set_surface_override_material is out of bounds when a softbody3d mesh has more than one material slot

[6b9BVecLBDvbEx]

Tested versions

reproducible in v4.4.1 and v4.5-dev3

System information

Fedora Linux 41

Issue description

debugger output:
E 0:00:00:510 set_surface_override_material: Index p_surface = 1 is out of bounds (surface_override_materials.size() = 1). <C++ Source> scene/3d/mesh_instance_3d.cpp:353 @ set_surface_override_material()

1.mp4

Steps to reproduce

1. use blender to create a simple mesh with 2 faces and 2 different materials for each face
2. attach this mesh to a softbody3d node in godot

Minimal reproduction project (MRP)

softbody.zip

[smix8]

A softbody mesh is single surface, it only reads surface index 0 at scene start to create the special dynamic update soft body mesh.

Why it still tries to copy the materials for nonexisting surfaces is a mystery.

[6b9BVecLBDvbEx]

I guess this loop is causing the problem

godot/scene/3d/physics/soft_body_3d.cpp

Lines 499 to 501 in 8f78e75

	for (int i = copy_materials.size() - 1; 0 <= i; --i) {
	set_surface_override_material(i, copy_materials[i]);
	}

Still, it's really annoying that I can't have more than one material on a softbody. Is there any technical reason behind this limitation? Can this be a feature request?

[smix8]

On the physics side because it is a single physics body. On the rendering side because it uses the mesh surface buffer directly.

Merging all the mesh surfaces to one wouldn't be that much of a problem but multi-materials ... nothing but problems. You need to remap all that geometry all the time and also end up juggling multiple buffers.