Step 3 - The Bird and the Ground

Let's create some ground for the Bird to collide with, make a new ground.ts file.

We can use the anchor property to tell Excalibur how to align the default graphics and collider (0, 0) means top left, by default graphics and collider are centered around the position pos.

typescript
// ground.ts
import * as ex from "excalibur";
export class Ground extends ex.Actor {
    moving = false;
    constructor(pos: ex.Vector) {
        super({
            pos,
            anchor: ex.vec(0, 0),
            height: 64,
            width: 400,
            color: ex.Color.fromHex('#bd9853'),
            z: 1 // position the ground above everything
        })
    }
    
    start() {
        this.moving = true;
    }
    stop() {
        this.moving = false;
    }
}

typescript
// ground.ts
import * as ex from "excalibur";
export class Ground extends ex.Actor {
    moving = false;
    constructor(pos: ex.Vector) {
        super({
            pos,
            anchor: ex.vec(0, 0),
            height: 64,
            width: 400,
            color: ex.Color.fromHex('#bd9853'),
            z: 1 // position the ground above everything
        })
    }
    
    start() {
        this.moving = true;
    }
    stop() {
        this.moving = false;
    }
}

We can make our Bird move by giving it some acceleration in onInitialize, it is generally recommended to initialize state in onInitialize. It makes testing easier and defers initialization until excalibur primitives are available. onInitialize is called once before the first update of the Actor.

BTW in Excalibur the positive y axis points down!

typescript
// bird.ts
import * as ex from 'excalibur';
export class Bird extends ex.Actor {
    ...
    override onInitialize(): void {
        this.acc = ex.vec(0, 1200); // pixels per second per second
    }
		start() {
      // later we'll use this to start our bird after game over
		}
		stop() {
      // later we'll use this to stop our bird after collision
		}
}

typescript
// bird.ts
import * as ex from 'excalibur';
export class Bird extends ex.Actor {
    ...
    override onInitialize(): void {
        this.acc = ex.vec(0, 1200); // pixels per second per second
    }
		start() {
      // later we'll use this to start our bird after game over
		}
		stop() {
      // later we'll use this to stop our bird after collision
		}
}

Now we want to collide with the Ground.

typescript
// bird.ts
import * as ex from 'excalibur';
import { Ground } from './ground';
export class Bird extends ex.Actor {
    ...
    override onCollisionStart(_self: ex.Collider, other: ex.Collider): void {
        if (other.owner instanceof Ground) {
            this.stop();
        }
    }
    stop() {
        this.vel = ex.vec(0, 0);
        this.acc = ex.vec(0, 0)
    }
}

typescript
// bird.ts
import * as ex from 'excalibur';
import { Ground } from './ground';
export class Bird extends ex.Actor {
    ...
    override onCollisionStart(_self: ex.Collider, other: ex.Collider): void {
        if (other.owner instanceof Ground) {
            this.stop();
        }
    }
    stop() {
        this.vel = ex.vec(0, 0);
        this.acc = ex.vec(0, 0)
    }
}

Let's put our Bird and Ground together in the default Scene

typescript
// main.ts
import * as ex from 'excalibur';
import { Bird } from './bird';
import { Ground } from './ground';
const game = new ex.Engine({...});
const bird = new Bird();
game.add(bird);
// drawHeight is the height of the visible drawing surface in game pixels
const ground = new Ground(ex.vec(0, game.screen.drawHeight - 64));
game.add(ground);
game.start();

typescript
// main.ts
import * as ex from 'excalibur';
import { Bird } from './bird';
import { Ground } from './ground';
const game = new ex.Engine({...});
const bird = new Bird();
game.add(bird);
// drawHeight is the height of the visible drawing surface in game pixels
const ground = new Ground(ex.vec(0, game.screen.drawHeight - 64));
game.add(ground);
game.start();