Elasticity parameter for collisions

[onnoeberhard]

Dear Brax community, is it possible to control the percentage of kinetic energy that is dissipated in a collision? I have noticed that the Ant, when all torques are set to a maximum such that it is very stiff, bounces as though the collisions with the ground are nearly elastic. It would be great to set such a parameter between 0 and 1 to make the collisions inelastic.

[btaba]

You can control elasticity depending on the physics backend used in brax. For the positional pipeline, you can use elasticity, as in

brax/brax/test_data/capsule.xml

Line 7 in ac2b94b

<tuple name="elasticity">

For the generalized pipeline you can use the constraint solver parameters solref as described in https://mujoco.readthedocs.io/en/latest/modeling.html#solver-parameters. These control the stifness and damping associated with the collision constraint