Alessio
2a3554c8a3
Contact of a flex with `passive` collisions enabled was applied as an explicit spring of fixed stiffness 1e4, which the timestep bounds: any stiffness worth having oscillates faster than the step can resolve, so the force was too soft to keep sheets apart and interpenetration was routine. Carry its curvature in the effective metric M + K instead, alongside the flex's own stretch and bending stiffness. The contact block k*J^T*J is appended to the per-vertex candidate list already assembled for the flex stencils, so it costs additional entries in an existing matrix rather than a new one, and the accompanying shift -h*K*v is what damps the stiff modes. At a 2 ms timestep this holds roughly 50x the stiffness an explicit force of the same step could. With the timestep no longer setting the bound, the stiffness is chosen as a natural frequency scaled by the participating vertex mass rather than left at a fixed 1e4, so one value suits models of any scale. Passive handling is scoped to contacts whose every dof is a flex vertex carried by the metric: flex against flex, flex against itself, and flex against static geometry, which contributes no dofs of its own. For those the Hessian is assembled in full. Contact with a body that can move would have that body's dofs dropped from it, and is left on the constraint solver. The feature now requires an integrator whose constraint solve runs in that metric, and is rejected with an error otherwise. Add model/flex/drape.xml as the example model, replacing sphere_passive, whose contacts no longer demonstrated the feature.
53 lines
2.6 KiB
XML
53 lines
2.6 KiB
XML
<!-- Copyright 2026 DeepMind Technologies Limited
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<mujoco model="Drape">
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<include file="scene.xml"/>
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<!-- Three cloths dropped over a sphere. Every contact here is handled by the passive path: cloth
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against cloth, each cloth against itself, and cloth against the sphere, which is static and
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so contributes no dofs of its own. Their stiffness is carried implicitly by the effective
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metric, which is why the sheets rest on each other without interpenetrating. -->
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<!-- The metric solve for qacc_smooth is iterative and its blocks do not see the vertex-to-vertex
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coupling that contact introduces, so a contact-rich scene like this one needs a larger
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iteration budget than the default to converge it. -->
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<option timestep="0.002" solver="CG" tolerance="1e-6" iterations="400" integrator="implicitfast"/>
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<size memory="50M"/>
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<worldbody>
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<geom name="ball" type="sphere" size=".3" pos="0 0 .3" rgba=".45 .45 .5 1"/>
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<flexcomp type="grid" count="13 13 1" spacing=".055 .055 .055" pos="0 0 .68"
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radius=".004" mass=".25" name="cloth1" dim="2" rgba=".85 .35 .25 1">
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<contact selfcollide="auto" passive="true" solref="0.01 1" solimp=".95 .99 .0001"/>
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<elasticity young="2e4" poisson=".2" thickness="1e-3" elastic2d="both" damping="1e-4"/>
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</flexcomp>
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<flexcomp type="grid" count="13 13 1" spacing=".055 .055 .055" pos=".06 -.04 .78"
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radius=".004" mass=".25" name="cloth2" dim="2" rgba=".25 .55 .8 1">
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<contact selfcollide="auto" passive="true" solref="0.01 1" solimp=".95 .99 .0001"/>
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<elasticity young="2e4" poisson=".2" thickness="1e-3" elastic2d="both" damping="1e-4"/>
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</flexcomp>
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<flexcomp type="grid" count="13 13 1" spacing=".055 .055 .055" pos="-.05 .05 .88"
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radius=".004" mass=".25" name="cloth3" dim="2" rgba=".95 .8 .3 1">
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<contact selfcollide="auto" passive="true" solref="0.01 1" solimp=".95 .99 .0001"/>
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<elasticity young="2e4" poisson=".2" thickness="1e-3" elastic2d="both" damping="1e-4"/>
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</flexcomp>
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</worldbody>
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</mujoco>
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