2D Platformer Collision Detection with Raycasts, Part 2: Lateral Movement

This is part 2 of a series; click here for part one.

Hello again, faithful squirrels - err, readers. This is the second post in a series wherein I discuss my home-brewed 2D platformer physics in Unity3D. Last time I talked about the basics of Raycasts, then applied them to movement in one direction (gravity). This time I'll be talking about applying my Raycasting method to lateral movement. The process, as you may have guessed, is the same for lateral movement, so it won't be as long to explain as gravity since we're already familiar with the method. I'll then explain jumping, which is also super easy (1-5 lines of code, tops).

DISCLAIMER: I discovered after writing the last article that Unity implemented native 2D physics and collision detection in a version of Unity I hadn't updated to yet. I've implemented my whole thing with 2D box colliders and 2D Raycasts, but it works the same either way. So I'm going to continue this tutorial with 3D box colliders which will be handy for anyone doing a 2.5D platformer, or for other fun experimentations.

Recap

To implement gravity and collision detection, I use the following steps:

1. Set up variables, like gravity and max fall speed, and gather information about the GameObject;
2. Apply acceleration to my movement: in this case, downward acceleration due to gravity;
3. Determine whether it makes sense for me to check below me for collisions (am I on the ground? or if not, am I falling?);
4. Determine where the rays I'm casting will begin (a few of them in a line);
5. Determine the length of all rays (they'll be the same, and check as far as my downward velocity);
6. Cast each ray and ask if it hit anything;
7. Use the result (if I'm on the ground and I didn't hit something, now I'm falling, and vice versa).

Here's what the rays would look like if we drew them:

For checking above, the lines will look identical but face up, and for lateral movement they will start along a vertical cross-section and reach out to the side.

Let's Boogie

Now let's take a look at the system applied to lateral movement. Here's what the code looks like:

There are lots of similarities but also some key differences. Let's go through them.

Input

Since we're now responding to the player's input, we have to get it from somewhere. My input code looks like this:

and that's it! Unity's inputs are generally set up well enough for this purpose, but if you want to play with it then go to Edit > Project Settings > Input to look at the data structure.

One thing to note about this is I use Input.GetAxisRaw instead of Input.GetAxis. The difference between the two is when it comes to keyboard input. All Input.Axis functions return a float between -1 and 1. When reading input from a keyboard, Input.GetAxis moves slowly toward the limit rather than snapping to it (which GetAxisRaw does). For example, if I want to move the character left using the keyboard, I'll press A or the left arrow. The first frame, using Input.GetAxis, will return something like -0.15. Then each frame it will go up by that amount until it caps at -1. This isn't what we want here, since we'll be multiplying things by our input. So, we use GetAxisRaw, which will only ever return -1, 0, or 1 when using a keyboard.

Applying movement

This was simpler with Gravity. We just checked if the player was grounded, and if they weren't, we subtracted from their vertical velocity (capping at a maximum fall speed). This is what I do to apply lateral movement.

I like to work with local variables - in this case newVelocityX. In C#, there's no way to alter one component of a Vector directly (in Javascript you could write "velocity.x += acceleration * input").  So, it's simpler to work with a new variable.

We're going to deal with an input of 0 differently, since that's deceleration and not an acceleration. Add acceleration * horizontalAxis to newVelocityX, then make sure it's not gone past the maximum speed. The principle is similar for deceleration.

Raycasts

This operation is nearly identical to that of gravity, with one exception: the direction of the rays is determined by our velocity. See here:

If we are going left, we check left, otherwise we check right. If there's no input we don't check at all. If the player holds a direction towards the wall, we know it connects on that side. There is the unfortunate side effect of doing a Translate of 0 every frame this happens but the pros outweigh the cons here. If I want to code Megaman's wall jump for example, I just have to set a variable like "bool wallHanging" to true if one of my rays connects. Then I can use the input and invert it to determine the direction he'll be jumping away from the wall (or hitInfo.normal for that matter).

Only the beginning

From here, the possibilities are endless. Try something fancy! For starters I suggest figuring out how to check above the character as it's jumping, to react to ceilings using this method.

I never even got into layer masks in this explanation. A layer mask will let you pick and choose what objects you want to detect. So if you want to make a platform you can jump through from underneath, just tell the rays you send up not to detect those platforms. Then when you're falling the downward ray will find it and you'll be golden.

Using this method you can implement: moving platforms; slanted surfaces; icy surfaces or other movement detriments; or a whole thwack of other things I can't think of right now.

I am really enjoying doing this sort of work, and I believe my next tutorial will be about bouncing off walls (like for an astronaut in a pinball machine, let's say).

Thank you so much for reading. Until next time, keep moving, kinetic beings!

-mysteriosum, the deranged hermit.