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Physics introduction

In game development, you often need to know when two objects in the game intersect or come into contact. This is known as collision detection. When a collision is detected, you typically want something to happen. This is known as collision response.

Godot offers a number of collision objects in 2D and 3D to provide both collision detection and response. Trying to decide which one to use for your project can be confusing. You can avoid problems and simplify development if you understand how each works and what their pros and cons are.

In this guide, you will learn:

• Godot's four collision object types

• How each collision object works

• When and why to choose one type over another

Note

This document's examples will use 2D objects. Every 2D physics object and collision shape has a direct equivalent in 3D and in most cases they work in much the same way.

Collision objects

Godot offers four kinds of collision objects which all extend CollisionObject2D. The last three listed below are physics bodies and additionally extend PhysicsBody2D.

• Area2D

  Area2D nodes provide detection and influence. They can detect when objects overlap and can emit signals when bodies enter or exit. An Area2D can also be used to override physics properties, such as gravity or damping, in a defined area.

• StaticBody2D

  A static body is one that is not moved by the physics engine. It participates in collision detection, but does not move in response to the collision. They are most often used for objects that are part of the environment or that do not need to have any dynamic behavior.

• RigidBody2D

  This is the node that implements simulated 2D physics. You do not control a RigidBody2D directly, but instead you apply forces to it (gravity, impulses, etc.) and the physics engine calculates the resulting movement. Read more about using rigid bodies.

• CharacterBody2D

  A body that provides collision detection, but no physics. All movement and collision response must be implemented in code.

Physics material

Static bodies and rigid bodies can be configured to use a PhysicsMaterial. This allows adjusting the friction and bounce of an object, and set if it's absorbent and/or rough.

Collision shapes

A physics body can hold any number of Shape2D objects as children. These shapes are used to define the object's collision bounds and to detect contact with other objects.

Note

In order to detect collisions, at least one Shape2D must be assigned to the object.

The most common way to assign a shape is by adding a CollisionShape2D or CollisionPolygon2D as a child of the object. These nodes allow you to draw the shape directly in the editor workspace.

Important

Be careful to never scale your collision shapes in the editor. The "Scale" property in the Inspector should remain (1, 1). When changing the size of the collision shape, you should always use the size handles, not the Node2D scale handles. Scaling a shape can result in unexpected collision behavior.

Physics process callback

The physics engine runs at a fixed rate (a default of 60 iterations per second). This rate is typically different from the frame rate which fluctuates based on what is rendered and available resources.

It is important that all physics related code runs at this fixed rate. Therefore Godot differentiates between physics and idle processing. Code that runs each frame is called idle processing and code that runs on each physics tick is called physics processing. Godot provides two different callbacks, one for each of those processing rates.

The physics callback, Node._physics_process(), is called before each physics step. Any code that needs to access a body's properties should be run in here. This method will be passed a delta parameter, which is a floating-point number equal to the time passed in seconds since the last step. When using the default 60 Hz physics update rate, it will typically be equal to 0.01666... (but not always, see below).

Note

It's recommended to always use the delta parameter when relevant in your physics calculations, so that the game behaves correctly if you change the physics update rate or if the player's device can't keep up.

Collision layers and masks

One of the most powerful, but frequently misunderstood, collision features is the collision layer system. This system allows you to build up complex interactions between a variety of objects. The key concepts are layers and masks. Each CollisionObject2D has 32 different physics layers it can interact with.

Let's look at each of the properties in turn:

• collision_layer

  This describes the layers that the object appears in. By default, all bodies are on layer 1.

• collision_mask

  This describes what layers the body will scan for collisions. If an object isn't in one of the mask layers, the body will ignore it. By default, all bodies scan layer 1.

These properties can be configured via code, or by editing them in the Inspector.

Keeping track of what you're using each layer for can be difficult, so you may find it useful to assign names to the layers you're using. Names can be assigned in Project Settings -> Layer Names.

GUI example

You have four node types in your game: Walls, Player, Enemy, and Coin. Both Player and Enemy should collide with Walls. The Player node should detect collisions with both Enemy and Coin, but Enemy and Coin should ignore each other.

Start by naming layers 1-4 "walls", "player", "enemies", and "coins" and place each node type in its respective layer using the "Layer" property. Then set each node's "Mask" property by selecting the layers it should interact with. For example, the Player's settings would look like this:

Code example

In function calls, layers are specified as a bitmask. Where a function enables all layers by default, the layer mask will be given as 0xffffffff. Your code can use binary, hexadecimal, or decimal notation for layer masks, depending on your preference.

The code equivalent of the above example where layers 1, 3 and 4 were enabled would be as follows:

# Example: Setting mask value for enabling layers 1, 3 and 4

# Binary - set the bit corresponding to the layers you want to enable (1, 3, and 4) to 1, set all other bits to 0.
# Note: Layer 32 is the first bit, layer 1 is the last. The mask for layers 4,3 and 1 is therefore
0b00000000_00000000_00000000_00001101
# (This can be shortened to 0b1101)

# Hexadecimal equivalent (1101 bi