Physics
*******

Summary
=======

A **Collider** is a component that defines the physical boundaries of a GameObject.
This way, to detect the collision between two GameObjects, each of them must have a Collider.

	
In Unity, colliders can function in two different ways:

 	* Normal Mode: the object physically interacts with other objects
	
		* Collider only
		
			* Property: **No movement, static object**
			* Useful for: walls, floor
			
		* Collider + RigidBody
		
			* Property: **Moves and responds to gravity or impulses**
			* Useful for: domino Challenge, rolling ball
			* Function: void OnCollisionEnter(Collision collision)

	* Trigger Mode: the object does not block others; no physical interaction => Ghost object
	
		* Collider + Trigger
		
			* Property: **detects when a GameObject passes through**
			* Useful for: the finish line in a car race, a trap in a cave
			* Function: void OnTriggerEnter(Collider other)
		

 

Colliders Shape
===============

Setup
-----

	* Navigate to Assets > Scenes
	* Create a new folder named "PhysX"
	* Create a new scene called "Colliders"
	* Double click to open it

	* Create a Sphere and assign it a color
	
		.. image:: sphere.png
			:scale: 40%
		
	* Select the sphere
	* In the Inspector, notice that a Collider is already present
	
		.. image:: spherecollider.png
			:scale: 40%
	
	* Uncheck the checkbox of the Mesh Renderer
	
	* Notice that the collider of the sphere is now visible on the screen
		
		.. image:: spherecol.png
			:scale: 50%	
	
	* Activate the sphere Mesh Renderer	
	
The Collider of the sphere perfectly matches its shape. Here are the colliders of other objects:

		.. image:: otherscolliders.png
			:scale: 30%
		

	* Sphere: sphere collider
	* Capsule: capsule collider
	* Cylinder: capsule collider
	* Cube: box collider
	 
		
Two remarks:

	* All basic 3D objects are delivered with a built-in collider
	* The cylinder uses a capsule collider (Lower computational cost)
	
.. warning:: We recommend not using a plane because it is defined by 200 triangles. Testing its collision is equivalent in complexity to testing the collision with 200/6 = 33 cubes. As a consequence, use a cube to create a floor.

 

	
RigidBody
=========

Basic setup
-----------

We begin our study by adding a RigidBody to the sphere.

	* Create a cube and and assign it a color
	
		* Scale the cube to create a ground

	* Select the sphere
	
		* In the Inspector, click on the button *Add Component*
	
		* Select RigidBody
		
		* In the RigidBody section, check: **Use Gravity**
		
		.. image:: usegravity.png
			:scale: 50%
		
		.. image:: fall.png
			:scale: 35%
			 
	* If you want the camera to shoot the same view as the scene view
	
		* In the Hierarchy, select the camera
		* Right click and select Align With View
			
	* Enter Play mode
	
		.. image:: fall.gif
			:scale: 40%

What happened?

	* The sphere has a *Rigidbody*, moreover it is affected by gravity so it falls
	* The ground don't have a *Rigidbody*, it's a static object, it cant move
	* Both the sphere and the ground have a Collider, allowing them to come into contact, which stops the sphere's fall
	
Simulation
----------

We add some new elements to the scene; try to reproduce the scene below.

Some hints:

	* Analyze which object has a RigidBody: is the object moving or not?
	* Detect which object is affected by gravity: it is falling or not ?
	* Enable all options correctly 
	
	.. image:: bascule.png
		:scale: 48%
		
	.. image:: vidbascule.gif
		:scale: 60%
		


Collider
========

Scene Setup
-----------

	* Navigate to Assets > Scenes > PhysX
	
	* Create a new Scene named *Detection*
	
	* Create a sphere, assign it a color and a RigidBody with gravity
	
	* Move the sphere upwards
		

Collider Setup
--------------

		
	* In the Hierarchy, right click and select **Create Empty**
	
		* Rename the object "BCollider"
		
		* In the Inspector, press the *Add Component* button and select **Box Collider**
		
		.. image:: scene1.png
			:scale: 40%	
		
		* In the Inspector, activate the **Is Trigger** option
		
		.. image:: istri.png
			:scale: 50%
		
		
		* In the Inspector, click on the **Edit Collider** icon:
		
		.. image:: edit.png
			:scale: 40%
	 
			 
		
		* Use the face gizmos to
		
			* Enlarge the box 
			* Make it thicker than the sphere 
			
		.. image:: enlarge.gif
			:scale: 70%
			
Collider script setup
---------------------

A collider trigger provides multiple functions to handle collisions:


	.. image:: onmethod.png
		:scale: 40%

	

		
	* In the PhysX folder, create a new script named *Colliz* 
	
	* Select the GameObject BCollider 

	* Drag and drop the script into the Inspector
	
		.. image:: script.png
			:scale: 50%
	
	
	* Copy paste the following code into the script
	
	.. code-block:: csharp


		using UnityEngine;

		public class Colliz : MonoBehaviour
		{
			public Material Mat_in,Mat_out;

			void OnTriggerEnter(Collider other)
			{
				Renderer objectRenderer = other.gameObject.GetComponent<Renderer>();
				objectRenderer.material = Mat_in;
			}
			
			void OnTriggerExit(Collider other)
			{
				Renderer objectRenderer = other.gameObject.GetComponent<Renderer>();
				objectRenderer.material = Mat_out;
			}
		}

	* Drag and drop two Solid Color Materials into the Inspector
	
		.. image:: plum.gif 
		
		
	.. warning:: By reading the script, you can see that we change the sphere’s color through the Collider using the reference named *other*. Why not place this script directly on the sphere? We could, but that would require setting its Collider as a trigger, turning it into a ghost object and we don’t want that.
		
		
Display collider
----------------

It is possible to display the collider in Play mode:


	* Select the Game window (by clicking on the tab, not by activating Play mode)
	
	* On the right of the Game tab, click on *Gizmos* to show the collider during Play mode
	
	* In the Hierarchy, select the BCollider
	
	.. image:: giz.png
		:scale: 60%
		
	.. image:: wire.png
		:scale: 60%

	
Test
----	
	
	* Enter Play mode:
	
		.. image:: fallll.gif
			:scale: 60%