Version: 2023.1
Language : English
Collision
Continuous collision detection (CCD)

# Introduction to collision

Unity handles collision between GameObjects with colliders, which attach to GameObjects and define the shape of a for the purposes of physical collisions. A collider is invisible, and does not need to be the exact same shape as the GameObject’s . A rough approximation of the mesh is often more efficient and indistinguishable in gameplay.

The simplest (and least processor-intensive) colliders are primitive collider types. In 3D, these are the , and . In 2D, you can use the Box Collider 2D and Circle Collider 2D. You can add any number of these to a single GameObject to create compound colliders.

## Compound colliders

Compound colliders approximate the shape of a GameObject while keeping a low processor overhead. To get further flexibility, you can add additional colliders on child GameObjects. For instance, you can rotate boxes relative to the local axes of the parent GameObject. When you create a compound collider like this, you should only use one component, placed on the root GameObject in the hierarchy.

Primitive colliders do not work correctly with shear transforms. If you use a combination of rotations and non-uniform scales in the Transform hierarchy so that the resulting shape is no longer a primitive shape, the primitive collider cannot represent it correctly.

## Mesh colliders

There are some cases, however, where even compound colliders are not accurate enough. In 3D, you can use to match the shape of the GameObject’s mesh exactly. In 2D, the Polygon Collider 2D does not match the shape of the graphic perfectly but you can refine the shape to any you like.

These colliders are much more processor-intensive than primitive types, so use them sparingly to maintain good performance. Also, a mesh collider cannot collide with another mesh collider (i.e., nothing happens when they make contact). You can get around this in some cases by marking the mesh collider as Convex in the . This generates the collider shape as a “convex hull” which is like the original mesh but with any undercuts filled in.

The benefit of this is that a convex mesh collider can collide with other mesh colliders so you can use this feature when you have a moving character with a suitable shape. However, a good rule is to use mesh colliders for geometry and approximate the shape of moving GameObjects using compound primitive colliders.

## Static colliders

You can add colliders to a GameObject without a Rigidbody component to create floors, walls and other motionless elements of a Scene. These are referred to as static colliders. At the opposite, colliders on a GameObject that has a Rigidbody are known as dynamic colliders. Static colliders can interact with dynamic colliders but since they don’t have a Rigidbody, they don’t move in response to collisions.

## Physics materials

When colliders interact, their surfaces need to simulate the properties of the material they are supposed to represent. For example, a sheet of ice will be slippery while a rubber ball will offer a lot of friction and be very bouncy. Although the shape of colliders is not deformed during collisions, their friction and bounce can be configured using Physics Materials. Getting the parameters just right can involve a bit of trial and error. A slippery material like ice, for example, has zero (or very low) friction. A grippy material like rubber has high friction and near-perfect bounciness. See the reference pages for Physic Material and Physics Material 2D for further details on the available parameters. Note that for historical reasons, the 3D asset is actually called (without the S) but the 2D equivalent is called (with the S).

## Triggers

The scripting system can detect when collisions occur and initiate actions using the `OnCollisionEnter` function. However, you can also use the simply to detect when one collider enters the space of another without creating a collision. A collider configured as a Trigger (using the Is Trigger property) does not behave as a solid object and will simply allow other colliders to pass through. When a collider enters its space, a trigger will call the `OnTriggerEnter` function on the trigger object’s .

## Collision callbacks for scripts

When collisions occur, the physics engine calls functions with specific names on any scripts attached to the objects involved. You can place any code you like in these functions to respond to the collision event. For example, you might play a crash sound effect when a car bumps into an obstacle.

On the first physics update where the collision is detected, the `OnCollisionEnter` function is called. During updates where contact is maintained, `OnCollisionStay` is called and finally, `OnCollisionExit` indicates that contact has been broken. Trigger colliders call the analogous `OnTriggerEnter`, `OnTriggerStay` and `OnTriggerExit` functions. Note that for 2D physics, there are equivalent functions with 2D appended to the name, eg, `OnCollisionEnter2D`. Full details of these functions and code samples can be found on the Script Reference page for the MonoBehaviour class.

