[X3D-Ecosystem] Research into MuJoCo XML for soft body physics

[John Carlson]

Here's the Claude intro to the MuJoCo <flexcomp>  element:  If someone
knows what an MJCF element is, cool.

John

Based on my research, I can provide you with a comprehensive summary of the
MuJoCo flexcomp element and its children.
MuJoCo flexcomp Element Summary

flexcomp is a meta-element in MuJoCo's MJCF format that procedurally
generates deformable objects Readthedocs
<https://mujoco.readthedocs.io/en/stable/XMLreference.html>. It was
introduced in MuJoCo 3.0 as a convenience macro for creating deformable
objects, similar to how composite works for rigid structures Readthedocs
<https://mujoco.readthedocs.io/en/3.1.0/changelog.html>. Like other
meta-elements, flexcomp disappears from the model upon saving the XML and
is replaced with the generated low-level MJCF elements Readthedocs
<https://mujoco.readthedocs.io/en/latest/modeling.html>.
Purpose

flexcomp is designed to simulate soft objects like cloth, soft bodies, and
deformable materials, replacing older composite types that were used for
these purposes Readthedocs
<https://mujoco.readthedocs.io/en/latest/modeling.html>.
Main Attributes

>From the examples in the search results, flexcomp supports the following
key attributes:

   - *type*: Defines the type of deformable object. Common values include:
      - "grid" - creates a grid-based structure
      - "gmsh" - imports tetrahedral meshes from GMSH format files
      - "mesh" - uses mesh-based deformation
   - *name*: Identifier for the flexcomp object
   - *count*: For grid types, specifies the number of elements in each
   dimension (e.g., "10 5 1" for a 2D cloth)
   - *spacing*: Defines the distance between elements (e.g., ".025 .025
   .025")
   - *pos*: Position in 3D space
   - *radius*: Visual radius of the deformable elements
   - *rgba*: Color and transparency
   - *dim*: Dimensionality (1, 2, or 3) - 2 for cloth/shells, 3 for solid
   deformables
   - *mass*: Total mass of the deformable object
   - *file*: For gmsh or mesh types, specifies the mesh file to load
   - *material*: References a material asset for appearance

Child Elements

flexcomp supports several child elements:

   1. *<contact>* - Defines collision properties
      - condim: Contact dimensionality (3 or 6)
      - solref: Solver reference parameters
      - solimp: Solver impedance parameters
      - selfcollide: Controls self-collision behavior
      - friction: Friction coefficients
   2. *<edge>* - Controls edge behavior
      - equality: Whether edges act as equality constraints (typically
      "true")
      - damping: Damping coefficient for edges
   3. *<plugin>* - Specifies elasticity model and material properties
      - Most commonly uses plugin="mujoco.elasticity.solid" (for 3D
      volumes) or plugin="mujoco.elasticity.shell" (for 2D surfaces)
      - Child <config> elements set material properties:
         - key="poisson" - Poisson's ratio
         - key="young" - Young's modulus (in Pascals)
         - key="thickness" - For shell elements, material thickness
         - key="damping" - Material damping

Example Usage

A typical cloth simulation might look like:
xml

<flexcomp type="grid" count="10 5 1" spacing=".025 .025 .025"
mass="0.1" name="cloth" radius="0.001" dim="2">
  <contact condim="6" solref="0.001"/>
  <edge equality="true" damping="0.1"/>
  <plugin plugin="mujoco.elasticity.shell">
    <config key="poisson" value="0"/>
    <config key="thickness" value="10e-3"/>
    <config key="young" value="3e4"/>
  </plugin></flexcomp>

A 3D soft body example:
xml

<flexcomp type="grid" count="5 5 5" spacing=".02 .02 .02"
pos="0 0 0" radius="0" rgba="0 .7 .7 1"          name="softbody"
dim="3" mass="1">
  <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001"
selfcollide="none"/>
  <edge damping="1"/>
  <plugin plugin="mujoco.elasticity.solid">
    <config key="poisson" value="0.2"/>
    <config key="young" value="5e5"/>
  </plugin></flexcomp>


On Wed, Dec 10, 2025 at 5:00 AM John Carlson <yottzumm at gmail.com> wrote:

> Poncho "IndexedTriangleSet" for MuJoCo:
>
> <flexcomp name="f1" type="direct" rgba=".8 .2 .2 1" radius="0.01" dim="2"
> pos="0 0 2"
>               mass="1"
>          point="...MFVec3F..."  (Coordinate.point)
> element="...MFInt32..." (IndexedTriangleSet coordIndex?)
>
>     <edge equality="true" damping="0.1"/>
>       <elasticity young="3e5" poisson="0" thickness="8e-3"
> elastic2d="bend"/>
>       <contact solref="0.003"/>
>     </flexcomp>
>
> Help converting this to something that works for X3D IndexedTriangleSet
> welcome.
>
> Hammock grid:
>
>  <flexcomp name="hammock" type="grid" count="11 9 1" spacing="0.2 0.2 0.2"
> material="hammock"
>               radius="0.04" dim="2">
>       <pin id="0 8 90 98"/>
>       <edge equality="true" solref="0.002 5"/>
>       <contact selfcollide="none" internal="false" solimp=".99 .999
> .00001"/>
>     </flexcomp>
>
> <!-- Copyright 2021 DeepMind Technologies Limited
>
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>      you may not use this file except in compliance with the License.
>      You may obtain a copy of the License at
>
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>
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> -->
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