Need Help Detecting Islands/Regions of Image Using Floodfill

(I moved this from the Forums, I think it's supposed to go here).

Hi All,

Using the alpha channel of the pixels of an image, I have created a 2-dimensional byte array where each byte is designated as a 0 or a 1 according to whether it's alpha is 0 (invisible) or not (!=0). The 1s are clustered in groups were the non-transparent parts of the image are, but these parts are not necessarily all connected to one another. What I'm trying to do is detect how many unique clusters (or islands) there are. This is sometimes known as blob extraction or connected component labeling. I'm currently going through all the bytes in the 2d array and when I find a 1, I start floodfilling.

My trouble is, when do I increment the regionCounter so that the next pass will skip the cluster I just labelled in the first pass. I figure the regionCounter should increment when a region is done being labelled, but how do I know when a region is done being labelled? In other words, where should I put the "regionCounter++"?

Note: I'm starting the regionCounter at 2 because I'm trying to turn the first batch of 1s into 2s, then the next into 3s through the increment step.

Any hints?

 public byte[,] map;
 public int regionCounter = 2;
  
 public void FindRegions () {
             for (int y = 1; y < this.map.GetLength(1)-1; y++) {
                 for (int x = 1; x < this.map.GetLength(0)-1; x++) {
                     FloodFill(x,y,1,(byte)regionCounter);
                 }
             }  
     }
  
  
     public void FloodFill(int x, int y, byte targetNumber, byte replacementNumber) {
         //if(x>0 && x < map.GetLength(0)-1 && y > 0 && y < map.GetLength(1)-1){ return; } //if you're within the array
         if(map[x,y] == replacementNumber) { return; } //if targetNum = replacementNum, return
         if(map[x,y] != targetNumber) { return; } //if number of Node != targetNum, return because you're not what we're looking for
         //if you've got this far...
         map[x,y] = replacementNumber; //set your current node to replacementNum
         //run function on it's neighbors...this is the floodfilling in action
         //WEST
         FloodFill(x-1,y, targetNumber, replacementNumber);
         //EAST
         FloodFill(x+1,y, targetNumber, replacementNumber);
         //NORTH
         FloodFill(x,y+1, targetNumber, replacementNumber);
         //SOUTH
         FloodFill(x,y-1, targetNumber, replacementNumber);
  
         return;
     }

Any help you could off would be fantastic.

Thanks so much! - Eric