Processing 1.0 - Processing Discourse - trouble calculating centroid

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trouble calculating centroid (Read 549 times)

oompa_l

trouble calculating centroid
Feb 5^th, 2009, 7:42pm

Hi

I 've been struggling implementing a calculation for a centroid - I had this working with another applet but with the one I'm currently working with, something just isn't working right.

the sketch is here : http://www.upsdn.ca/processing/animal_particles_06d/applet/

you can see a big ugly red circle in the top left hand corner which should be located at the centroid of all those little circles jumping around.

here's the code:

Quote:

Vector3D centroid () {
// ArrayList tempParticles = new ArrayList();

    float neighbordist = 500000.0f;
    Vector3D sum = new Vector3D(0,0,0); // Start with empty vector to accumulate all locations
    int count = 0;

    for (int i = 1; i < z.animals.size(); i++) {
      Animal currentAnimal = (Animal) z.animals.get(i-1);
      Animal other = (Animal) z.animals.get(i);
//      println(other.getPosition().getX());
      float d = distance(currentAnimal.position,other.position);

      if ((d > 0) && (d < neighbordist)) {
//        println(other.getX());
        sum.add(other.position); // Add location
        count++;

      }
    }
    if (count > 0) {
//      println("yes?");
println(sum.getY());
      sum.div((float)count);
    }
//    println(sum.getX());
    return sum;
  }

  float distance(Vector3D v1, Vector3D v2) {

    float dx = v1.getX() - v2.getX();
    float dy = v1.getY() - v2.getY();
    float dz = v1.getZ() - v2.getZ();
    return (float)Math.sqrt(dx * dx + dy * dy + dz * dz);

  }

can't figure out what's wrong. I think I have tried everything I cold possibly think of. if you have any ideas, I would really appreciate it...

thanks

cloister

Re: trouble calculating centroid
Reply #1 - Feb 5^th, 2009, 7:55pm

Ok, maybe I'm missing something, but I can't figure out why you're calculating the centroid in the way that you are.

If I try to distill that piece of code into english, here's what I come up with:

For each successive pair of Animals in the array, that is with adjacent array positions, calculate the distance between them. If the distance is less than a very large threshhold, add the second animal's position to the "sum" vector, and count how many such positions have been put into "sum". At the end, scalar-divide "sum" by that count.

While this should be, in practice, close to the average position of all the animals it won't ever be their actual average position because animal[0] won't ever have its position accounted for. Why not just this:

Vector3D centroid () {
Vector3D sum = new Vector3D(0,0,0);
for (int i = 0; i < z.animals.size(); i++)
{
Animal a = (Animal) z.animals.get(i);
sum.add(a.position); // Add location
}
return sum.div((float)z.animals.size());
}

oompa_l

Re: trouble calculating centroid
Reply #2 - Feb 5^th, 2009, 8:10pm

it never completely made sense to me...but sometimes things just go over my head and I accept that..bad practice.

so i just put your code in and got a "Cannot return a void result".

but I broke it into two lines and that seemed to work. but this all still doesnt address why sum is still returning 0. No matter how much I try to debug I can't find where things are not working.

my gut says that there's something wrong with the sum.add(Vector3D) line...I can check to see that a.getX() is actually returning values but there's no real way to see whether the full vector is working, is there?

oompa_l Full Member Offline Posts: 212	Re: trouble calculating centroid Reply #3 - Feb 5^th, 2009, 8:18pm switched to sum.addToThis(Vector3D) and it worked...thanks for the help

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