Unity Import MuJoCo Scene Error

[Lrogic]

Intro

Hi!

I am a graduate student at UBC using MuJoCo for robotics research in Unity. I am trying to import a Unitree G1 MJCF model into the MuJoCo Unity plugin and am running into mesh loading failures during import.

My setup

MuJoCo version tested: 3.6.0 and 3.3.7

API: MuJoCo Unity plugin

OS: Windows 11 Pro

Architecture: x64

Unity Editor version: 6000.3.9f1

What's happening? What did you expect?

When importing an MJCF model with mesh assets in the MuJoCo Unity importer, the import fails with:

IOException: Error loading the model: Error: no decoder found for mesh file '.../pelvis.STL'
Element name 'pelvis', id 0, line 20
Mujoco.MjEngineTool.LoadModelFromFile (...)
Mujoco.MjImporterWithAssets.ImportFile (...)
Mujoco.MjImporterEditorWindow.Apply ()

I expected the model to import successfully, or at least for supported mesh formats such as STL / OBJ to be recognized by the decoder system.

I tried the following:

• MuJoCo 3.6.0
• MuJoCo 3.3.7
• original .STL meshes
• converting the STL meshes to binary STL
• converting meshes to .obj
• using content_type="model/stl" and content_type="model/obj"

Steps for reproduction

Steps for reproduction

1. Create a blank Unity project.
2. Add the MuJoCo Unity package from a tagged MuJoCo repo checkout. I tried both 3.6.0 and 3.3.7.
3. Copy the matching Windows mujoco.dll into C:\Users\YourName\MuJoCo as described in the docs.
4. Open the MuJoCo importer in Unity.
5. Clone the unitree_mujoco repo and try to import this MJCF file: https://github.com/unitreerobotics/unitree_mujoco/blob/main/unitree_robots/g1/g1_23dof.xml.
6. Observe the import failure with no decoder found for mesh file.
7. I also reproduced the same behavior after converting the referenced meshes to OBJ and updating the MJCF accordingly.

Minimal model for reproduction

Clone the unitree_mujoco repo and try using this xml file: https://github.com/unitreerobotics/unitree_mujoco/blob/main/unitree_robots/g1/g1_23dof.xml

unitree_mujoco-main.zip

Code required for reproduction

No custom code is needed beyond using the Unity importer UI.

The failure occurs when clicking import/apply in the MuJoCo Unity importer.

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[YashwantMoharil]

I am having the same problem
@Balint-H any idea?

[Balint-H]

Will look into this tomorrow! STL and OBJ meshes are expected to be supported, so this looks like a bug

[Balint-H]

Hello, thanks for the bug report!

I think I found the culprit! We currently detect if an STL file is an ascii file or not if it starts with the string "solid". This is apparently a common way to test for this but it is not foolproof , as some binary STLs are exceptions to this rule, including the unitree meshes.

Reenconding the file could fix it (e.g. with Blender). Converting it to OBJ was also a good idea, but apparently for an unrelated reason the OBJ mesh parsing seems to have broken in the last couple releases. I'll look into that!

I'll also include a more robust way to check for an ASCII stl.

Immediately now, you can try loading and resaving your mesh file with e.g. blender, which in my tests that seems to resolve it. Please save with a lowercase file extension ".stl". Case sensitivity is another idiosyncrasy to fix in the next PR.

Alternatively, you can maybe check out this branch where the obj importer last worked, and convert the files to obj (importantly, this branch uses MuJoCo 3.2.7 so don't forget to verify you have that version of the library if you are switch to it!):
https://github.com/Balint-H/mujoco/tree/feature/unity-obj

[Lrogic]

Hi @Balint-H, thanks for the reply!

I tried two different approaches based on your suggestions:

---

1. MuJoCo 3.6.0 + binary STL conversion

First, I converted all the Unitree meshes from ASCII STL to binary STL using PyMeshLab. I also renamed them to use lowercase .stl instead of .STL. My setup was:

• MuJoCo 3.6.0
• Windows 11
• Unity 6
• Same setup as described in my original post

Unfortunately, even after converting all meshes to binary, I still get the same error:

Error: no decoder found for mesh file '.../pelvis.STL'

---

2. MuJoCo 3.2.7 + feature/unity-obj branch

Since that didn’t work, I then tried your alternative approach:

• Checked out the feature/unity-obj branch
• Used MuJoCo 3.2.7 as mentioned
• Switched to .obj meshes instead of .stl

With this setup, the robot loads successfully in Unity.

However, the rendering seems broken, the entire robot appears pink:

---

Questions

• Do you have any idea what might still be causing the decoder issue with MuJoCo 3.6.0 + binary STL files?
• And separately, any idea why the robot appears fully pink when using the .obj workflow with MuJoCo 3.2.7?

---

Thanks again for the help!

[Balint-H]

Hello!

In the meantime @havess cleared it up that this is caused by mujoco now treating mesh decoding as extendable plugins, but that didnt get reflected in the Unity plugin. It is expected to be solved in the next release!

The pink material is due to the default material using the old built-in renderer of Unity, which is being phases out. By inspecting the geom object and its associated material, you can change the material's shader to use the "Lit" shader of whichever render pipeline your project is using (likely the universal render pipeline).

Thats what I did in my screenshot as well, sorry for not mentioning it!

[Lrogic]

Thanks for the quick reply! I have just two final questions:

1. Do you have an estimated release date for the version that includes the decoder fix?
2. I updated all the materials to use the Universal Render Pipeline/Lit shader, but now the robot appears completely white, as shown in the screenshot below. This isn’t a functional issue, of course, but now I’m just curious how you managed to get the colours to display correctly in your simulator.

Thanks again!

[Balint-H]

Releases are roughly about once a month. While of course it is good to have the recent fixes and features it is not usually necessary. For using the Unity plugin I recommend sticking with a stable version and only upgrading if there is a new feature or fix available that you need.

In the inspector, when you select the material, below where you can change the shader you have options to configure the material. You can edit PBS properties, add textures and heightmaps or change its albedo color. If you click on the white rectangle that you see next to the Base map field, you will be able to edit it with a colour picket tool!

Playing around with materials is one of the nice features you have access to with Unity!

You also have the option of just picking one materials and assign it to multiple parts of the robot (e.g. by drag and dropping it in the scene view)

[Balint-H]

#3202 I think now fixes the extension case sensitivity, and uses URP by default (as the built-in pipeline is getting deprecated). Newly imported scenes should have materials with compatible shaders now!