With normal, non-trigger collisions, there is an additional detail that at least one of the objects involved must have a non-kinematic Rigidbody (ie, Is Kinematic must be switched off). If both objects are kinematic Rigidbodies then `OnCollisionEnter`, etc, will not be called. With trigger collisions, this restriction doesn’t apply and so both kinematic and non-kinematic Rigidbodies will prompt a call to `OnTriggerEnter` when they enter a trigger collider.

## Collider interactions

Colliders interact with each other differently depending on how their Rigidbody components are configured. The three important configurations are the Static Collider (ie, no Rigidbody is attached at all), the Rigidbody Collider and the Kinematic Rigidbody Collider.

### Static Collider

A static collider is a GameObject that has a Collider but no Rigidbody. Static colliders are mostly used for level geometry which always stays at the same place and never moves around. Incoming Rigidbody objects collide with static colliders but don’t move them.

In particular cases, the physics engine optimizes for static colliders that never move. For instance, a vehicle resting on top of a static collider remains asleep even if you move this static collider. You can enable, disable, or move static colliders in runtime without specially affecting the physics engine computation speed. Also, you can safely scale a static Mesh Collider as long as the scale is uniform (not skewed).

### Rigidbody Collider

This is a GameObject with a Collider and a normal, non-kinematic Rigidbody attached. Rigidbody colliders are fully simulated by the physics engine and can react to collisions and forces applied from a script. They can collide with other objects (including static colliders) and are the most commonly used Collider configuration in games that use physics.

### Kinematic Rigidbody Collider

This is a GameObject with a Collider and a kinematic Rigidbody attached (ie, the IsKinematic property of the Rigidbody is enabled). You can move a kinematic rigidbody object from a script by modifying its but it will not respond to collisions and forces like a non-kinematic rigidbody. Kinematic rigidbodies should be used for colliders that can be moved or disabled/enabled occasionally but that should otherwise behave like static colliders. An example of this is a sliding door that should normally act as an immovable physical obstacle but can be opened when necessary. Unlike a static collider, a moving kinematic rigidbody will apply friction to other objects and will “wake up” other rigidbodies when they make contact.

Even when immobile, kinematic rigidbody colliders have different behavior to static colliders. For example, if the collider is set to as a trigger then you also need to add a rigidbody to it in order to receive trigger events in your script. If you don’t want the trigger to fall under gravity or otherwise be affected by physics then you can set the IsKinematic property on its rigidbody.

A Rigidbody component can be switched between normal and kinematic behavior at any time using the IsKinematic property.

A common example of this is the “ragdoll” effect where a character normally moves under animation but is thrown physically by an explosion or a heavy collision. The character’s limbs can each be given their own Rigidbody component with IsKinematic enabled by default. The limbs move normally by animation until IsKinematic is switched off for all of them and they immediately behave as physics objects. At this point, a collision or explosion force will send the character flying with its limbs thrown in a convincing way.

## Collision action matrix

When two objects collide, a number of different script events can occur depending on the configurations of the colliding objects’ rigidbodies. The charts below give details of which event functions are called based on the components that are attached to the objects. Some of the combinations only cause one of the two objects to be affected by the collision, but the general rule is that physics will not be applied to an object that doesn’t have a Rigidbody component attached.

Collision detection occurs and messages are sent upon collision
Static Collider Rigidbody Collider Kinematic Rigidbody Collider Static Trigger Collider Rigidbody Trigger Collider Kinematic Rigidbody Trigger Collider
Static Collider   Y
Rigidbody Collider Y Y Y
Kinematic Rigidbody Collider   Y
Static Trigger Collider
Rigidbody Trigger Collider
Kinematic Rigidbody Trigger Collider
Trigger messages are sent upon collision
Static Collider Rigidbody Collider Kinematic Rigidbody Collider Static Trigger Collider Rigidbody Trigger Collider Kinematic Rigidbody Trigger Collider
Static Collider         Y Y
Rigidbody Collider       Y Y Y
Kinematic Rigidbody Collider       Y Y Y
Static Trigger Collider   Y Y   Y Y
Rigidbody Trigger Collider Y Y Y Y Y Y
Kinematic Rigidbody Trigger Collider Y Y Y Y Y Y
Collision
Continuous collision detection (CCD)