data/model/flex/sphere_trilinear.xml

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<mujoco model="Trilinear flex spheres">
  <include file="scene.xml"/>

  <option solver="CG" tolerance="1e-6" timestep="1e-3" integrator="implicitfast"/>

  <size memory="10M"/>

  <visual>
    <map stiffness="100"/>
    <!-- The trilinear bounding box is visualized by turning on "Flex Tree" (mjVIS_FLEXBVH).
         In order to make the visualization clearer,
         we make the regular collision BVH visualization invisible: -->
    <rgba bv="0 0 0 0" bvactive="0 0 0 0"/>
  </visual>

  <worldbody>
    <geom type="box" pos="1.5 0 0.25" size=".5 2 .25"/>
    <geom type="box" pos="0 0 0.25" size="2 2 .05" euler="0 15 0"/>
    <flexcomp type="ellipsoid" count="8 8 8" spacing=".07 .07 .07" pos="-.5 -.5 1" dim="3"
              radius=".001" rgba="0 .7 .7 1" mass="5" name="soft" dof="trilinear">
      <elasticity young="1e3" poisson="0.49" damping="0.1"/>
      <contact selfcollide="none" internal="false"/>
    </flexcomp>
    <flexcomp type="ellipsoid" count="8 8 8" spacing=".07 .07 .07" pos="-.5 .5 1" dim="3"
              radius=".001" rgba="0 .7 .7 1" mass="5" name="rubber" dof="trilinear">
      <elasticity young="1e4" poisson="0.4" damping="3e-3"/>
      <contact selfcollide="none" internal="false" solref="-5000 -10"/>
    </flexcomp>
  </worldbody>
</mujoco>