Rigidbody 2D

A Rigidbody 2D component places an object under the control of the physics engine. Many concepts familiar from the standard Rigidbody component carry over to Rigidbody 2D; the differences are that in 2D, objects can only move in the XY plane and can only rotate on an axis perpendicular to that plane.

How a Rigidbody 2D works

Usually, the Unity Editor’s Transform component defines how a GameObject (and its child GameObjects) is positioned, rotated and scaled within the Scene. When it is changed, it updates other components, which may update things like where they render or where colliders are positioned. The 2D physics engine is able to move colliders and make them interact with each other, so a method is required for the physics engine to communicate this movement of colliders back to the Transform components. This movement and connection with colliders is what a Rigidbody 2D component is for.

The Rigidbody 2D component overrides the Transform and updates it to a position/rotation defined by the Rigidbody 2D. Note that while you can still override the Rigidbody 2D by modifying the Transform component yourself (because Unity exposes all properties on all components), doing so will cause problems such as GameObjects passing through or into each other, and unpredictable movement.

Any Collider 2D component added to the same GameObject or child GameObject is implicitly attached to that Rigidbody 2D. When a Collider 2D is attached to the Rigidbody 2D, it moves with it. A Collider 2D should never be moved directly using the Transform or any collider offset; the Rigidbody 2D should be moved instead. This offers the best performance and ensures correct collision detection. Collider 2Ds attached to the same Rigidbody 2D won’t collide with each other. This means you can create a set of colliders that act effectively as a single compound collider, all moving and rotating in sync with the Rigidbody 2D.

When designing a Scene, you are free to use a default Rigidbody 2D and start attaching colliders. These colliders allow any other colliders attached to different Rigidbody 2Ds to collide with each other.

Tip

Adding a Rigidbody 2D allows a sprite to move in a physically convincing way by applying forces from the scripting API. When the appropriate collider component is also attached to the sprite GameObject, it is affected by collisions with other moving GameObjects. Using physics simplifies many common gameplay mechanics and allows for realistic behavior with minimal coding.

Body Type

The Rigidbody 2D component has a setting at the top labelled Body Type. The option you choose for this affects the other settings available on the component.

There are three options for Body Type; each defines a common and fixed behavior. Any Collider 2D attached to a Rigidbody 2D inherits the Rigidbody 2D’s Body Type. The three options are:

• Dynamic
• Kinematic
• Static

The option you choose defines:

• Movement (position & rotation) behavior
• Collider interaction

Note that although Rigidbody 2Ds are often described as colliding with each other, it is the Collider 2Ds attached to each of those bodies which collide. Rigidbody 2Ds cannot collide with each other without colliders.

Changing the Body Type of a Rigidbody 2D can be a tricky process. When a Body Type changes, various mass-related internal properties are recalculated immediately, and all existing contacts for the Collider 2Ds attached to the Rigidbody 2D need to be re-evaluated during the GameObject’s next FixedUpdate. Depending on how many contacts and Collider 2Ds are attached to the body, changing the Body Type can cause variations in performance.

Body Type: Dynamic

A Dynamic Rigidbody 2D is designed to move under simulation. It has the full set of properties available to it such as finite mass and drag, and is affected by gravity and forces. A Dynamic body will collide with every other body type, and is the most interactive of body types. This is the default body type for a Rigidbody 2D, because it is the most common body type for things that need to move. It’s also the most performance-expensive body type, because of its dynamic nature and interactivity with everything around it. All Rigidbody 2D properties are available with this body type.

Do not use the Transform component to set the position or rotation of a Dynamic Rigidbody 2D. The simulation repositions a Dynamic Rigidbody 2D according to its velocity; you can change this directly via forces applied to it by scripts, or indirectly via collisions and gravity.

Body Type: Kinematic

A Kinematic Rigidbody 2D is designed to move under simulation, but only under very explicit user control. While a Dynamic Rigidbody 2D is affected by gravity and forces, a Kinematic Rigidbody 2D isn’t. For this reason, it is fast and has a lower demand on system resources than a Dynamic Rigidbody 2D. Kinematic Rigidbody 2D is designed to be repositioned explicitly via Rigidbody2D.MovePosition or Rigidbody2D.MoveRotation. Use physics queries to detect collisions, and scripts to decide where and how the Rigidbody 2D should move.

A Kinematic Rigidbody 2D does still move via its velocity, but the velocity is not affected by forces or gravity. A Kinematic Rigidbody 2D does not collide with other Kinematic Rigidbody 2Ds or with Static Rigidbody 2Ds; it only collides with Dynamic Rigidbody 2Ds. Similar to a Static Rigidbody 2D (see below), a Kinematic Rigidbody 2D behaves like an immovable object (as if it has infinite mass) during collisions. Mass-related properties are not available with this Body Type.

Body Type: Static

A Static Rigidbody 2D is designed to not move under simulation at all; if anything collides with it, a Static Rigidbody 2D behaves like an immovable object (as though it has infinite mass). It is also the least resource-intensive body type to use. A Static body only collides with Dynamic Rigidbody 2Ds. Having two Static Rigidbody 2Ds collide is not supported, since they are not designed to move.

