Orthogonal menu will work around the issue:

This option only applies in the editor. To disable LOD in the running project, set the rendering/mesh_lod/lod_change/threshold_pixels project setting to 0.0.

[orosmatthew]

I did a little investigation and found this is because the preview orthogonal camera is placed ~2000m away from the origin thus resulting in a low auto LOD. Instancing a Camera3D set to orthographic works as expected and lowers LOD when moved farther away.

2022-01-29.13-10-15.mp4

I'm not completely sure what the solution would be. Either bring the preview orthographic camera closer to the origin (which is not a great solution) or disable auto LOD only when the preview camera is used and not instanced by a node. Thoughts?

Edit:
I'm actually not even sure that auto LOD even makes sense for an orthographic camera. Might be worth just having it not be taken into account during rendering when an orthographic camera is used.

[TokageItLab]

We may need to consider the effect not only on Automatioc LOD, but also on the MSFT_LOD that glTF has.

[Calinou]

I'm actually not even sure that auto LOD even makes sense for an orthographic camera. Might be worth just having it not be taken into account during rendering when an orthographic camera is used.

Automatic LOD calculations could be made to work with an orthogonal camera, but it would have to be based on the effective screen size occupied by the object rather than distance. This is in fact how it works in perspective mode already, but I suppose the calculation is broken in orthogonal mode.

In general, orthographic cameras require different distance formulas for shaders and other effects to still look correct.

[orosmatthew]

Right now, there isn't any differentiation in the code with perspective and orthogonal cameras. It just looks like it's based on distance, which inherently is proportional to screen size in perspective but not in orthogonal. I tried just disabling auto LOD in orthogonal and it does "fix" the issue now if I were to submit a PR, but it might just be better to recalculate how auto LOD is determined with screen size instead.

Edit: nvm, it does look like it does take screen space into account, but also distance. I will experiment but maybe ignoring distance in an orthographic camera and only taking into account screen space is the right way to go

[Calinou]

See also #54885.