Path: chuka.playstation.co.uk!news
From: James Burns <james.w.burns@talk21.com>
Newsgroups: scee.yaroze.freetalk.english
Subject: Re: HELP! Year 5 Maths....
Date: Mon, 27 Feb 2006 14:33:51 +0000
Organization: PlayStation Net Yaroze (SCEE)
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References: <dtg8tq$881@www.netyaroze-europe.com>
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In-Reply-To: <dtg8tq$881@www.netyaroze-europe.com>

Hi Chris,

I've been working on something like this at work.
An optimisation that may make a difference, is to remove the sqrt(),
where you comparing the mag to dist, use mag2 < (dist * dist).
I had done some research into the theory (its out there in plentiful
supply), I also read up on some of the optimisations as I was
considering my target platform as well.
I then had a quick look at a link to MSDN, it had an article about
Collision Detection with the following sample code:

// Simple sphere test to start with
BOOL bSphereTest( CObject3D *obj1, CObject3D *obj2 )
{
    D3DVECTOR relPos = obj1->prPosition - obj2->prPosition;
    float dist = relPos.x * relPos.x +
                 relPos.y * relPos.y +
                 relPos.z * relPos.z;
    float minDist = obj1->fRadius + obj2->fRadius;

    return dist <= minDist * minDist;
}

You can also calculate if the vectors will intersect over a time, so you
can calculate a collision in the future.
Another optimisation is to break down the area you apply the test into a
grid, thereby you only do the detections relevant to the sector.

Hope this interests you...

Regards,
James

Chris Wallace wrote:
> Ok, I'm trying to make my cars avoid objects that nasty people leave in the 
> middle of the track....
> 
> Of course the first step to avoiding objects is knowing that they are there, 
> so I thought to myself:
> I have a target location and a current location, if I draw a line between 
> the two I can test it with
> a collision sphere around objects and if they meet then I can deal with it..
> 
> Sounded good to me anyway - to make it a little more effiicient I check 
> first what the closest object is and then only check with that object, but I 
> have tried it with testing all objects and I get the same result.
> 
> Essentually, the cars seem to know what they are closest too, and as I am 
> drawing to screen all collision boundarys (currently spheres) and the 
> targeting lines I can see that they are intersecting.... but the code 
> doesn't seem to notice. Below is the code.... if anyone can see any glaring 
> errors it would be a big help!!!
> 
> ---------
> 
> int closest=0;
> double dist=100000.0;
> 
> for(int i=0;i<track->num_objects;i++)
> {
> //calculate distance between this car and object:
> 
> double xdiff = (x - track->Objects[i]->x);
>  double zdiff = (z - track->Objects[i]->z);
> 
>  double mag2 = (xdiff*xdiff)+(zdiff*zdiff);
>  double mag = sqrt(mag2);
> 
>  if(mag < dist)
>  {
>   dist = mag;
>   closest = i;
> 
>  }
> 
> }
> 
> // we now know the closest object and its distance from us
> //move the target away from the close object, depending on HOW close the 
> object is
> 
> //we know which object is close, closest, so
> //check if it is in our way
> 
> 
> double x1 = x;
> double y1 = 10;
> double z1 = z;
> 
> double x2 = targetx;
> double y2 = 10;
> double z2 = targetz;
> 
> double x3 = track->Objects[closest]->x;
> double y3 = 10;
> double z3 = track->Objects[closest]->z;
> 
> double r = track->Objects[closest]->radius;
> 
> 
> double a =  square(x2 - x1) + square(y2 - y1) + square(z2 - z1);
> 
> double b =  2* ( (x2 - x1)*(x1 - x3)
>    + (y2 - y1)*(y1 - y3)
>    + (z2 - z1)*(z1 - z3) );
> 
> double c =  square(x3) + square(y3) +
>    square(z3) + square(x1) +
>    square(y1) + square(z1) -
>    2* ( x3*x1 + y3*y1 + z3*z1 ) - square(r);
> 
> double answer = square(b) - (4 * a * c);
> 
> if(answer >= 0)
> {
> 
> printf("%i : %f - EEEEEEK!!!!!!\n",closest,answer);
> 
> } 
> 
>