Jacobian for a body chain

[Tadinu]

Intro

I'm a student and trying to calculate Jacobian between dofs of a specific body chain (eg: wrist to a finger end body on a robot composed of manipulator + a multi-fingered hand) and one point on the finger (eg: fingertip)

The most relevant APIs seem to be mj_bodyChain() & mj_jacSparse(). However, the first one currently assumes the chain goes all down to root body (id 0).
Would it be possible to add base_body_id as an optional parameter, with minor changes:

• body==0 -> body==base_body
• while (da >= 0) -> while (da >= m->body_dofadr[base_body])?

Besides, they are also not exposed yet by MJAPI.
If there are other alternative API, may you help with a pointer?
Thanks!

My setup

MuJoCo 3.3.1, API (C), Ubuntu 22.04

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[yuvaltassa]

You can get Jacobians using mj_jac and related functions. I think what you want is equivalent to the Jacobian of the finger minus the jacobian of the wrist, is it not?

[Tadinu]

Yeah it could be done that way. So in a certain task, if the arm (including the wrist) does not move at all & only the hand dofs are controlled, the resulting Jacobian will have all zeros at arm dofs. Just debugging-wise, it'd be easier on a compact Jacobian of active-only dofs.

[yuvaltassa]

Easier still, you can just manually zero out the columns you don't care about 🙂

I agree that active-only dofs is more efficient and as you noted, MuJoCo does indeed do this internally, but for 99% of users having dense matrices with mjModel.nv columns is much easier to handle. It's actually quite rare that there is a significant performance gain to be had...