Physics Interpolation is in conflict with MultiMesh (3D)

[Soydab]

Tested versions

• Reproducible in: 4.5.beta1
• Reproducible in: 4.4 and 4.4.1

System information

Godot v4.4.1.stable - Windows 11 (build 26100) - Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 5080 (NVIDIA; 32.0.15.7680) - AMD Ryzen 7 9800X3D 8-Core Processor (16 threads)

Issue description

When we tried to migrate our project from Godot 4.3 to Godot 4.4.1 we encountered issues with all of our MultiMeshInstance3D. When we want to interact with the MultiMesh (set_instance_transform) to change the position/rotation/scale the instances of the MultiMesh act weirdly. For example, we have a terrain (plane mesh) with grass on it, when we tried to modify one of the instance (one piece of grass), nothing happened (in the _ready func) or the instance got modified but all the others instances Transform3D were set to 0 (in the _process func).
We added comments in the script to explain.

It's working with the same context in Godot 4.3.
If like us you have this problem, you can disable Physics Interpolation in the Node property of the MultiMeshInstance3D (Inherit to Off) or in the project settings (globally) while waiting for a fix.

Example in 4.3 :

Example in 4.4.1 (_ready func):

Example in 4.4.1 (_process func):

Steps to reproduce

• Enable Physics Interpolation in your project settings
• Create a scene with a plane (MeshInstance3D), a Camera3D, Light, Environment
• Add a cube (MeshInstance3D) with a particular color
• Add a MultiMeshInstance3D
• Populate the plane with the cube as the mesh (amount of instances set to 15 in our tests)
• Try to use set_instance_transform in the _ready or _process (or use our examples projects/scripts) on few instances (if you set the amount to 15, try instance 1/9/12 for example)

Minimal reproduction project (MRP)

bugmultimesh_4_3.zip
bugmultimesh_4_4.zip

In the projects you will find :

• a script with the use case : test.gd
• a scene with the MultiMeshInstance3D (with all you need to reproduce the problem) : terrain.tscn

[lawnjelly]

Am AFK until tomorrow so cant try yet, but be sure to read the physics interpolation documentation, specifically the multimesh section.

For non moving, set the physics interpolation mode to OFF, for moving, call reset_physics_interpolation on the node AND the instances.

[lawnjelly]

This did turn out to be a bug - I've done a PR #108109 to fix. This will address the situation with moving MultiMeshes.

But actually (as you have figured out), in the situation of static MultiMeshes it is recommended to just turn off physics interpolation (via physics_interpolation_mode) for the branch. This is simpler for the user, and more performant. I've mentioned this in a few previous issues but must get it added to the docs, as it could be non-obvious to users.

[Soydab]

We looked at the documentation and we tried some things but we still have the bug so thank you for the PR 👍 .

We searched for a while before opening the issue and we didn't find any issue that matched our problem, so I assume we missed your comments about the bypass for this problem.

Again thank you !