Unable to add Contact Geometry: ContactMesh obj file

[bfugger]

Hi there, I am so far unable to add a contactmesh to my project. I typed the path to the obj file and I ensured that it is watertight and closed and i selected the proper frame. When I click "add" I don't have any contact geometry added. There are no error or info lines in the log. Any help is greatly appreciated

[adamkewley]

Hi @bfugger

I confirmed the bug on my side. From what I can see:

1. After adding a ContactMesh to a model and choosing an STL or OBJ file
2. The component is added to the model fine (as you found)
3. And the resulting osim will load sucessfully in both OpenSim Creator and OpenSim GUI with no error message
4. But the mesh will not be shown in either UI

---

The bug appears to be from OpenSim:

• ContactMesh sourcecode in OpenSim is here:
  • https://github.com/opensim-org/opensim-core/blob/4.4/OpenSim/Simulation/Model/ContactMesh.h
• Contains two mesh pointers: one for contact and one for decoration:
  • https://github.com/opensim-org/opensim-core/blob/4.4/OpenSim/Simulation/Model/ContactMesh.h#L114
• The decorative one is only populated via either:
  • The full-fat constructor (won't be used by either OpenSim Creator or OpenSimGUI): https://github.com/opensim-org/opensim-core/blob/4.4/OpenSim/Simulation/Model/ContactMesh.cpp#L55
  • loadMesh, which might be called when adding the contact mesh into the physics system: https://github.com/opensim-org/opensim-core/blob/4.4/OpenSim/Simulation/Model/ContactMesh.cpp#L96
• loadMesh is called by ContactMesh::createSimTKContactGeometry, but createSimTKContactGeometry is only called by forces that use contact geometries
  • e.g. here: https://github.com/opensim-org/opensim-core/blob/4.4/OpenSim/Simulation/Model/HuntCrossleyForce.cpp#L87
• So the only way to visualize a ContactMesh in the 3D scene is to add the ContactMesh to a force, because the side-effect of doing that is that the decorative geometry will also be loaded

So, @bfugger, the way I was able to move forward on this was to add a ContactMesh to the model (works fine) and then add a HuntCrossleyForce to the model and set one of its geometry properties to the name of the ContactMesh (defaults to ContactMesh).

However, I couldn't move forward from there because SimTK seems to prescribe that any mesh file that is provided must contain triangles with edges that share exactly two faces and I couldn't find an appropriate mesh during testing.

Your bug (not being able to see the contact geometry until a force is added) isn't in OpenSim Creator. It will need to be upstreamed to OpenSim. The potential later "bug" (of having to provide a perfect mesh file - not necessarily a bug but might be very annoying) appears to originate from simbody, which is another upstream library that OpenSim uses:

• https://simtk.org/api_docs/molmodel/api_docs22/Simbody/html/classSimTK_1_1ContactGeometry_1_1TriangleMesh.html#_details
• Error source: https://github.com/simbody/simbody/blob/master/SimTKmath/Geometry/src/ContactGeometry_TriangleMesh.cpp#L464

[bfugger]

Thank you so much for the detailed write up. I also think that I may be attempting to use a noncompliant mesh. Some further digging on the forum and the links you provided show my mesh also needs to be onesided? and with proper vertices directions. So i am working on cleaning that up. In the meantime, I will try the approach you described as I need to assign forces anyways. I am still getting the hang of opensim so I appreciate all your input.

[adamkewley]

Moving this to a discussion because the core issue is related to how OpenSim works, rather than something that OpenSim Creator can solve in-tree.