Proposal: system for setting initial position / velocity

[scpeters]

Desired behavior

I'd like to be able to specify initial model/link velocity and initial joint position/velocity. For joints, I believe this can be done by setting JointPositionReset and JointVelocityReset components programmatically. It would be consistent with this pattern to add LinearVelocityReset / AngularVelocityReset and/or WorldLinearVelocityReset / WorldAngularVelocityReset components that could be used to set initial model / link velocities.

Alternatives considered

I had started an alternative approach in #2265 based on using the existing AngularVelocityCmd and LinearVelocityCmd components with the VelocityControl system, but I think the *Reset components make more sense.

Implementation suggestion

<model name="example_model">
  <link name="parent_link" />
  <link name="child_link" />
  <joint name="multiaxis_joint" type="universal">
    <axis>
      <xyz>0 0 1</xyz>
    </axis>
    <axis2>
      <xyz>1 0 0</xyz>
    </axis2>
  </joint>
  <plugin name="SetInitialComponents" ...>
    <!-- different syntax ideas to set initial velocity for this model -->
      <!-- specify //entity/@type="model" and //entity/@name="__model__" -->
      <entity type="model" name="__model__">
        <set_component_value type="WorldLinearVelocityReset">1.0 4.0 9.0</set_component_value>
      </entity>
      <!-- specify //model/@name="__model__" -->
      <model name="__model__">
        <set_component_value type="WorldLinearVelocityReset">1.0 4.0 9.0</set_component_value>
      </model>
      <!-- or just treat set_component_value tags at the root scope as referring to the model -->
      <set_component_value type="WorldLinearVelocityReset">1.0 4.0 9.0</set_component_value>
  
    <!-- different syntax ideas to set initial velocity for a link -->
      <!-- specify //entity/@type="link" and //entity/@name="parent_link" -->
      <entity type="link" name="parent_link">
        <set_component_value type="WorldLinearVelocityReset">1.0 4.0 9.0</set_component_value>
      </entity>
      <!-- specify //link/@name="parent_link" -->
      <link name="parent_link">
        <set_component_value type="WorldLinearVelocityReset">1.0 4.0 9.0</set_component_value>
      </link>

    <!-- different syntax ideas to set initial position / velocity for a joint -->
      <!-- specify //entity/@type="joint" and //entity/@name="multiaxis_joint" -->
      <entity type="joint" name="multiaxis_joint">
        <set_component_value type="JointPositionReset">0.1 0.4</set_component_value>
        <set_component_value type="JointVelocityReset">1.0 -0.5</set_component_value>
      </entity>
      <!-- specify //joint/@name="multiaxis_joint" -->
      <joint name="multiaxis_joint">
        <set_component_value type="JointPositionReset">0.1 0.4</set_component_value>
        <set_component_value type="JointVelocityReset">1.0 -0.5</set_component_value>
      </joint>
  
  </plugin>

Thoughts are welcome for the right syntax here. I think we would have to hard-code a relationship between the //set_component_value/@type string and a gz::sim::components:: class since the component Factory doesn't allow constructing just from a string. It does have namesById and runtimeNamesById data structures but not the inverse (IdsByName). Even if we did, I'm not sure how it could be implemented.

I suggested the system name of SetInitialComponents in this example to go along with the <set_component_value> elements, but we could also use names with more specific semantics (similar to the InitialVelocityPlugin from gazebo-classic).

Additional context

[scpeters]

consider expressing the model state using the syntax of the model_state

[arjo129]

This would be really useful for reducing our test times. In particular we could use it to test plugins like the hydrodynamics/LiftDrag etc.