Only a very limited set of properties are available for this Body Type.

There are two ways to mark a Rigidbody 2D as Static:

1. For the GameObject with the Collider 2D component not to have a Rigidbody 2D component at all. All such Collider 2Ds are internally considered to be attached to a single hidden Static Rigidbody 2D component.

2. For the GameObject to have a Rigidbody 2D and for that Rigidbody 2D to be set to Static.

Method 1 is a shorthand for making Static Collider 2Ds. When creating large numbers of Static Collider 2Ds, it is easier not to have to add a Rigidbody 2D for each GameObject with a Collider 2D.

Method 2 exists for performance reasons. If a Static Collider 2D needs to be moved or reconfigured at run time, it is faster to do so when it has its own Rigidbody 2D. If a group of Collider 2Ds needs to be moved or reconfigured at run time, it is faster to have them all be children of one parent Rigidbody 2D marked as Static than to move each GameObject individually.

Note: As stated above, Static Rigidbody 2Ds are designed not to move, and collisions between two Static Rigidbody 2D objects that intersect are not registered. However, Static Rigidbody 2Ds and Kinematic Rigidbody 2Ds will interact with each other if one of their Collider 2Ds is set to be a trigger. There is also a feature that changes what a Kinematic body will interact with (see Use Full Kinematic Contacts, below).

Rigidbody 2D properties

Simulated

Use the Simulated property to stop (unchecked) and start (checked) a Rigidbody 2D and any attached Collider 2Ds and Joint 2Ds from interacting with the 2D physics simulation. Changing this property is much more memory and processor-efficient than enabling or disabling individual Collider 2D and Joint 2D components.

When the Simulated box is checked, the following occurs:

• The Rigidbody 2D moves via the simulation (gravity and physics forces are applied)
• Any attached Collider 2Ds continue creating new contacts and continuously re-evaluate contacts
• Any attached Joint 2Ds are simulated and constrain the attached Rigidbody 2D
• All internal physics objects for Rigidbody 2D, Collider 2D & Joint 2D stay in memory

When the Simulated box is unchecked, the following occurs:

• The Rigidbody 2D is not moved by the simulation (gravity and physics forces are not applied)
• The Rigidbody 2D does not create new contacts, and any attached Collider 2D contacts are destroyed
• Any attached Joint 2Ds are not simulated, and do not constrain any attached Rigidbody 2Ds
• All internal physics objects for Rigidbody 2D, Collider 2D and Joint 2D are left in memory

Why is unchecking Simulated more efficient than individual component controls?

In the 2D physics simulation, a Rigidbody 2D component controls the position and rotation of attached Collider 2D components, and allows Joint 2D components to use these positions and rotations as anchor points. A Collider 2D moves when the Rigidbody 2D it is attached to moves. The Collider 2D then calculates contacts with other Collider 2Ds attached to other Rigidbody 2Ds. Joint 2Ds also constrain Rigidbody 2D positions and rotations. All of this takes simulation time.

You can stop and start individual elements of the 2D physics simulation by enabling and disabling components individually. You can do this on both Collider 2D and Joint 2D components. However, enabling and disabling individual elements of the physics simulations has memory use and processor power costs. When elements of the simulation are disabled, the 2D physics engine doesn’t produce any internal physics-based objects to simulate. When elements of the simulation are enabled, the 2D physics engine does have internal physics-based objects to simulate. Enabling and disabling of 2D physics simulation components means internal GameObjects and physics-based components have to be created and destroyed; disabling the simulation is easier and more efficient than disabling individual components.

NOTE: When a Rigidbody 2D’s Simulated option is unchecked, any attached Collider 2D is effectively ‘invisible’, that is; it cannot be detected by any physics queries, such as Physics.Raycast.

Use Full Kinematic Contacts

Enable this setting (check the checkbox) if you want the Kinematic Rigidbody 2D to collide with all Rigidbody 2D Body Types. This is similar to a Dynamic Rigidbody 2D, except the Kinematic Rigidbody 2D is not moved by the physics engine when contacting another Rigidbody 2D; it acts as an immovable object, with infinite mass.

When this setting is disabled (unchecked), a Kinematic Rigidbody 2D only collides with Dynamic Rigidbody 2Ds; it does not collide with other Kinematic Rigidbody 2Ds or Static Rigidbody 2Ds (note that trigger colliders are an exception to this rule). This means that no collision scripting callbacks (OnCollisionEnter, OnCollisionStay, OnCollisionExit) occur.

This can be inconvenient when you are using physics queries (such as Physics.Raycast) to detect where and how a Rigidbody 2D should move, and when you require multiple Kinematic Rigidbody 2Ds to interact with each other. Enable Use Full Kinematic Contacts to make Kinematic Rigidbody 2D components interact in this way.

Use Full Kinematic Contacts allows explicit position and rotation control of a Kinematic Rigidbody 2D, but still allows full collision callbacks. In a set-up where you need explicit control of all Rigidbody 2Ds, use Kinematic Rigidbody 2Ds in place of Dynamic Rigidbody 2Ds to still have full collision callback support.