Accessing Site Names for Spatial Tendons in MjSpec

[nbadie]

Intro

Hi!

I am a graduate student at University of Stuttgart and I use MuJoCo for my research on musculoskeletal simulation and RL.

My setup

I am using mujoco 3.2.7.

My question

I'm currently working on modifying tendons and muscles using MjSpec, but I’ve run into a limitation: spec.tendons doesn’t seem to expose site names for spatial tendons. Is there a recommended way to access or work around this limitation?

Minimal model and/or code that explain my question

No response

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]

hmm @quagla what's the right way to do this? I expected the mjsTendon to point me to a list of mjsWrap which would have what @nbadie is looking for, but I looked and now I'm not sure how one would get this from the mjSpec API...

As a workaround, I can tell you how to do this from the mjModel, would that be helpful?

[quagla]

The thing is that the names are not part of the mjsWrap C-struct because they cannot be set by the user.

[Balint-H]

We'd like this feature mainly for procedural scene editing logic. The specific case that prompted this was deleting bodies that had tendon insertion sites, and wanting to automatically remove all tendons that wrapped the now missing sites.

Interestingly in spec there are scripts for adding new wrap sites/geoms, but no way to read what is the existing sequence of wrap elements associated with a spatial tendon.

[quagla]

So would it be enough if I exposed a functionality to read the wrap elements but not their names?

[Balint-H]

What information would be available from the wrap elements? Would we be able in some way to match against the corresponding sites (and maybe geoms)? I.e. if we have the list of wrap elements, could we identify to geoms or sites it came from?

[Balint-H]

Of course, if you have an alternative approach recommended for the use case I mentioned that would also be helpful! (One that is preferably flexible and doesn't need identifying and storing the names explicitly)

[yuvaltassa]

The alternative approach (which I realize is not ideal, since it requires compilation), is to read all this info from mjModel. Everything you want can be gotten from tendon_adr, tendon_num, wrap_type and wrap_objid. I think it should be obvious how these correspond to what you are looking for. If not, let me know.

That's said, I agree this should be possible via the mjSpec API. We'll think about it!