How can I make paths wrap over wrap objects?

[adamkewley]

Question originally from mikeh7791 on YouTube: https://www.youtube.com/watch?v=SHfWK-SlZys&lc=UgzsIoQsLOo5EUzJ62J4AaABAg

How can I get the muscle paths to move around a wrap object?

[adamkewley]

The OpenSim engine handles wrapping using three separate concepts:

• The path (e.g. the path component within a muscle)
• A wrap object (e.g. a cylinder)
• An association between the path (or multiple paths) and one-or-many wrap objects

Your question is probably the 3rd one, but I'll also outline how the others are added just in case.

Paths

Paths are almost always added via components that use them. E.g. when you add a muscle component (e.g. Millard2012AccelerationMuscle ) or PathSpring there will be a path property where you list the points you'd like to add to the path. Concrete example:

• Right-click in the scene, Add > Force > PathSpring
• Edit the path property in the popup
• Add a couple of points to the path

Wrap Objects

At time of writing (osc 0.5.13, OpenSim 4.5), a wrap object is almost always a child of a Body in the model. You can add them via osc by:

• Right-clicking a body in the model, Add > Wrap Object > WrapSphere/WrapCylinder/...
• The wrap object should be added to the model

Wrap Associations

Each GeometryPath in the model--regardless of whether it originated via a muscle, or spring--can be associated to N WrapObjects in the model. As a model designer, you need to create this association. The reason this step is necessary is because one path wrap may be used by multiple paths, and a path may wrap other multiple path wraps.

At time of writing (osc 0.5.13), the procedure for adding a path wrap is:

• Select the GeometryPath you want to associate with a WrapObject. E.g. for muscles, use the Navigator window to find the path component within the muscle of interest.
• Right-click the path component, go to Add > Path Wrap > ... and select the WrapObject (addition explained above) you want the path to wrap over
• The path should start wrapping over the wrap object

Other Notes

• The direction in which the path wraps over the wrap object is dictated by the quadrant property on the WrapObject itself. E.g. if you want to require that the path wraps "behind" a cylinder then it's wise to select a quadrant the represents the "behind" of it (e.g. -Y - it depends on your model what you define as "behind" or "in front"). You can usually play around with the quadrant setting to see the effect
• Path points must never intersect the wrap object. The wrapping algorithm(s) tend to assume that the line between the path points may intersect the surface of the wrap object. They cannot handle figuring out how to wrap something that's embedded inside of the wrap geometry.
• OpenSim's wrapping implementation can be slow/buggy sometimes. The worst offences are when your path crosses multiple quadrants and the algorithm can't quite figure out how to handle it without "hopping" the path to a completely different shape. OpenSim is currently developing a better algorithm, though!