From: Roman Suzi <nuser AT rsuzi DOT pgu DOT karelia DOT ru>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: collision detection
Date: Mon, 20 Oct 97 18:51:41 +0300
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To: djgpp AT delorie DOT com
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Hi!


>>
>>Rob Farley wrote:
>>> >Andrew Deren wrote:
>>> >>
>>> >> Does anyone know how to detect if two rectangles overlap each
>other at
>>> >> some point. Let's say we have something like this:
>>
>>
>>Given two rectangles, A and B, check to see if any of the vertices of B
>>lie on A.  If this is true, then there must be overlap.
>>
>>For collision-detection, you probably want to intersect your velocity
>>vectors with the rectangle to determine where the collision occurs.
>>
>
>
>Well, let's 'C',
>
>I've used rectangle collision detection before.  It worked something
>like this:
>
>RECT a, b;
>
>a.top = TopA;
>a.left = LeftA;
>a.bottom = BottomA;
>a.right = RightA;
>
>b.top = TopB;
>b.left = LeftB;
>b.bottom = BottomB;
>b.right = RightB;
>
>..
>
>int collision(RECT *a, RECT *b)
>{
>   // check if left side of a is inbetween b's right & left
>   if (a.left > b.left  &&  a.left < b.right)
>   {   // check if the top of a is between b's top and bottom
>       if (a.top < b.top && a.top > b.top)
>           return TRUE;  // collision!
>
>       // check if the bottom of a is between b's top and bottom
>       if (a.bottom < b.top && a.bottom > b.top)
>           return TRUE;  // collision!
>   }
>   // check if right side of a is between b's left and right
>   if (a.right > b.left  &&  a.right < b.right)
>   {   // check if the top of a is between b's top and bottom
>       if (a.top < b.top && a.top > b.top)
>           return TRUE;  // collision!
>
>       // check if the bottom of a is between b's top and bottom
>       if (a.bottom < b.top && a.bottom > b.top)
>           return TRUE;  // collision!
>   }
>
>// Then do the same thing for b:
>   // check if left side of a is inbetween b's right & left
>   if (b.left > a.left  &&  b.left < a.right)
>   {   // check if the top of a is between b's top and bottom
>       if (b.top < a.top && b.top > a.top)
>           return TRUE;  // collision!
>
>       // check if the bottom of a is between b's top and bottom
>       if (b.bottom < a.top && b.bottom > a.top)
>           return TRUE;  // collision!
>   }
>   // check if right side of a is between b's left and right
>   if (b.right > a.left  &&  b.right < a.right)
>   {   // check if the top of a is between b's top and bottom
>       if (b.top < a.top && b.top > a.top)
>           return TRUE;  // collision!
>
>       // check if the bottom of a is between b's top and bottom
>       if (b.bottom < a.top && b.bottom > a.top)
>           return TRUE; // collision!
>   }
>   return FALSE;  // no collision!
>}
>
>
>I know it's probably not the best method, but of the top of my mind, I
>can't think of any others...

  The solution is much simpler:

int collision(RECT *a, RECT *b)
{
    return
          a.right >= b.left    &&  b.right >= a.left
       && a.top   >= b.bottom  &&  b.top   >= a.bottom;
};

Please, check!
The generalization to more objects is quite obvious.

I hope, this will help.


Sincerely,
Roman





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--  Roman A. Suzi * Petrozavodsk Karelia Russia --
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