[Collisions] Test before collision

Newbie & Debugging Questions
#1

I am having a little trouble here, I can easily fix it but the code is messy and horrible.

Basically I need to invert the X or Y velocity if I touch the edge of the screen, I can do this by doing:

			if (ball.getX() + ball.getWidth() > Engine.graphics.getBox2DCam().viewportWidth) {
				ball.velocity.set(-3, ball.velocity.y);
			}
			if (ball.getX() < 0) {
				ball.velocity.set(3, ball.velocity.y);
			}
			if (ball.getY() < 0) {
				ball.velocity.set(ball.velocity.x, 3);

			}
			if (ball.getY() + ball.getHeight() > Engine.graphics.getBox2DCam().viewportHeight) {
				ball.velocity.set(ball.velocity.x, -3);
			}

This however frustrates the crap out of me, I don’t want to manipulate the ball in the collision code, I simply want to check for collisions then report back to the entities that “hey, you just collided, do something”.

I want something like this, the problem being that because I am checking for collisions when they happen, the ball collides and remains colliding; resulting in a constantly flip of the velcocity:

				if (ball.getX() > Engine.graphics.getBox2DCam().viewportWidth
						|| ball.getX() < 0) {
					event = Event.BOUNDARY_VIOLATION;
					event.setProjection(CollisionProjection.X);
					notify(ball, event);
				}
				if (ball.getY() < 0
						|| ball.getY() > Engine.graphics.getBox2DCam().viewportHeight) {
					event = Event.BOUNDARY_VIOLATION;
					event.setProjection(CollisionProjection.Y);
					notify(ball, event);
				}

This basically checks what axis we are colliding on and sends an notification to the Ball itself to let it know it just hit something, then in the ball code:

@Override
	public void onNotify(Entity entity, Event event) {
		switch (event) {
		case BOUNDARY_VIOLATION:
			if(event.getProjection() == CollisionProjection.X){
				velocity.set(-velocity.x, velocity.y);				
			}else if(event.getProjection() == CollisionProjection.Y){
				velocity.set(velocity.x, -velocity.y);
			}
			break;
		default:
			break;
		}
	}

As you can see this is just going to constantly flip the velocity vector because I check for collision as they happen, is there a simple algorithm I can implement to check for collisions just before they happen? No idea what to google, came up results that mostly just check for collisions when they overlap.

#2

That’s not why it’s not working (and for what it’s worth, there’s nothing wrong at all with handling the collision directly in the collision code - your way is just making things more complex for the sake of it I think).

All you’ve forgotten to do is move the ball again before checking for collisions once more. You’ll find that having reversed the ball’s X or Y direction its next move should take it back out of the collision area (the edge of the screen) and so it shouldn’t collide again.

Cas :slight_smile:

#3

Yep, I am practising apply design patterns to code lol.

Damn, shoot me lol.

#4

So I sort of re-done the code for this, after doing some reading on AABB and the proper way to do it.

I am using the MTD way, I am however having a bit of a problem adjusting the balls position after a collision, it seems to go full on crazy when it collides with something.

I know why this is, it is this line of code here:

ball.getPos().add(mtd);

However, when I remove this code the ball collides and stops on the object and slides along it, like said before its doing this because I have not adjusted the position due to the collision. Can anyone tell me what I am doing wrong?

Here is my collision check code:

if (ball.getBounds().overlaps(block.getBounds())) {
					Vector2 mtd = ball.getBounds().minimumTranslation(
							block.getBounds());
					ball.getPos().add(mtd);
					if (mtd.x != 0) {
						ball.velocity.x = -ball.velocity.x;
					} else if (mtd.y != 0) {
						ball.velocity.y = -ball.velocity.y;

					}
				}

Here is the minimumTranslation method, I followed a tutorial to create it and it original used Vectors for the min/max points but I changed that around and instead just done the vertices calculation in the variable deceleration, afaik it works out the same.

	public Vector2 minimumTranslation(BoundingBox other) {

		/* The collision vector */
		Vector2 mtd = new Vector2();

		/* The left side of the AABB */
		float left = this.getX() - (other.getX() + other.getWidth());
		/* The right side of the AABB */
		float right = (this.getX() + this.getWidth()) - other.getX();
		/* The top side of the AABB */
		float top = this.getY() - (other.getY() + other.getHeight());
		/* The bottom side of the AABB */
		float bottom = (this.getY() + this.getHeight()) - other.getY();


		 /* If any of the edges are zero, there was no collision */
		 if (left > 0 || right < 0 || top > 0 || bottom < 0)
		 return Vector2.Zero;

		/* If right is less than left, we collided left */
		if (Math.abs(left) < right) {
			mtd.x = left;
		} else {
			/* Else we collided right */
			mtd.x = right;
		}

		/* If top is less than bottom, we collided top */
		if (Math.abs(top) < bottom) {
			mtd.y = top;
		} else {
			/* Else we collided bottom */
			mtd.y = bottom;

		}

		/*
		 * See which component of our collision vector is smallest
		 */
		if (Math.abs(mtd.x) < Math.abs(mtd.y)) {
			mtd.y = 0;
		} else {
			mtd.x = 0;
		}

		/* Return our collision vector */
		return mtd;

	}

Don’t tell me exactly what is wrong if you can help it, give me a hint lol.