Invisible geometry importing glTF with more than 4 bone weights

[punto-]

Godot version

v3.4.3.rc.custom_build [dd00219]

System information

windows 10, gles3

Issue description

When importing the attached glb, the geometry is invisible, seems to be empty, no vertices, When importing in 3.3 or 4.0, it loads correctly. The model has a skeleton and some animations, both seem to load fine

Steps to reproduce

import glb into project, load scene

Minimal reproduction project

bird.zip

[fire]

Can confirm.

Photo is with the fix.

The bug happens when the skeleton is at the root.

Attached fixed bird.

bird_rev.zip

[lyuma]

Well, I tried the model and got this error:

 modules/gltf/gltf_document.cpp:2684 - Condition "a.has("JOINTS_0") && a.has("JOINTS_1")" is true. Continuing.

This error means this model is not supported in Godot 3.4, because it uses more than 4 bone weights per vertex.

During the development of the glTF module, I remember that it was decided that models with more than 4 bones could not be reasonably imported (based on some chat conversations I had with @fire and others), because some pathological cases exist which require more than 4 bone weights to function. My opinion is that the top 4 bones could be selected and renormalized, but I would need to understand more to know if this is a viable approach.

I don't actually know how Godot <=3.3 dealt with models with >4 bone weights, such as JOINTS_0 and JOINTS_1. it's possible it just ignored JOINTS_1, but did it renormalize the remaining bones?

[fire]

My last attempt at normalizing bones is here. #48360

[fire]

Discussed what happens when the number of godot weights is above 8 like 16 weights with @punto-

1. want to use the same algorithm as normalizing 4 weights
2. normalize with the constants of 8 weights to solve more weights
3. can also make @reduz 's once per frame compute shader support arbitrary weights

[RockyMadio]

I can also confirm this. Either some rotation values are added into some bones (usually spines). After reimporting the gltf a few times, all meshes disappear.

[akien-mga]

Fixed by #61912.