Warn if no joints on Root Body (#5387)

com.unity.ml-agents.extensions/Editor/RigidBodySensorComponentEditor.cs

             so.Update();
 
             var rbSensorComp = so.targetObject as RigidBodySensorComponent;
+            if (rbSensorComp.IsTrivial())
+            {
+                EditorGUILayout.HelpBox(
+                    "The Root Body has no Joints, and the Virtual Root is null or the same as the " +
+                    "Root Body's GameObject. This will not generate any useful observations; they will always " +
+                    "be the identity values. Consider removing this component since it won't help the Agent.",
+                    MessageType.Warning
+                );
+            }
+
             bool requireExtractorUpdate;
 
             EditorGUI.BeginDisabledGroup(!EditorUtilities.CanUpdateModelProperties());

com.unity.ml-agents.extensions/Runtime/Sensors/RigidBodySensorComponent.cs

         {
             GetPoseExtractor().SetPoseEnabled(index, enabled);
         }
+
+        internal bool IsTrivial()
+        {
+            if (ReferenceEquals(RootBody, null))
+            {
+                // It *is* trivial, but this will happen when the sensor is being set up, so don't warn then.
+                return false;
+            }
+            var joints = RootBody.GetComponentsInChildren<Joint>();
+            if (joints.Length == 0)
+            {
+                if (ReferenceEquals(VirtualRoot, null) || ReferenceEquals(VirtualRoot, RootBody.gameObject))
+                {
+                    return true;
+                }
+            }
+            return false;
+        }
     }
 
 }

com.unity.ml-agents.extensions/Tests/Runtime/Sensors/RigidBodySensorTests.cs

             var gameObj = new GameObject();
 
             var sensorComponent = gameObj.AddComponent<RigidBodySensorComponent>();
+            Assert.IsFalse(sensorComponent.IsTrivial());
             var sensor = sensorComponent.CreateSensors()[0];
             SensorTestHelper.CompareObservation(sensor, new float[0]);
         }
                 UseLocalSpaceTranslations = true,
                 UseLocalSpaceRotations = true
             };
+            Assert.IsTrue(sensorComponent.IsTrivial());
 
             var sensor = sensorComponent.CreateSensors()[0];
             sensor.Update();
                 UseLocalSpaceLinearVelocity = true
             };
             sensorComponent.VirtualRoot = virtualRoot;
+            Assert.IsFalse(sensorComponent.IsTrivial());
 
             var sensor = sensorComponent.CreateSensors()[0];
             sensor.Update();

com.unity.ml-agents/CHANGELOG.md

 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Added a fully connected visual encoder for environments with very small image inputs. (#5351)
 ### Bug Fixes
+- RigidBodySensorComponent now displays a warning if it's used in a way that won't generate useful observations. (#5387)
 
 
 ## [2.0.0-exp.1] - 2021-04-22