Error "XML Error: Element size must be a multiple of dim+1 mujoco"

[Nick20500]

Intro

Hi!

I am a undergrad student at ETH Zurich, I use MuJoCo for my research on Multimaterial deformable objects.

My setup

I use MuJoCo 3.2.3 with python bindings.

My question

When loading the file "parts_3800_3.xml" in the mujoco viewer, the error
"XML Error: Element size must be a multiple of dim+1 mujoco
Element 'flexcomp', line 46"
is displayed.

This error should imply, that the file "part-1.41.msh" has an error, i.e. one of the $Elements$ hasn't the right format/dimension.
But after manual inspection, manual comparison of the files "part-0.41.msh" and "part-1.41.msh" and automated inspection via python script, I can't find an error in the "part-1.41.msh" file, nore a difference in the format between the two files. And by the mujoco viewer, the file "part-0.41.msh" is correct since I can view it and the error only appears for the "part-1.41.msh" not for the "part-0.41.msh".

When I remove the flexcomp with the name "part 2", I can view the "parts_3800_3.xml" without any error in the mujoco viewer.
I am also able to load all 3 .msh files referenced in the "parts_3800_3.xml" file, in the gmsh viewer without any problems.

Is there actually a problem with the file "part-1.41.msh" which I and my code have overlooked or is there a bug in mujoco?

I have attached all 4 files in the follwoing zip folder:

mujoco_files_from_issue.zip

Thank you for your help.

Minimal model and/or code that explain my question

Model:
mujoco_files_from_issue.zip

<!-- 3 PARTS
    
    created by Sebastian Heckers on 25.04.2024
    
    Example of multi-material deformable objects with 3 parts/meshes.
    With Bottle (ID 3800), body, neck and lid.
-->

<mujoco model="Sphere Drop">

    <include file="../drop/scene.xml"/>


    <asset>
        <texture name="tex" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .1 .9" rgb2=".9 .3 .3"/>
        <material name="tex" texture="tex" texrepeat="1 1" texuniform="true" reflectance=".2"/>
    </asset>

    <!-- set default units -->
    <compiler angle="radian" meshdir="../../../assets/"/>

    <extension>
        <plugin plugin="mujoco.elasticity.solid" />
    </extension>

    <compiler autolimits="true"/>

    <option solver="CG" tolerance="1e-6" timestep=".0001"/>

    <size memory="240M"/>

    <worldbody>

        <flexcomp name="part1" type="gmsh" file="partnet/3800/part-0.41.msh" rgba=".8 .2 .2 1" pos="0 0 5" euler="0 0 -1.57075" mass="500" scale="3 3 3" >
            <!-- <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001" selfcollide="none" friction="5 0.5 0.01"/> -->
        
            <plugin plugin="mujoco.elasticity.solid">
                <!--Units are in Pa (SI)-->
                <config key="young" value="1e5"/>
                <config key="poisson" value="0.45"/>
                <config key="damping" value="0.0"/>
            </plugin>

        </flexcomp>

        <flexcomp name="part2" type="gmsh" file="partnet/3800/part-1.41.msh" rgba=".2 .2 .8 1" pos="0 0 5" euler="0 0 -1.57075" mass="500" scale="3 3 3" >
            <!-- <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001" selfcollide="none" friction="5 0.5 0.01"/> -->
        
            <plugin plugin="mujoco.elasticity.solid">
                <!--Units are in Pa (SI)-->
                <config key="young" value="1e7"/>
                <config key="poisson" value="0.45"/>
                <config key="damping" value="0.0"/>
            </plugin>

        </flexcomp>

        <flexcomp name="part3" type="gmsh" file="partnet/3800/part-2.41.msh" rgba=".2 .8 .2 1" pos="0 0 5" euler="0 0 -1.57075" mass="500" scale="3 3 3" >
            <!-- <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001" selfcollide="none" friction="5 0.5 0.01"/> -->
        
            <plugin plugin="mujoco.elasticity.solid">
                <!--Units are in Pa (SI)-->
                <config key="young" value="1e8"/>
                <config key="poisson" value="0.45"/>
                <config key="damping" value="0.0"/>
            </plugin>

        </flexcomp>

    </worldbody>

</mujoco>

Code:

import mujoco
import mediapy as media
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

Confirmations

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

[quagla]

Do the meshes contain only tetrahedra?

Cc @mohammad200h as he knows more about the GMSH format.

[Nick20500]

The statistics, which can be viewed with gmsh, confirms that there only should be tetrahedra.

[quagla]

Have you tried to set https://mujoco.readthedocs.io/en/latest/XMLreference.html#body-flexcomp-dim in the XML?

[mohammad200h]

You need to set dim=3 in xml.
Here is a simplified model that loads.

mujoco_model_solution.zip

if you look at the content of part-0.41.msh:

$Nodes
1 1663 1 1663
3 0 0 1663

you can see 3 0 0 1663.
3 indicates that this is a volumetric mesh. Therefore you need to set the dim=3 in xml.