Issue with move_and_slide in basic platformer movement

dr_nir

Hey,

I'm just starting out and I'm following a tutorial on how to make a 2d platformer.

I'm using a KinematicBody2D for the player, and a TileMap for... well, the tiles.

I have the most bare bones Player movement, but I get a weird vertical jitter when walking left and right. Here is a video of how the player moves, and on the side are the position.y values: https://youtu.be/nWrIXY9yld4 I'm only pressing left and right here.

here is my code:

extends KinematicBody2D

const SPEED = 60
const GRAVITY = 10
const JUMP_V = -250
const UP_DIR = Vector2(0, -1)

var velocity = Vector2()

var y # for debugging

func _physics_process(delta):
	if y != position.y:
		y = position.y
		print(y)
	
	if Input.is_action_pressed("ui_right"):
		velocity.x = SPEED
	elif Input.is_action_pressed("ui_left"):
		velocity.x = -SPEED
	else:
		velocity.x = 0
		
	if Input.is_action_just_pressed("ui_select"):
		if is_on_floor():
			velocity.y = JUMP_V

	if not is_on_floor():
		velocity.y += GRAVITY
	
	velocity = move_and_slide(velocity, UP_DIR)

besides the visual jitter, I'm also unable to jump 50% of the times.

I'm using rectangles for all the collision shapes.

What can I possibly be doing wrong to cause this effect?

I'm on a mac with godot 3.2.3

EJFudd

I may be wrong, but to me it looks like your velocity.y never gets reset to 0. In the first line of _physics_process try adding velocity = Vector2.ZERO Or something like this before you check for a jump: if is_on_floor(): velocity.y = 0

dr_nir

according to documentation and verified by debug prints, velocity = move_and_slide(velocity, UP_DIR) sets y to zero when I'm not moving

dr_nir

I asked the same question on reddit and they asked for velocity prints, so I'm sharing them here as well:

https://youtu.be/o0T4A7KeeKM

TwistedTwigleg

I think this is due to the gravity being toggled on and off quickly, similar to the issue in this topic: Why is Floor Detection very inconsistent

I would recommend adding the following after line 28:

if not is_on_floor():
	velocity.y += GRAVITY
else:
	# add just a little gravity (1/4 strength)
	velocity.y += GRAVITY * 0.25

That might push the character down just enough to keep them "grounded". :smile:

Edit: Also, welcome to the forums!

dr_nir

Thank you very much, nice to be here :)

I've tried different things since I posted, and one of them was to just completely remove the if statement. Unfortunately it didn't help. I even started a whole new project and the bug was still there.

The only thing that stopped the jittering for me was to use move_and_slide_with_snap.

Honestly I'm very surprised not to hear about this from other people - I followed three different tutorials in the past day, including the official one about KinematicBody2D, and I always get this jumping. I don't think I'm doing something fundamentally wrong.

I will try to create the project on a windows machine, maybe godot has weird physics on a mac?

Edit: Tried the same thing on windows and got the same jittering.

Maybe all games have this, but we can't see it through all the animations?

TwistedTwigleg

Hmm, strange. I would expect the official tutorial on KinematicBody2D nodes to not exhibit this issue. What collision shape are you using? Are you using a box, a capsule, or something else? Maybe the shape is leading to the jumpy movement, though I am just guessing at this point.