Kinematic Friction

Problem

You want to add friction and acceleration to your kinematic character, giving it a smoother feel.

Solution

For most games, we’re not necessarily interested in a perfect physics simulation. We want action, responsiveness, and arcade feel. This is why you choose a kinematic body over a rigid one: so that you can control its behavior directly. However, some amount of physics is good - it means an object doesn’t instantly change direction or come to a stop.

Below is the code for a no-frills kinematic platformer character:

extends KinematicBody2D

var speed = 1200
var jump_speed = -1800
var gravity = 4000

var velocity = Vector2.ZERO

func get_input():
    velocity.x = 0
    if Input.is_action_pressed("ui_right"):
        velocity.x += speed
    if Input.is_action_pressed("ui_left"):
        velocity.x -= speed

func _physics_process(delta):
    get_input()
    velocity.y += gravity * delta
    velocity = move_and_slide(velocity, Vector2.UP)
    if Input.is_action_just_pressed("ui_select"):
        if is_on_floor():
            velocity.y = jump_speed

If you run this code, you’ll see that the character’s x velocity changes instantaneously. To fix this, we’ll use lerp() to gradually increase/decrease the velocity.

Using lerp

lerp(start_value, end_value, amount)

lerp(), aka linear interpolate, finds a “blended” value between two given numbers. See Interpolation for details.

In the code below, friction represents how quickly the character comes to a stop, while acceleration determines how quickly it gets up to full speed. Both are values between 0.0 and 1.0.

Replace the get_input() code with the following:

var friction = 0.1
var acceleration = 0.5

func get_input():
    var input_dir = 0
    if Input.is_action_pressed("ui_right"):
        input_dir += 1
    if Input.is_action_pressed("ui_left"):
        input_dir -= 1
    if dir != 0:
        # accelerate when there's input
        velocity.x = lerp(velocity.x, dir * speed, acceleration)
    else:
        # slow down when there's no input
        velocity.x = lerp(velocity.x, 0, friction)

Explanation

We’re using friction and acceleration as the amount to blend. For acceleration, we want to find a value between the current speed and the maximum, speed. When decelerating, we’re ramping the current speed down to 0.

Related Recipes

• Platform Character