Tilting on Slopes

In the tutorial about handling slope movement, @intelligenced asked about tilting the character parallel to the slope. So I decided to make a quick experiment reusing the Slope Movement, which also just got updated with a small refactoring to encapsulate the moving logic into a _move method.

Now in this experiment, I’ve approached tilting the character based on the slope’s angle. But how to get the slope angle, you might be thinking, right? We’ll see in this new tutorial.

What is a slope?

Well, the first thing to understand when we are dealing with a slope is…what a slope is?

The cool thing about programming and game development especially is that things are what you want them to be. That’s what a variable or a method declaration is, you name something and describe what this thing is.

In our context, the KinematicBody2D’s move_and_slide_with_snap allows us to pass an argument (floor_max_angle) to describe when the floor stops…being floor. In the documentation, it states that:

floor_max_angle is the maximum angle (in radians) where a slope is still considered a floor (or a ceiling), rather than a wall. The default value equals 45 degrees.

It’s very interesting how programming is essentially a philosophical debate. Imagine you’re debating how an object should move on a slope and the person asks you “What is a slope?” you argue (give an argument) that a slope is a part of the floor or ceiling with an angle lower than 45 degrees. Now that this person knows the definition of a slope, you can keep your debate. The same thing goes here.

In our case, our definition of a slope uses the value from a const that I’ve declared right below the SNAP_LENGTH. I’ve called it SLOPE_LIMIT because beyond this angle the KinematicBody2D can consider that it is handling a wall. I’ve set the SLOPE_LIMIT to be 46 degrees because I want to consider slopes up to 45 degrees.

I’ve used the deg2rad built-in function to convert degrees to radians since the floor_max_angle works in radians.

extends KinematicBody2D

const FLOOR_NORMAL = Vector2.UP
const SNAP_LENGTH = 64.0
const SLOPE_LIMIT = deg2rad(46)

This is something that also got to the Slope Movement latest update.

Getting the Slope angle

Do you know how collisions work in Godot? Well…neither do I, but I do know that we can have access to a lot of information regarding them. But here comes the trick…we better not use the KinematicBody2D’s get_slide_collision because it doesn’t always update us since the character doesn’t always slide. Here is how this implementation behaves:

We need a reliable way to always be updated about the floor’s angle. And for that, I present you [RayCast2D](https://docs.godotengine.org/en/stable/classes/class_racom aycast2d.html). A RayCast2D allows us to constantly check the floor. It does that by casting a Vector2 and checking for physical bodies, or even areas. If it intersects with one of those it immediately triggers a collision and stores data about this collision.

One advantage of RayCast2Ds is that they don’t accumulate collisions, so if there are 3 possible intersections between the casting point and the target point, it only triggers the first collision.

To tell the RayCast2D where it should cast to…we use the cast_to property. I love how explicit Godot is. In this experiment, I’ve used a value of Vector2(0, 32).

But why all that? WHY?! Why are we still here, just to suffer?

Turns out that, as I’ve mentioned, the RayCast2D stores information about the current collision, including the most meaningful for us in this context: the collision normal.

Using the get_collision_normal method we can…get…the collision’s normal…see? It’s not even funny. You try to explain the thing and it’s pointless because Godot’s engineers did it already. So here goes the code, guess by yourself.

func _tilt():
	if is_on_floor() and raycast.is_colliding():
		var normal = raycast.get_collision_normal()

Ohh, wait. I forgot to tell what a collision normal is right? So, briefly, a collision normal is a vector that only contains the collision’s direction data, its values can range from -1 to 1 and it always has a length of 1, meaning it is a unitary vector. You can check the Godot documentation about working with vectors for more information.

Why having the collision normal is so important? READ THE HEADER!

Yes, using the collision normal is the secret to get the slope’s angle. But how?! How a vector will give me an angle? You might be asking yourself, right? Well, turns out that all Vector2 have a fun property of having an angle.

See, in Godot a Vector2 that points straight to the right, in other words Vector2.RIGHT or rather Vector2(1, 0), independently from its length will always have an angle of…ZERO degrees! Yes, in Godot Engine we start measuring angles with Vector2.RIGHT as base. From there, since vectors are essentially lines, you just need to know the angle between your Vector2 and Vector2.RIGHT. For instance, Vector2.DOWN, a.k.a Vector2(0, 1), gives us an angle of 90 degrees.

To know the angle of a Vector2 you can simply call angle() on any Vector2 and it returns an angle in radians.

There we have it. Now that we know the slope’s angle, let’s tilt the character accordingly.

Tilting the character

Now that we know the slope’s angle we just need to rotate accordingly, right? Well…kinda.

See, this tilting, ideally, is just a visual cue for the player, in another post I’ll comment the design implications of that, but what you need to know is that you, ideally, shouldn’t rotate the whole KinematicBody2D but instead just the visual components, in this case, the Sprite.

BUT WAIT! You can’t simply apply the slope angle as the Sprite.rotation. Had you forgotten how we count angles in Godot? Yes, with Vector2.RIGHT being 0.0 degrees. And why is this a problem? Because unless your character is drawn like this:

It won’t rotate properly. So we need to add an offset angle…unless you really draw characters like that.

So, we are going to offset the slope’s angle by 90.0 degrees in order to properly tilt our character parallel to the slope. And this is how the tilt method looks like at the end.

func _tilt():
	if is_on_floor() and raycast.is_colliding():
		var normal = raycast.get_collision_normal()
		sprite.rotation = normal.angle() + deg2rad(90)

For full context, you can download the Tilting on Slopes files. And we that we finished this tutorial.

You can discuss these topics and more at the project’s community. Also, you can make requests there for future experiments.

That’s it, thank you so much for reading. Keep developing and until the next time!


Slope Movement 12 kB
Version 1.0.1 Jun 29, 2020
Tilting on Slopes 20 kB
Version 1.0.0 Jun 29, 2020

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I know it's been a while since you posted this, but would this translate to 3D?

Wow! that was fast. I was going to ask for  it in the community.  You have beaten me to it. 

This is very very helpful.  Exactly what I needed. Thanks a lot. I will be keeping a keen eye on your recipes. 

Good stuff.  Keep it going.