Are the path points of a PathSpring are mis-rendered?

[adamkewley]

AK: This question was ported from akewley@tudelft.nl . Some details were REDACTED or edited because the original question was private.

I have recently created a model through OpenSimCreator. It consists of a pendulum with a pathspring attached to it. I have added PhysicalOffsetFrames to the bodies that act as the pathpoints of the PathSpring.

However, when I open the same model in OpenSim the PathSpring (green in the model) is not correctly connected (see image). It seems like the PhysicalOffsetFrames are ignored and the parent bodies are taken as PathPoints of the PathSpring.

[adamkewley]

I looked through your model and I believe that OSC’s rendering is (for once 😉) to be expected. The two frames you’ve specified in the GeometryPath attribute of ExoTendon are attached at (0,0,0) to:

• /bodyset/pendulum_base/pendulum_base_exotendon_attach
• /bodyset/pendulum_head/pendulum_head_exotendon_attach

Which are both in the expected locations as-rendered. Even simulating it, which is separate from OSC’s renderer, seems to pull the head along the spring (if I tweak the stiffness, rest length, etc.).

This appears to be a bug in OpenSim GUI. I don’t have a definite explanation, but I would guess that the bug is related to the fact that OSC implements its own custom way of rendering GeometryPaths (to support e.g. custom muscle styles, better mouse hit testing, etc.) whereas OpenSim GUI uses OpenSim core.

Quickly reading through the differences:

• OpenSim core: https://github.com/opensim-org/opensim-core/blob/4.4/OpenSim/Simulation/Model/GeometryPath.cpp#L216
• OpenSim Creator: https://github.com/ComputationalBiomechanicsLab/opensim-creator/blob/0.5.0/src/OpenSimCreator/Utils/OpenSimHelpers.cpp#L1222

They’re both doing a very similar thing (OSC’s approach is simplified), but one difference is that OpenSim seems to tag the “BodyID” of the line element as 0 (hard-coded) and sets the index of the line of the body as i, which might have some bizzare downstream impact on rendering if you have attached a path to something that isn’t a body (in your case, you’re attaching to an offset frame that is attached to the body where the path incorrectly renders). So, technically speaking, the problem is probably that OpenSim assumes paths are expressed within bodies not frames (even though your usage is technically also correct).