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  Idea for collision detection between arbitrary shaped objects in 2D  (Read 2657 times)
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Offline DavidW

Junior Devvie


Medals: 3
Exp: 7 years



« Posted 2011-10-21 18:14:32 »

How viable is this idea in a game?  You give each sprite (bouncing ball, bullet, monster, etc...) a java.awt.geom.Area object which represents it's bounds.  You could even have a list of them if you want to for animations which change shape.  Then to test for a collision you can just see if the areas union equals their xor.  If it does then no collision happened.

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public boolean collision(Area a, Area b) {
 Area union = new Area(a); // you make copies of a since Area's add() and exclusiveOr() methods will edit the object
 Area xor = new Area(a);
 union = aa.add(b);
 xor.exclusiveOr(b);

 return !union.equals(xor); // equals is built into Area
}


I don't know how fast or slow these Area methods are (no doubt they are slower than Rectangle.contains()), but I don't see why this wouldn't work, especially for a small number of sprites on the screen.  You could even have the Area object created automatically from your sprites Image.  Anyone have any thoughts on this method? I came up with this while working on my Tower Defense game and trying to figure out a way to set certain parts of the map "unbuildable" (like where the enemies walk, and where you have already built something).

Hello!
Offline DavidW

Junior Devvie


Medals: 3
Exp: 7 years



« Reply #1 - Posted 2011-10-21 18:29:52 »

So After thinking about it for 2 seconds I realized it's better to just test if their intersection is empty.  DOH!  Tongue

Hello!
Offline Kova

Senior Devvie





« Reply #2 - Posted 2011-10-21 21:48:57 »

dont' know about java.awt.geom.Area but Slick2d's Polygon class has intersect() and contains() and it's all implemented to be fast as possible. I suggest using circle vs circle or rectangle vs rectangle for (extreamly) fast result.
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Offline ra4king

JGO Kernel


Medals: 355
Projects: 3
Exp: 5 years


I'm the King!


« Reply #3 - Posted 2011-10-22 02:22:31 »

Area is realllllllllly slow, I don't recommend doing that at all.

If you really need pixel perfect collision detection, I would first check if their rectangle bounds intersect, THEN you do the Area checking:

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public boolean collision(Area a, Area b) {
    if(a.getBounds().intersects(b.getBounds())) {
        Area union = new Area(a);
        union.intersect(b);
        return !union.isEmpty();
    }
   
    return false;
}

Offline dishmoth
« Reply #4 - Posted 2011-10-22 08:50:04 »

How viable is this idea in a game?

That's how I usually end up doing things.  If the game entities can't be represented well by (axis-aligned) rectangles or circles (or by a collection of rectangles or circles), then Area objects are a simple way of getting things working.
Simon

Online pjt33
« Reply #5 - Posted 2011-10-22 18:15:53 »

Be warned that although this allows you to detect collisions, it doesn't give you the normals to handle a collision response. So it's fine if you want to overlap the monsters and take continual damage, but not if you want to bounce off them.
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