Using KinematicBody2D

Introduction

Godot offers several collision objects 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 of them works and what their pros and cons are. In this tutorial, we'll look at the KinematicBody2D node and show some examples of how to use it.

Note

This document assumes you're familiar with Godot's various physics bodies. Please read Physics introduction first.

What is a kinematic body?

KinematicBody2D is for implementing bodies that are controlled via code. Kinematic bodies detect collisions with other bodies when moving, but are not affected by engine physics properties, like gravity or friction. While this means that you have to write some code to create their behavior, it also means you have more precise control over how they move and react.

Tip

A KinematicBody2D can be affected by gravity and other forces, but you must calculate the movement in code. The physics engine will not move a KinematicBody2D.

Movement and collision

When moving a KinematicBody2D, you should not set its position property directly. Instead, you use the move_and_collide() or move_and_slide() methods. These methods move the body along a given vector and instantly stop if a collision is detected with another body. After a KinematicBody2D has collided, any collision response must be coded manually.

Warning

You should only do Kinematic body movement in the _physics_process() callback.

The two movement methods serve different purposes, and later in this tutorial, you'll see examples of how they work.

move_and_collide

This method takes one parameter: a Vector2 indicating the body's relative movement. Typically, this is your velocity vector multiplied by the frame timestep (delta). If the engine detects a collision anywhere along this vector, the body will immediately stop moving. If this happens, the method will return a KinematicCollision2D object.

KinematicCollision2D is an object containing data about the collision and the colliding object. Using this data, you can calculate your collision response.

move_and_slide

The move_and_slide() method is intended to simplify the collision response in the common case where you want one body to slide along the other. It is especially useful in platformers or top-down games, for example.

Tip

move_and_slide() automatically calculates frame-based movement using delta. Do not multiply your velocity vector by delta before passing it to move_and_slide().

In addition to the velocity vector, move_and_slide() takes a number of other parameters allowing you to customize the slide behavior:

• up_direction - default value: Vector2( 0, 0 )

  This parameter allows you to define what surfaces the engine should consider being the floor. Setting this lets you use the is_on_floor(), is_on_wall(), and is_on_ceiling() methods to detect what type of surface the body is in contact with. The default value means that all surfaces are considered walls.

• stop_on_slope - default value: false

  This parameter prevents a body from sliding down slopes when standing still.

• max_slides - default value: 4

  This parameter is the maximum number of collisions before the body stops moving. Setting it too low may prevent movement entirely.

• floor_max_angle - default value: 0.785398 (in radians, equivalent to 45 degrees)

  This parameter is the maximum angle before a surface is no longer considered a "floor."

• infinite_inertia - default value: true

When this parameter is true, the body can push RigidBody2D nodes, ignoring their mass, but won't detect collisions with them. If it's false the body will collide with rigid bodies and stop.

move_and_slide_with_snap

This method adds some additional functionality to move_and_slide() by adding the snap parameter. As long as this vector is in contact with the ground, the body will remain attached to the surface. Note that this means you must disable snapping when jumping, for example. You can do this either by setting snap to Vector2.ZERO or by using move_and_slide() instead.

Detecting collisions

When using move_and_collide() the function returns a KinematicCollision2D directly, and you can use this in your code.

When using move_and_slide() it's possible to have multiple collisions occur, as the slide response is calculated. To process these collisions, use get_slide_count() and get_slide_collision():

GDScriptC#

# Using move_and_collide.
var collision = move_and_collide(velocity * delta)
if collision:
    print("I collided with ", collision.collider.name)

# Using move_and_slide.
velocity = move_and_slide(velocity)
for i in get_slide_count():
    var collision = get_slide_collision(i)
    print("I collided with ", collision.collider.name)

Note

get_slide_count() only counts times the body has collided and changed direction.

See KinematicCollision2D for details on what collision data is returned.

Which movement method to use?

A common question from new Godot users is: "How do you decide which movement function to use?" Often, the response is to use move_and_slide() because it's "simpler," but this is not necessarily the case. One way to think of it is that move_and_slide() is a special case, and move_and_collide() is more general. For example, the following two code snippets result in the same collision response:

GDScriptC#

# using move_and_collide
var collision = move_and_collide(velocity * delta)
if collision:
    velocity = velocity.slide(collision.normal)

# using move_and_slide
velocity = move_and_slide(velocity)

Anything you do with move_and_slide() can also be done with move_and_collide(), but it might take a little more code. However, as we'll see in the examples below, there are cases where move_and_slide() doesn't provide the response you want.

In the example above, we assign the velocity that move_and_slide() returns back into the velocity variable. This is because when the character collides with the environment, the function recalculates the speed internally to reflect the slowdown.

For example, if your character fell on the floor, you don't want it to accumulate vertical speed due to the effect of gravity. Instead, you want its vertical speed to reset to zero.

move_and_slide() may also recalculate the kinematic body's velocity several times in a loop as, to produce a smooth motion, it moves the character and collides up to five times by default. At the end of the process, the function returns the character's new velocity that we can store in our velocity variable, and use on the next frame.

Examples

To see these examples in action, download the sample project: using_kinematic2d.zip.

Movement and walls

If you've downloaded the sample project, this example is in "BasicMovement.tscn".

For this example, add a KinematicBody2D with two children: a Sprite and a CollisionShape2D. Use the Godot "icon.png" as the Sprite's texture (drag it from the Filesystem dock to the Texture property of the Sprite). In the CollisionShape2D's Shape property, select "New RectangleShape2D" and size the rectangle to fit over the sprite image.

Note

See 2D movement overview for examples of implementing 2D movement schemes.

