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.

Using values of 1.0 would recreate the “instant” movement we started with.

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