Dynamic Geometry (Mesh) Changes post-plant finalize

Question

This is a follow-up after reading this:

Adding New Geometries Post-Finalize

I was wondering about the way to change the geometry of meshes, for perception and illustration roles. It was mentioned that it is possible to get deeper into the SceneGraph object and manipulate the objects. However, I was not able to find the appropriate API calls to retrieve and set the geometry.

In the example that I am working on, I would like to update a mesh of a link as part of a multi-body plant.

To be more specific, I have a fixed link with a mesh that represents a rough terrain.I would like to visualize some sort of arbitrary change to each vertice on the mesh and move them. (somewhat similar to a deformable body, but explicitly moving the points rather than involving forces)

Side note for context: These are my early steps to integrate a soil simulator for terra-mechanics simulation.

I am a novice Drake user and I appreciate the help and support.

I loaded up an SDF model with a .dae mesh file and I was successful in creating the right plan and scene. Yet, I did not know how to change a mesh during simulation run time.

Answer 1

So, the only recourse you have is redefining the shape associated with the geometry. We currently don't have any mechanism by which you can perturb particular vertex locations. The workflow would look something like this:

1. Your original ground terrain is defined on disk as "ground.obj" (or some such thing).
2. You've added a Mesh shape to SceneGraph with illustration or perception properties. You need its GeometryId and (currently) the SourceId that was used. Most likely, if you parsed it from an sdf/urdf file, it was registered with the MultibodyPlant's source id (MultibodyPlant::get_source_id()).
3. Given the new mesh (with repositioned vertices), you'll have to write that mesh out to disk as ground1.obj.
4. Call SceneGraph::ChangeShape() pass where you provide a Mesh which references ground1.obj.
   • Make sure you call the version of ChangeShape() whose first parameter is your current Context.

That's it. It's cumbersome (having to go to disk), but, currently, that's the only mechanism we have for changing the geometry. Fortunately, MultibodyPlant is not sensitive to the visual (illustration/perception) representations of the bodies, so it doesn't interfere with these actions.