cocos2d for iPhone

A fast, easy to use, free, and community supported 2D game engine

1. 

   neigaard
   Member

   Hi

   My objects are not moving that fast, so I think that makes a little simpler. Anyways here is my attempt for a collision detection algorithm for a circle hitting a square, please comment on it, can it be made faster/better?

   
//cPx = x coordinate of circle
//cPy = y coordinate of circle
//cr = radius of circle
//sqW = with of square
//sqH = height of square
//sqX = x coordinate of square
//sqY = y coordinate of square
-(BOOL)gcCircleCenterRadiusCollidesSquare: (float) cPx : (float) cPy : (float) cr : (float) sqW : (float) sqH : (float) sqX : (float) sqY {
float sqRx = sqX + sqW;
float sqLx = sqX - sqW;
float sqTy = sqY + sqH;
float sqBy = sqY - sqH;

   if(sqrt(pow(sqRx - cPx, 2) + pow(sqBy - cPy, 2)) < cr ||
   sqrt(pow(sqRx - cPx, 2) + pow(sqTy - cPy, 2)) < cr ||
   sqrt(pow(sqLx - cPx, 2) + pow(sqTy - cPy, 2)) < cr ||
   sqrt(pow(sqLx - cPx, 2) + pow(sqBy - cPy, 2)) < cr ||
   ((cPx < sqRx && cPx > sqLx) && (cPy - cr <= sqTy && cPy - cr > sqBy)) ||
   ((cPx < sqRx && cPx > sqLx) && (cPy + cr <= sqTy && cPy + cr > sqBy))) {
   return TRUE;
   }

   return FALSE;
}


   Posted 2 years ago #
2. 

   pabloruiz55
   Member

   I don't know if this works or not, but having those many sqrt and pow is really intensive and think you will be calling that function every update call...

   Posted 2 years ago #
3. 

   MikeSz
   Member

   agreed.

   also - why are you trying to reinvent the wheel ? you can easily find the best possible solution on google...

   yeah, I know it's fun to do things on your own, but where efficiency is concerned - it's usually better to just stick to proven solutions ;)

   Posted 2 years ago #
4. 

   noshbar
   Member

   It's really quite late in the day to be trying to sound smart, but MAYBE you could change:
   sqrt(pow(sqRx - cPx, 2) + pow(sqBy - cPy, 2)) < cr
   to be something like
   (pow(sqRx - cPx, 2) + pow(sqBy - cPy, 2)) < (cr * cr)
   that should save on the sqrt's... and maybe change the pow's to be manually typed out (x * x), just in case (for some reason) pow isn't optimised to convert pow2 to be sqr.

   You could also do a quick test to abort early, treating the square as a sphere too, because if the two sphere's don't collide (one single if comparison [radius, without sqrt's]), then you needn't do all those other checks.

   Annnyway, I didn't explain things properly at all, feel free to ask for more if you think I'm typing anything worthwhile...

   Posted 2 years ago #
5. 

   Mitch Lindgren
   Member

   The function you posted looks like it would be quite slow, just based on intuition. I'd agree with pabloruiz that's pow and sqrt are probably going to be quite CPU-intensive on a mobile device. That said, the iPhone is relatively powerful so you might be able to get away with it if you're not checking collisions on too many objects each frame.

   It's probably best to just use Chipmunk or Box2D unless you only need extremely basic collision detection. If you do want to implement your own collision detection, you can apply the separating axis theorem to get good collision detection for basic rigid bodies. I THINK it would be faster if implemented properly but I could be wrong.

   Posted 2 years ago #
6. 

   Adam Allen
   Member

   Ported to obj-c from: http://cgp.wikidot.com/circle-to-circle-collision-detection

   //Returns true if the circles are touching, or false if they are not
   -(BOOL) circlesColliding: (int) x1 y1: (int) y1 radius1: (int) radius1 x2: (int) x2 y2: (int) y2 radius2: (int) radius2 {
       //compare the distance to combined radii
       int dx = x2 - x1;
       int dy = y2 - y1;
       int radii = radius1 + radius2;
       if ( ( dx * dx )  + ( dy * dy ) < radii * radii ) {
           return YES;
       } else {
           return NO;
       }
   }

   Posted 1 year ago #
7. 

   abitofcode
   Moderator

   You can drop out a little earlier on this one. Useful for simple circle collision but the method above is for the collision of a square and a circle :s

   //Returns true if the circles are touching, or false if they are not
   -(BOOL) circlesColliding: (int) x1 y1: (int) y1 radius1: (int) radius1 x2: (int) x2 y2: (int) y2 radius2: (int) radius2 {
       int radii = radius1 + radius2;
   
       //compare the distance to combined radii
       int dx = x2 - x1;
       if(dx > radii) return NO;
       int dy = y2 - y1;
       if(dy > radii) return NO;
   
       if ( ( dx * dx )  + ( dy * dy ) < radii * radii ) {
           return YES;
       } else {
           return NO;
       }
   }

   Posted 1 year ago #

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