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RobotLocomotion / drake

Model-based design and verification for robotics.
https://drake.mit.edu
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multibody: `ModelInstanceIndex` as an optional argument to the Jacobian methods #14033

Open RussTedrake opened 4 years ago

RussTedrake commented 4 years ago

Working with Jacobians in a MBP that has multiple instances is a pain -- it's up the user to pull out the elements of the jacobian that they actually need. (here is an example)

It seems quite reasonable to support an optional argument for ModelInstanceIndex that would just compute/return the gradients wrt a subset of the qdot or v for a particular model. This could have even more value/meaning for the CalcJacobianCenterOfMassTranslationalVelocity method (if it would compute CM for just that model instance).

xuchenhan-tri commented 2 years ago

@RussTedrake Is the request to provide sugar that extracts relevant parts of the Jacobian or to provide a more efficient implementation (aka avoid irrelevant calculations)? My understanding is the former, but a confirmation would be helpful.

If so, a fix similar to that in #15900 should be easy and I can take on this issue in the coming weeks.

BTW, CalcJacobianCenterOfMassTranslationalVelocity for selected model instances is already supported via #15387.

xuchenhan-tri commented 2 years ago

Per f2f, @mitiguy will pick up this ticket upon confirmation from OP that this is still relevant.

RussTedrake commented 2 years ago

The request was to do the same as #15387 for any remaining jacobians in multibody plant. Of course saving calculations is great, but this was a usability issue first not a performance issue. If computing and then filtering is the best first approach, that could be ok.

xuchenhan-tri commented 2 years ago

Thanks for the confirmation. The usability issue is an easy fix as shown in #15900. I put it on the MbP hitlist.

This is slightly different from #15387 though. There we were cutting down the number of functions being differentiated whereas here the request is to cut down the w.r.t. variables.