Attach a script to the KinematicBody2D and add the following code:

GDScriptC#

extends KinematicBody2D

var speed = 250
var velocity = Vector2()

func get_input():
    # Detect up/down/left/right keystate and only move when pressed.
    velocity = Vector2()
    if Input.is_action_pressed('ui_right'):
        velocity.x += 1
    if Input.is_action_pressed('ui_left'):
        velocity.x -= 1
    if Input.is_action_pressed('ui_down'):
        velocity.y += 1
    if Input.is_action_pressed('ui_up'):
        velocity.y -= 1
    velocity = velocity.normalized() * speed

func _physics_process(delta):
    get_input()
    move_and_collide(velocity * delta)

Run this scene and you'll see that move_and_collide() works as expected, moving the body along the velocity vector. Now let's see what happens when you add some obstacles. Add a StaticBody2D with a rectangular collision shape. For visibility, you can use a sprite, a Polygon2D, or turn on "Visible Collision Shapes" from the "Debug" menu.

Run the scene again and try moving into the obstacle. You'll see that the KinematicBody2D can't penetrate the obstacle. However, try moving into the obstacle at an angle and you'll find that the obstacle acts like glue - it feels like the body gets stuck.

This happens because there is no collision response. move_and_collide() stops the body's movement when a collision occurs. We need to code whatever response we want from the collision.

Try changing the function to move_and_slide(velocity) and running again. Note that we removed delta from the velocity calculation.

move_and_slide() provides a default collision response of sliding the body along the collision object. This is useful for a great many game types, and may be all you need to get the behavior you want.

Bouncing/reflecting

What if you don't want a sliding collision response? For this example ("BounceandCollide.tscn" in the sample project), we have a character shooting bullets and we want the bullets to bounce off the walls.

This example uses three scenes. The main scene contains the Player and Walls. The Bullet and Wall are separate scenes so that they can be instanced.

The Player is controlled by the w and s keys for forward and back. Aiming uses the mouse pointer. Here is the code for the Player, using move_and_slide():

GDScriptC#

extends KinematicBody2D

var Bullet = preload("res://Bullet.tscn")
var speed = 200
var velocity = Vector2()

func get_input():
    # Add these actions in Project Settings -> Input Map.
    velocity = Vector2()
    if Input.is_action_pressed('backward'):
        velocity = Vector2(-speed/3, 0).rotated(rotation)
    if Input.is_action_pressed('forward'):
        velocity = Vector2(speed, 0).rotated(rotation)
    if Input.is_action_just_pressed('mouse_click'):
        shoot()

func shoot():
    # "Muzzle" is a Position2D placed at the barrel of the gun.
    var b = Bullet.instance()
    b.start($Muzzle.global_position, rotation)
    get_parent().add_child(b)

func _physics_process(delta):
    get_input()
    var dir = get_global_mouse_position() - global_position
    # Don't move if too close to the mouse pointer.
    if dir.length() > 5:
        rotation = dir.angle()
        velocity = move_and_slide(velocity)

And the code for the Bullet:

GDScriptC#

extends KinematicBody2D

var speed = 750
var velocity = Vector2()

func start(pos, dir):
    rotation = dir
    position = pos
    velocity = Vector2(speed, 0).rotated(rotation)

func _physics_process(delta):
    var collision = move_and_collide(velocity * delta)
    if collision:
        velocity = velocity.bounce(collision.normal)
        if collision.collider.has_method("hit"):
            collision.collider.hit()

func _on_VisibilityNotifier2D_screen_exited():
    queue_free()

The action happens in _physics_process(). After using move_and_collide(), if a collision occurs, a KinematicCollision2D object is returned (otherwise, the return is Nil).

If there is a returned collision, we use the normal of the collision to reflect the bullet's velocity with the Vector2.bounce() method.

If the colliding object (collider) has a hit method, we also call it. In the example project, we've added a flashing color effect to the Wall to demonstrate this.

Platformer movement

Let's try one more popular example: the 2D platformer. move_and_slide() is ideal for quickly getting a functional character controller up and running. If you've downloaded the sample project, you can find this in "Platformer.tscn".

For this example, we'll assume you have a level made of StaticBody2D objects. They can be any shape and size. In the sample project, we're using Polygon2D to create the platform shapes.

Here's the code for the player body:

GDScriptC#

extends KinematicBody2D

export (int) var run_speed = 100
export (int) var jump_speed = -400
export (int) var gravity = 1200

var velocity = Vector2()
var jumping = false

func get_input():
    velocity.x = 0
    var right = Input.is_action_pressed('ui_right')
    var left = Input.is_action_pressed('ui_left')
    var jump = Input.is_action_just_pressed('ui_select')

    if jump and is_on_floor():
        jumping = true
        velocity.y = jump_speed
    if right:
        velocity.x += run_speed
    if left:
        velocity.x -= run_speed

func _physics_process(delta):
    get_input()
    velocity.y += gravity * delta
    if jumping and is_on_floor():
        jumping = false
    velocity = move_and_slide(velocity, Vector2(0, -1))

When using move_and_slide(), the function returns a vector representing the movement that remained after the slide collision occurred. Setting that value back to the character's velocity allows us to move up and down slopes smoothly. Try removing velocity = and see what happens if you don't do this.

Also note that we've added Vector2(0, -1) as the floor normal. This vector points straight upward. As a result, if the character collides with an object that has this normal, it will be considered a floor.

Using the floor normal allows us to make jumping work, using is_on_floor(). This function will only return true after a move_and_slide() collision where the colliding body's normal is within 45 degrees of the given floor vector. You can control the maximum angle by setting floor_max_angle.

This angle also allows you to implement other features like wall jumps using is_on_wall(), for example.