How to detect if an actor is on the ground?

[tenpaMk2]

Hi, everyone. Please advise.

I want to detect if an actor is on the ground, because I want to make the player jumpable only when he is on the ground.
I have a problem to achieve it.

sample code

• index.ts

Physics.gravity = Vector.Down.scale(300);

• game-scene.ts

import {
  Actor,
  CollisionType,
  Color,
  Engine,
  PreCollisionEvent,
  Scene,
  Vector,
} from "excalibur";

export class GameScene extends Scene {
  onInitialize(engine: Engine): void {
    const ground1 = new Actor({
      pos: new Vector(200, 700),
      width: 300,
      height: 200,
      color: Color.Cyan,
      collisionType: CollisionType.Fixed,
    });
    engine.add(ground1);

    const ground2 = new Actor({
      pos: new Vector(400, 400),
      width: 100,
      height: 600,
      color: Color.Magenta,
      collisionType: CollisionType.Fixed,
    });
    engine.add(ground2);

    const player = new Actor({
      x: engine.halfDrawWidth,
      y: engine.halfDrawHeight,
      width: 20,
      height: 20,
      color: Color.Chartreuse,
      collisionType: CollisionType.Active,
    });
    engine.add(player);

    player.vel = new Vector(30, 0);
    player.on("precollision", (event: PreCollisionEvent): void => {
      console.log(event.side);
    });
  }
}

result

• console.log

Bottom
Right
Bottom
Right
Bottom
Right
Bottom
Right
...

problem

I get the log that repeats Bottom and Right .
It means 2 events are emitted each frame.

Therefore, I cannot detect if an actor is on the ground as following code.

    let isOnGround = false;
    player.on("precollision", (event: PreCollisionEvent): void => {
      if (event.side === Side.Bottom) {
        isOnGround = true;
      } else {
        isOnGround = false;
      }
    });

Does anyone knows good solution?

[tenpaMk2]

Here are all code.

https://github.com/tenpaMk2/excalibur-examples/tree/main/examples/detect-ground

[eonarheim]

@tenpaMk2 This is a good question!

There are a couple tricks that can be used to detect on ground in a situation like this:

1. You could use your most of your existing code and keep track of all sides collided in a frame, if Side.Bottom is in the list of sides that collided that frame you are on the ground.

export class GameScene extends Scene {
  playerOnGround = false;
  playerSideCollisions: Side[] = [];
  onPreUpdate() {
    playerSideCollisions.length = 0; // clear last frame's side collisions
  }
  ...

  isPlayerOnGround() {
    return playerSideCollisions.includes(Side.Bottom);
  }

  onInitialize() { 
    ...
    player.on("postcollision", (event: PostCollisionEvent): void => {
      playerSideCollisions.push(event.side);
    });
    engine.input.pointers.primary.on("down", (event: PointerEvent): void => {
      if (this.isPlayerOnGround()) player.vel = player.vel.add(new Vector(0, -100)); // jump!!
    });
  }

2. Use a child entity positioned just below the player and if it is currently colliding consider the player on the ground.

const playerGroup = ex.CollisionGroupManager.create('player'); // things in the same group do not collide
const player = new Actor({
  x: engine.halfDrawWidth,
  y: engine.halfDrawHeight,
  width: 20,
  height: 20,
  color: Color.Chartreuse,
  collisionType: CollisionType.Active,
  collisionGroup: playerGroup
});

const groundColliderActor = new ex.Actor({
  x: 0,
  y: offsetBelowPlayer,
  width: playerWidth,
  height: 32,
  color: ex.Color.Green, // color there just to visualize where the collider is
  collisionType: ex.CollisionType.Passive,
  collisionGroup: playerGroup
});

let activeGroundCollisions = 0;
groundColliderActor.on('collisionstart', () => {
  activeGroundCollisions++;
});
groundColliderActor.on('collisionend', () => {
  activeGroundCollisions--;
});
player.addChild(groundColliderActor);

const isPlayerOnGround = () => {
  return activeGroundCollisions > 0;
}

3. For fancier platforming, some people use ray casting to detect the floor but this can be a more complex approach. This has a couple advantages, mostly it's useful if you need more control over how your player interacts with the ground, like if you need to detect a slope, handle a character partially over an edge, or you need to know the distance to the ground. For Excalibur you currently need a reference to every collider you want to raycast against, which could get expensive or hard to use, this is something we want to fix in the next version of Excalibur.

const floorActors: ex.Actor[] = [...];
const onGround = () => {
  for (let floor of floorActors) {
    const hit = floor.collider.get().rayCast(
      new ex.Ray(player.pos, ex.Vector.Down),
      100); // length to cast
    if (hit) {
      return true;
    }
    return false;
  }
}

[tenpaMk2]

@eonarheim Thank you for many solutions!!

Solution 1

I love it. Very simple.

However, the player sometimes jumps too high.
I don't know why.

Solution 2

Actually, I knew this because I use it in dino-runner .

This also have a problem.
That is that the child emits the event when hit the right wall.
Therefore, the player can jump even though when not hit on the ground (bottom wall) .

I get around this problem by shortening the child.
However, this workaround occurs that incorrect detect...fmm...

Solution 3

This is completely new idea for me!!
I can understand the concept by your code. Thanks.

appendix

I implemented all solutions.

https://github.com/tenpaMk2/excalibur-examples/tree/main/examples/detect-ground