I assume that by biggest blob you mean the one that covers the most pixels.

I have looked at the source code for the library and although it is not at straightforward as I thought it might be I have a solution. The library will calculate the 'weight' of a blob and looking at the source code the weight of a blob is the number of pixels in the blob.

The code below creates an array of blobs recording the blob number and its weight and then sorts them in reverse order by weight (heaviest first) and then returns the id of the heaviest.

In your code you have a Detector object called bd. To get the id number of the 'heaviest' blob use
int id = getBiggestBlobIndex(bd);
after you have called findBlobs or imageFindBlobs

Note: the method returns -1 if no blobs have been detected.
Note: the id must be re-calculated each frame.

HTH

1. import java.util.Arrays; // This may not be needed
2. 
   
3. public int getBiggestBlobIndex(Detector d) {
4.   int nbrBlobs = d.getBlobsNumber();
5.   // If we have no blobs return
6.   if (nbrBlobs ==0)
7.     return -1;
8.   d.weightBlobs(false);
9.   BlobWeight[] blobs = new BlobWeight[nbrBlobs];
10.   for (int i = 0; i < blobs.length; i++)
11.     blobs[i] = new BlobWeight(i, d.getBlobWeight(i));
12.   Arrays.sort(blobs);
13.   return blobs[0].id;
14. }
15. 
    
16. public class BlobWeight implements Comparable<BlobWeight> {
17.   public int id;
18.   public Integer weight;
19. 
    
20.   public BlobWeight(int id, Integer weight) {
21.     this.id = id;
22.     this.weight = weight;
23.   }
24. 
    
25.   public int compareTo(BlobWeight b) {
26.     // Reverse so heaviest are first in list
27.     return new Integer(b.weight).compareTo(weight);
28.   }
29. }

Hey thanks for that code! 

Once you have got the index value for the heaviest blob using

int id = getBiggestBlobIndex(bd);

Once you have the blob id then the library has a number of methods to get further information about the blob, here are some of them :-

public void drawBlobContour(int blobnumber,int contourColor, float thickness)
public float getCentroidX(int blobnumber)
public float getCentroidY(int blobnumber)
public int getBlobWidth(int blobnumber)
public int getBlobHeight(int blobnumber)
public int getBlobWeight(int blobNum)
public PVector[] getEdgePoints(int blobnumber)

To get the centroid x and y position you can use

cx = bd.getCentroidX(id);
cy = bd.getCentroidY(id);

where cx and cy are integer variables.

HTH if you want further information about these methods then look at the HTML reference pages 
that came with the library.

Thanks for the reply... I tried using the id and the getCentroidX and got a strange error:

The error is in the for loop (line 132). You have
                            v  empty for-loop so it does nothing
for(int i = 0; i < bd.; i++){}
                     ^ full stop needs to be followed by something

The empty for-loop will not cause an error but then why have it. The error is created by the full-stop. What you should have is :-

for(int i = 0; i < bd.getBlobsNumber(); i++){
   // we can do something here with each blob detected
      // this will print the blob number and its weight
      println(i + "    weight= " + bd.getBlobWeight(i));
}

You have made a reasonable assumption that the heaviest blob (index = id from line 129) is the person, so you need to change lines 134-135, to

personX = bd.getCentroidX(id);
personY = bd.getCentroidY(id);

I would also recommend that you make id a global variable to do that add the following statement at line 26 or 29 (anywhere near there)
int id;
and change line 129 to
id = getBiggestBlobIndex(bd);

I see from the comment at line 128 you want to draw all the bounding boxes. Although the library will draw the blob outline for you it won't draw the bounding box so you have to some extra work.

The following code will draw the bounding boxes for all the blobs in green and the heaviest in red. It replaces the code you highlighted in yellow)

1. int col;
2. pushStype();          // remember current drawing style
3. rectMode(CENTER);
4. noFill();             // empty boxes
5. strokeWeight(1.0);
6. for(int i = 0; i < bd.getBlobsNumber(); i++){
7.       if(i == id)
8.             col = color(255,0,0); // red for body blob
9.        else
10.             col = color(0,255,0); // green for all the others
11.       float bmx = bd.getBoxCentX(i);
12.       float bmy = bd.getBoxCentY(i);
13.       float bwidth = bd.getBlobWidth(i);
14.       float bheight = bd.getBlobHeight(i);
15.       // Draw the bounding box in the appropriate color
16.       stroke(col);
17.       rect(bmx,bmy,bwidth,bheight);
18. }
19. popStyle(); // restore the original drawing style
20. // Remember the body blob (heaviest)
    
21. personX = bd.getCentroidX(id);
22. personY = bd.getCentroidY(id);

There maybe the odd syntax error as I have not tried the code myself.

HTH

Thanks again for the reply... the empty for loop was left in by mistake; the line 

You have made a reasonable assumption that the heaviest blob (index = id from line 129) is the person, so you need to change lines 134-135, to

personX = bd.getCentroidX(id);
personY = bd.getCentroidY(id);

Since id will be -1 if there are no blobs then change lines 136-137 to

1. if(id >= 0){
   
2.   personX = bd.getCentroidX(id);
3.   personY = bd.getCentroidY(id);
4. }
   

Hmm new error and a black screen rather than grey, so we're makin progress!!! thanks again!

This error is related to the Kinect library it seems that it can't see your Kinect device.

I don't have an X-box or a Kinect device and have never used this library. Perhaps you should send a private message to the library author Thomas Diewald with a link to this post and ask for help.

Ok thanks!

I did respond to this yesterday but it appears to have been lost so I will try again:

Change the code from my last post to

1. bd.loadBlobsFeatures();
2. bd.findCentroids(false, false);
3. if(id >= 0){
4.   personX = bd.getCentroidX(id);
5.   personY = bd.getCentroidY(id);
6. }

In the blob detection library some methods have to be called first before other methods will work. You need to read the documentation that comes with the library to find out which and the order they are to be called.

If that doesn't work then I can't do much more because I cannot run your code.

Good luck
 

At first that didn't work, but it looks like it was a matter of order; had to call some methods first, THEN call your function and THEN get the centroids!