Disable collisions between attached bodies

[azeey]

🦟 Bug fix

Fixes #

Summary

Even with #632 , it looks like objects that are attached to each other continue to generate contacts. Even though we've explicitly marked the two objects not to collide, looking at the bullet code (https://github.com/bulletphysics/bullet3/blob/e9c461b0ace140d5c73972760781d94b7b5eee53/src/BulletDynamics/Featherstone/btMultiBodyLinkCollider.h#L79-L80), that is ignored because checkCollideWithOverride will always return true if the two objects are not in the same btMultiBody.

@iche033 I haven't checked this thoroughly, but the solution I came up with is to call the base class's checkCollideWithOverride as well as btMultiBodyLinkCollider::checkCollideWithOverride and return the result of anding the two. This does behave much better. I tried with the SDF below by uncommenting only one of the <plugin> tags. Without this PR, the objects fly up or down together even though gravity is 0. Visualizing contacts in gz-sim also shows that they are in contact. With the PR, the boxes stay in place and generate no contacts.

SDF File

<?xml version="1.0"?>
<sdf version="1.11">
  <world name="default">

    <gravity>0 0 0</gravity>

    <model name="ground_plane">
      <static>true</static>
      <link name="link">
        <collision name="collision">
          <geometry>
            <plane>
              <normal>0 0 1</normal>
            </plane>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <material>
            <ambient>0.8 0.8 0.8 1</ambient>
            <diffuse>0.8 0.8 0.8 1</diffuse>
            <specular>0.8 0.8 0.8 1</specular>
          </material>
        </visual>
      </link>
    </model>
    <model name="M1">
      <pose>0 2 3.0 0 0.0 0.0</pose>
      <link name="body">
        <inertial auto="true"/>
        <collision name="box_collision">
          <geometry>
            <box>
              <size>1 1 0.5</size>
            </box>
          </geometry>
        </collision>

        <visual name="box_visual">
          <geometry>
            <box>
              <size>1 1 0.5</size>
            </box>
          </geometry>
          <material>
            <ambient>1 0 0 1</ambient>
            <diffuse>1 0 0 1</diffuse>
            <specular>1 0 0 1</specular>
          </material>
        </visual>
      </link>
      <plugin filename="gz-sim-detachable-joint-system"
              name="gz::sim::systems::DetachableJoint">
       <parent_link>body</parent_link>
       <child_model>M2</child_model>
       <child_link>body2</child_link>
      </plugin>
    </model>

    <model name="M2">
      <pose>0.3 2 3.0 0 0.0 0</pose>
      <link name="body2">
        <inertial auto="true"/>
        <collision name="box_collision">
          <geometry>
            <box>
              <size>1 1 0.5</size>
            </box>
          </geometry>
        </collision>

        <visual name="box_visual">
          <geometry>
            <box>
              <size>1 1 0.5</size>
            </box>
          </geometry>
          <material>
            <ambient>0 1 0 1</ambient>
            <diffuse>0 1 0 1</diffuse>
            <specular>1 0 0 1</specular>
          </material>
        </visual>
      </link>
      <!-- <plugin filename="gz-sim-detachable-joint-system" -->
      <!--         name="gz::sim::systems::DetachableJoint"> -->
      <!--  <parent_link>body2</parent_link> -->
      <!--  <child_model>M1</child_model> -->
      <!--  <child_link>body</child_link> -->
      <!-- </plugin> -->
    </model>
  </world>
</sdf>

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[iche033]

oh nice, thanks for looking into this! #632 made it work only for static bodies. I mentioned in that PR that setIgnoreCollisionCheck does not seem have an effect. This should do it!