STEP4: Update interaction model for physical interaction

In the last step the physical representations for the entities in the virtual world were added. Now
we will have a look at the interaction implementation. The current implementation was designed to
work with standard EntityTypes which have no physical representation. In order to be able to
manipulate physically simulated entities we have to change the manipulation model used by the
Interaction module.

The first thing we have to do is to register the factory for the new manipulation model at the
Interaction module. Therefore we add following lines in our source code:

...
+ #include <inVRs/Modules/3DPhysics/PhysicsHomerManipulationActionModel.h>
...
	void initModuleCallback(ModuleInterface* moduleInterface) {
		if (moduleInterface->getName() == "Physics") {
...
		} // if
+		else if (moduleInterface->getName() == "Interaction") {
+			((Interaction*)moduleInterface)->registerStateActionModelFactory(
+					new PhysicsHomerManipulationActionModelFactory);
+		} // if
	} // initModuleCallback
...

This causes the PhysicsHomerManipulationActionModelFactory to be registered at the Interaction
module right before the interaction configuration file is loaded. In order to use the registered
model now we have to change the configuration of the interaction module (interaction.xml):

-	<manipulationActionModel name="HomerManipulationActionModel">
-		<arguments>
-			<arg key="usePickingOffset" type="bool" value="true" />
-		</arguments>
-	</manipulationActionModel>
+	<manipulationActionModel name="PhysicsHomerManipulationActionModel"/>

We remove the previous entry for the manipulationActionModel and replace it with a new one using the
PhysicsHomerManipulationActionModel.

When you rebuild and restart your application now the interaction with the objects should look
better now. What you may have recognized is that the manipulated object is always a little behind 
the cursor when moving. So let me explain what happens here exactly:

When you change to manipulation state by pressing the mouse button the cursor will be animated in
direction of the object by the HomerCursorModel. As soon as the hand has reached the object the
CreateCursorSpringConnectorEvent is triggered. This event creates (as the name says) a
CursorSpringConnector object, which is composed of a RigidBody representing the cursor and a joint
which connects the cursor RigidBody with the RigidBody of the entity which should be manipulated.
The created RigidBody for the cursor has no collision geometry so that it can't collide with any
objects. The mass of this RigidBody is set to a very high value so that the RigidBody of the Entity
which should be manipulated can't affect the cursor's transformation (this should always be steered
by the user input). The created joint acts like a spring and damper system. This allows to pick up
objects with a low mass easier than objects with a high mass (like it should be in real world). But
because of this spring/damper system the manipulated objects always lag behind the cursor a bit.

I think this is enough for now, for more details like on the porting of this application on the
CAVE/CurvedScreen or on details about the physics module just write a mail to rlander@inVRs.org