A platform game written with Excalibur

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The last remaining graphical element with a boring default is the background. This was just set to black, which looks extremely low-effort. Even though our character is some sort of space alien, it just doesn’t look inviting.

I went back to the Kenney assets and picked up a background sprite, which looks like this:


There doesn’t seem to be a simpler way to do this than just to create it as a new actor:

import { backgroundSprite } from "../resources";

export class Background extends ex.Actor {
  constructor() {
      x: 512,
      y: 512,
      width: 2048,
      height: 2048,
      collisionType: ex.CollisionType.PreventCollision,



Note the ex.CollisionType.PreventCollision, which prevents any of our other elements from colliding with it, and also prevents the system wasting any time calculating collisions with it.

This looks kind of flat though; we want to give the impression that this background is a long way away and not nailed to the back of our platforms. A simple way to do that is with parallax, i.e. making the background move depending on where the camera is.

At first thought it seems odd to have to move the background, because we’re trying to make it look like it’s fixed in place, and things that are fixed in place don’t move. But of course we’re trying to make an effect that implies depth in a 2D game engine, which means something will have to be faked.

Long story short, it’s simplest if the platform and player objects are fixed in world coordinates and we fake the depth by moving the background (which is a very simple object and doesn’t interact with anything). This means that as the camera moves right, the platforms won’t move at all but the background will move right a little bit, but less than the camera. In real 3D you’d transform things into camera space where the camera would be fixed. We can compare these cases:

Real 3D in camera coordinates Fake 3D in 2D engine
Camera doesn’t move Camera moves right
Platform moves left a long way Platform doesn’t move
Background moves left a little Platform moves right a little

Obviously implementing this is going to involve getting access to the camera object somehow; at the moment all camera movement is being automatically handled for us by Excalibur, and the code I’ve written doesn’t so much as read it. Luckily we can read the camera’s current location really easily: