Bullet Collision Loop

Newbie & Debugging Questions
#1

I’m looping through an ArrayList to check if an enemy is colliding with one of the player’s bullets. It runs smoothly with only a few enemies on the screen but when there’s a bunch of them then the game starts to lag because every enemy is looping through every bullet.

Code:


	public void checkCollision(ArrayList<ShipBullet> bullets){
		for (int i = 0; i < bullets.size(); i++){
			ShipBullet bul = bullets.get(i);
			Rect rect1, rect2;
			rect1 = new Rect(bul.x,bul.y,bul.x+1,bul.y+4);
			rect2 = new Rect(x, y, x+graphic.getWidth(), y+graphic.getHeight());
			if (rect1.intersect(rect2)){
				destroyed = true;
				break;
			}
		}
	}

Is there any better way of doing this? I was thinking of pooling the bullets but I’m not familiar with pooling just yet (pretty much just found it on Google). Just wondering if there are other methods that I can try.

The method is checked every frame and there can be up to 10~ bullets on the screen from the player alone. And it starts lagging when 6 or so enemies are present. Not to mention I’m working on a cheap Android device but I’d still like to optimize it.

Thanks!

#2

Have you tried profiling it? How many bullets are we talking about?

You could probably save a lot of time if you didn’t create two new Rect objects to test for each intersection. Either do the math manually or just create some Rects earlier and reuse them.

#3

Is this collision method run for each entity? Because that seems wrong to me. Also just use an arraylist just like enemies.

But a short fix is to perform a distance check before collision (avoid sqrt)

#4

A distance check isn’t going to be much more expensive than a rect-rect check, you’re just adding extra work. The ‘new’ overheard (especially on an Android VM) is going to dwarf that.

#5

That was just some sloppy code on my behalf, I didn’t think it would have affected the performance that much though? (I’ll smooth it out a bit now)
And well bullets… uhm, can be around 20 maximum at any one time I think, by which time they leave the screen and get removed from the ArrayList. That’s just for the player bullets minus any additional ones from enemies (which I have yet to add).
Edit: Nope, no profiling - yet.

I run that method for every enemy ship in the enemy ArrayList.
Like this:


			//Enemies
			for (int i = 0; i < enemyList.size(); i++){
				Enemy enemy = enemyList.get(i);
				enemy.checkCollision(player.getBullets());
				
				if (enemy.getDestroyed()){
					propList.add(new GameExplosion(enemy.x,enemy.y));
					enemyList.remove(i);
					continue;
				}
				enemy.step(g);
			}

The bullets original ArrayList is held within the player object, and I just pass it to each enemy every step. If that’s a bad idea how otherwise would I get each enemy to check for collisions against bullets?

Edit 2:
Thanks for the suggestion about creating the Rects early on and reusing them. Got a noticeable performance boost. I don’t detect any lag unless I fill the screen up with about 40 enemies - the actual game won’t get that half as many enemies on the screen at once.

#6

For optimized performance I wouldn’t use ArrayLists if items are removed frequently - just take a look into the source code to see what happens if its not the last item you remove.