Entity knockback

Game Mechanics
#1

Hi, can someone please tell me how to add knock-back? My entities have x,y,velocityX and velocityY variables, and i have just no idea how to add it. I can like add 5 to velocityX for example, but how do i decrease it? And if the entity has bumped into something and it’s velocity is set to 0, how do i make it so that it stops decreasing?Maybe add separate fields, but that destroys the purpose of one velocity vector, doesn’t it? Please help me :c

PS: Using Artemis.

#2

It’s probably a bad way of doing it, but this is my code for knockback to give you an idea


	public void knockback(float r, float force){
		r = r - 180;
		
		force *= knockbackResistance; // how heavy the entity is (so lesser knock back)
		
		knockbackTimer = force * 8;
		knockbackTo = new Vector2((force * (float) MathUtils.sin(r * MathUtils.degreesToRadians)),
						       (-force * (float) MathUtils.cos(r * MathUtils.degreesToRadians)))
			                              .add(position);
					
	}

Then I have this on the update loop for the entity


	public void update(){
		if(knockbackTimer > 0){
			position = Tools.lerp(position, knockbackTo, 0.2f);
			knockbackTimer -= Gdx.graphics.getDeltaTime() * 1000;
		}
        }
#3

Whoa i didn’t expect code! Thanks ;D ! I think i will add knockbackTimer an knockbackTo to the Velocity component and do the logic in some system. Thanks again ;D !

PS : r is radians , right? Or degrees?

#4

I would recommend against having knockback specifically in the entity. Instead you should have an acceleration attribute. You could just use a vector for acceleration. You then apply acceleration to speed every update. Then you apply friction to acceleration right after. This way anything that affects the entity can just change acceleration.

#5

Please note that applying friction to acceleration makes no sense. Apply it to velocity. Acceleration (caused by a force) is immediately zero after it is processed, as the knockback force is instantaneous.

Instead of having an acceleration vector as field(s), I’d do this:


class Entity
{
   Vec2 position;
   Vec2 velocity;
   float mass;
   Vec2 force;
   float drag;

   public void tick() {
      Vec2 acceleration = new Vec2(force).div(mass);
      force.set(0,0,0);
      velocity.add(acceleration);
      position.add(velocity);
      velocity.mul(1.0f - drag);
   }
}

Whether this fits in your game logic, is another matter :slight_smile:

#6

According to Riven’s logic, also ensure tick() is being called at a consistent rate (like 30fps or something).

#7

Ah, yes. I have this assumption that everybody is sensible enough to have a fixed logic rate, and a dynamic render rate.

(Please don’t let this derail in a fixed/delta step discussion!)