Getting Multiple Contact Force Vectors for a Single Capsule Geom/Body

[heleiduan]

My simulation model has a body with a single geom attached to it. However, in Mujoco Sim, when pressing 'C', I see two contacts showing up. Similarly with pressing 'F', I can see two force vectors coming from two end of the geom/body, potentially from the two-half spheres. I am able to get the single force vector applied onto the body. But how do I read the two distinct force vectors that are shown in the simulation? Thank you!

[yuvaltassa]

Please attach your model so we can inspect.

[heleiduan]

Here is the model file zipped. The force vectors are referring to the ones coming from the contact on the feet ("left-foot" at line 147 and "right-foot" at line 211) in the model. Thank you!
model_file.zip

[yuvaltassa]

Your model has the unfortunate line <material name='collision' rgba='0.1 0.5 0.9 0'/>, so your collision geometries are hard-coded to be invisible. Set the alpha value to 1 and then add group="1" to this default: <geom material='collision' contype='1' user='2'/>. Now, clicking the '0' and '1' keys will respectively show/hide the visual and collision geometries.

[heleiduan]

Thank you! The collision geom for feet is a single capsule geom. We are able to get the single contact force vector using mj_contactForce() with the corresponding geom_id. But looking at the image when pressing 'F' and 'C', we can also see the two distinct force vectors/contacts at both ends of the capsule. How do we get the values for two distinct vectors?

[yuvaltassa]

The id in mj_contactForce is not a geom id but rather a contact id. You're supposed to iterate over the data.contact buffer and look at the id's of the the two geoms that are contacting.