How to define collision filter groups across model instances?

Question

When constructing a MultiBodyPlant from multiple ModelInstances each loaded from a separate URDF, is there a way to define collision filter groups to exclude collisions across model instances? In the multibody URDF parser, loading collision filter groups is restricted to the those present in the same model instance.

What I would like to do is in URDF A specify:

  <drake:collision_filter_group name="model_a_group">
    <drake:member link="link1"/>
    <drake:member link="link2"/>
    <drake:ignored_collision_filter_group name="model_b_group"/>
  </drake:collision_filter_group>

and in URDF B specify:

  <drake:collision_filter_group name="model_b_group">
    <drake:member link="link4"/>
    <drake:member link="link5"/>
    <drake:ignored_collision_filter_group name="model_a_group"/>
  </drake:collision_filter_group>

But currently attempting to do this results in the following error:

abort: Failure at multibody/parsing/detail_urdf_parser.cc:238 in abort: 
Failure at multibody/parsing/detail_urdf_parser.cc:238 in ParseCollisionFilterGroup(): 
condition 'collision_filter_group_b != collision_filter_groups.end()' failed.

Answer 1

The scope of drake:collision_filter_group is limited to the scope of the file. So, it's sufficient to prevent "fake" collision at a robot's joints but insufficient to prevent two robots that get parsed independently from colliding.

In order to achieve that end, you'll have to parse the two robots, and then algorithmically declare the collision filters.

I don't have a great deal of time right now, so I'll give a rough overview of what it looks like, and then provide code when I free up later:

The approach.

1. Parse both URDFs (SDFs).
2. Grab the bodies for model instance "a" (link1 and link2), and similarly the bodies for model instance "b" (link4 and link5).
3. Using the MBP interface get the geometry::FrameId associated with each of those bodies. (We'll call them frameA1, frameA2, frameB4, frameB5, respectively.
4. Create two instances of geometry::GeometrySet. geometry_set_A = {frameA1, frameA2} and geometry_set_B = {frameB4, frameB5}.
5. Invoke SceneGraph::ExcludeCollisionsBetween(geometry_set_A, geometry_set_b).

Done

This should give you the first glimmer of how to proceed. I'll come by for a second pass to give concrete code to accomplish the task later.