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How do you check collision against 1000x1000 objects?

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Started by Assassinbeast December 02, 2012 10:16 PM

12 comments, last by Burnt_Fyr 11 years, 4 months ago

Assassinbeast

271

Author

December 02, 2012 10:16 PM

Hey, i just need some information on how to check collision if theres 1000 objects on the field.
And all 1000 objects are enemies to eachother.

So, do i for example have to make two for loops like this?:

[source lang="cpp"]
for(int i = 0; i < objects.size(); ++i) //objects.size is the size of the vector(if i use that)
for(int j = 0; j < objects.size(); ++i)
collision(objects, objects[j]);
[/source]
But thats 1000x1000 = a million loops in a single statement

So, i just want to know if thats how you do it... cant see any other way of doing this so far

ultramailman

1,720

December 02, 2012 10:24 PM

The idea is to seperate the map into several regions. An object in region B cannot possibly be colliding with an object in region C. So if your 1000 objects are evenly distributed in four regions, you only need to do 4 * (1000 / 4)^2 = 250000 checks. Even less if you split the map into more regions.

C0lumbo

4,415

December 02, 2012 10:27 PM

Broadphase collision detection is the general term you need to be googling.

Common solutions are spatial partitioning and sweep-and-prune.

Rapture - World Conquest

Cornstalks

7,033

December 02, 2012 10:27 PM

There are usually two steps in collision detection: broad phase and narrow phase. The first one, broad phase, is basically going through the objects and creating "groups" of objects that might be colliding. In the narrow phase, each group is individually processed and the actual collisions are detected and resolved. The broad phase is really important, as it allows you to avoid N[sup]2[/sup] comparisons.

As for what broad phase algorithm to use... that depends. You could create a spatial hash (works pretty well if you fine tune it and have lots of objects all of about the same size), binary space partitioning (BSP) trees, sweep and prune, axis-aligned bounding box (AABB) trees, etc. Bullet has a nice page on a couple of broad-phase algorithms.

I don't know what's easiest, but I've found dynamic AABB trees seems to work pretty well (which Chipmunk physics uses by default, albeit it's a little tweaked/fine tuned).

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Stroppy Katamari

1,416

December 02, 2012 10:32 PM

Before doing anything else, you'll probably want to prevent 500000 useless comparisons by switching the "j=0" in your code into "j=i+1"...

cardinal

909

December 02, 2012 10:33 PM

Yep, subdivide the problem!!!

It's a little more complicated that JUST dividing into regions (i.e. players on the boundaries of two different regions might be able to collide, or a bullet/player might cross from one region into another in one time-step). You'll need to handle these edge cases in addition to what ultramailman suggests.

Octrees/quadtrees are common ways to hierarchically subdivide your world although depending on your requirements you can optimize this in different ways.

Assassinbeast

271

Author

December 02, 2012 10:40 PM

ahh ok, thanks for all the help and suggestions

Burnt_Fyr

1,668

December 02, 2012 10:53 PM

in addition to what was said,ie: using some form of broad phase culling of objects that can't possibly collide, your loops are naive in that they check each item with each other item... twice. Better would be to change the second loop to:



for(int i=0; i< objects.size() ; i++)

{

   for(int j=0; j<i; j++)

   {

	   // check collisions

   }

}

Your implementation results in n^2 tests, the above in a little over 1/2 that.

Paradigm Shifter

5,832

December 02, 2012 11:06 PM

Stroppy Katamari already said that, and the j = i+1 method saves checking i == j as well.

You can also sort the coordinates in each axis and check 1 dimension at a time, if something is at x = 1 and moving with an x velocity of +2, it's never going to collide with anything that isn't in the interval [1, 3] in the x coordinate (need to account for the width of both objects in the x axis as well, of course). You need to have a system for reoordering the objects when they move which doesn't involve sorting the whole lot though. Space partitioning systems are just more advanced versions of that kind of thing.

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

__UFNHGGI_H__

305

December 03, 2012 07:24 AM

hehe
1000 * 1000 == BOOOM

The first solution ::



class Object

{

bool receiveCollision;

};

//player.receiveCollision = true;

//block.receiveCollision = false;

//items.receiveCollision = false;

for(int i = 0; i < objects.size(); ++i)

{

Object* obj =  objects;

if(obj->receiveCollision)

{

  for(int j = 0; j < objects.size(); ++i)

  {

   collisionCheck(obj, objects[j])

  }

}

}

The second solution ::



enum Tags { TagPlayer, TagEnemy, TagBlock, TagWeapons};

class Object

{

Tags tag;

std::vector<Tags> collisionTags;

};

for(int i = 0; i < objects.size(); ++i)

{

Object* obj =  objects;

for(int j = 0; j < obj->collisionTags.size(); ++i)

{

  for(int k = 0; k < objects.size(); ++k)

  {

   if(obj->collisionTags[j] == objects[k].tag )

   {

    collisionCheck(obj, objects[k]);

   }

  }

}

}

How do you check collision against 1000x1000 objects?

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How do you check collision against 1000x1000 objects?

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