[JaxSelfCollisionBarrier] Error in Jacobian calculation given empty collision_pairs

[Tadinu]

Hi,
Thanks alot for the very awesome work!

May you help confirm:
For a JaxSelfCollisionBarrier added to the problem, with certain collision_bodies and excluded_collisions but resulting in no collision_pairs as computed here
https://github.com/based-robotics/mjinx/blob/main/mjinx/components/barriers/_self_collision_barrier.py#L281
-> Then is that the jacobian cannot be computed & the default one should be returned here?
https://github.com/based-robotics/mjinx/blob/main/mjinx/components/barriers/_self_collision_barrier.py#L110

[domrachev03]

Hello, sorry for the late response.

I think you are right, and the behaviour of the self-collision barrier when no collision bodies is detected is undefined. Moreover, it is undefined whenever len(collision_pairs) < n_closest_pairs.

I think two checks should be added:

1. Check that len(collision_pairs) >= n_closest_pairs
2. If n_closest_pairs is not specified, len(collision_pairs)!=0. However, if len is indeed zero, I may argue that the code would still work as expected, just having an empty Jacobian. Anyway, we should investigate this case better.

I would try to address this issue, but I could not do it right now, so it would take some time. Alternatively, you are more than welcome to open a PR with fix, I would do my best to merge it ASAP.