Intermediate Unity 3D for iOS: Part 1/3

Unity Physics: Colliders and Bodies

It’s time to add Components to your Scene’s GameObjects so that they can react to each other!

Your goal is to have your objects react to each other when they collide. Luckily, Unity contains a fully-integrated physics engine and has packaged it up into a suite of Components that can easily dock onto your GameObjects.

Before you add ‘physics’ capabilities to your objects, you’ll need to first take a look into what ‘Physics’ means in Unity.

Click on the Components -> Physics menu (top toolbar) and have a quick look through the types of components readily available to you.

Colliders define the physical dimensions of your object, which can be independent of the visual shape of the object. In general, the colliders are ordered by complexity, and the more complex the object, the performance cost of using these objects rises.

Where possible, use Box/Sphere to encapsulate your objects, as these Colliders have the least computational load for your application. The other common Collider you’ll use frequently is the Mesh Collider. This uses the 3D model’s mesh to define the boundary of your object. In this case, the visual dimensions will equal the physical dimensions.

In addition to physically colliding with other objects, a Collider can be set up as a trigger that can detect collisions (so you can make something happen programmatically), but not actually cause any collision responses. This will be useful so you can detect when the ball goes through the basketball net.

In order for objects to react, each object must have some form of body in addition to a collider. This is done by either adding a Rigidbody or CharacterController to the GameObject.

The best way to get comfortable with Physics is to play around – let’s make that basketball bounce!

Bounce that Basketball!

Select the basketball, and add a Rigidbody onto it by selecting Component > Physics > Rigidbody. Then hit the Play button in the upper center of Unity to preview the gameplay – you’ll see the basketball fall below the floor.

If your basketball “warps” to the middle of the scene when you click play, select the basketball and unclick the Animation checkbox in the properties and try again. It should stay in the right spot now.

But it wouldn’t be much of a game if the ball was allowed to fly out of the scene! :] You’ll need to create a set of boundaries that will constrains the ball to the playing area.

To do this, select the scene.Wall object and select Component > Physics > Mesh Collider. Repeat for the scene.Ground and scene.Background objects. Then select the scene.court object and select Component > Physics > Box Collider.

If you play the scene again, you’ll see that it still falls through the floor. This is because you still haven’t set up a collider for the basketball!

So select the basketball and go to Component > Physics > Sphere Collider to set up a sphere collider for your basketball. It will default to the right size, but you can change the radius if you want in the Inspector’s Sphere Collider section.

Along with the ball reacting to the environment, you’ll also want it to bounce when it collides with an object. To do this, you’ll need to assign a special type of Material that Unity provides called a Physic Material.

Where Materials affect how objects look, Physic Materials determine how the object behaves when a collision occurs. This is associated with the Material property of the GameObjects Collider component.

The following image shows the properties of the Rigidbody attached to the basketball:

On the Project panel, select the Create dropdown menu and then select Physic Material to create a Physic Material , and name it BallPhyMat.

Now set the properties of the Physic Material as shown below. Details on the function of each of the properties shown below is out of scope for this tutorial, but further information can be found at http://docs.unity3d.com/Documentation/ScriptReference/PhysicMaterial.html.

In order to allow the ball to bounce, friction is set fairly low.

To associate the newly created Physic Material to the basketball, select the Physic Material you just created and drag it to the to the basketball’s Collider material property.

Click the play button again, and this time it falls to the floor and bounces, w00t! :]

As for the hoop, you want it to react to the ball as well as detect when the ball goes through the net. Add a Mesh Collider to the hoop mesh (hoop.LeftHoop).

Also, you want to set up a sensor or “trigger” to detect when the ball goes through the hoop. To do this, add a Box Collider to hoop.LeftHoop_001, but shorten the box and position it so it’s just below the net (you can do this by tweaking the values in the Inspector – I changed Center Z to -1.4 and Size Z to 0.2). Also, click the checkbox to set the trigger property to true.

Note: to visually resize colliders using the mouse, select the collider’s object and hold down Shift. This will show the handles of the collider, allowing you to resize it using the mouse.

Okay! That takes care of the ball — time to take a look at the Player object! :]

Meeting the Team

In this game, we want the player to bounce the ball and for the ball to not roll through him. Stopping the ball rolling through the Player is easy enough; go ahead and add a Capsule Collider to the player GameObject.

You’ll then need to position and size the capsule – I changed the height to 3.8 and the Center Y to 1.8.

Note: Colliders are shown in the Scene panel in a green outline — unless they are Mesh Colliders , in which case it’s the mesh that shows the collision boundaries.

The approach used to bounce the ball requires that the game can detect when the ball collides with the player’s hand. When this occurs, you will push the ball back down to the ground, just like it happens in real life! To attach the collider at the correct position, you’ll drill down to the player’s skeleton and add the collider to the hand which will be used to bounce the ball.

The above screenshot shows you the children of the player GameObject. Where do these children come from? Good question! :]

These children form the skeleton built to animate the player – the parent is the pelvis which also has the Skeleton Mesh Renderer component attached to it, which is responsible for rendering the mesh,. The children are the bones of the skeleton which were built in Blender. ArmIK_L, ArmIK_R, LegIK_L, LegIK_R are just handles used in Blender and have no function in your app.

Now add a Box Collider to player\BPlayerSkeleton\Pelvis\Hip\Spine\Shoulder_R\UpperArm_R\LowerArm_R\Hand_R and resize the Collider similar to what is shown below, and set the trigger flag to true.