PImage img;
void setup(){
    img = loadImage("testimage.png");
    image(img, 0, 0);
    size(1000, 1000);
}
void draw(){
}

int visited[] = new int[100000];
int vcount = 0;
void floodFill(int x, int y){
  img.loadPixels();
  visited[0] = x + y * width;
  vcount = 1;
  while(vcount > 0){
     vcount--;
     int curr = visited[vcount];
     img.pixels[curr] = color(0, 255, 0);
     if(curr+1 < img.width*img.height &&  green(img.pixels[curr + 1]) < 128){
        visited[vcount] = curr+1;
        vcount++;
     }
     if(curr-1 >= 0 &&  green(img.pixels[curr - 1]) < 128){
        visited[vcount] = curr-1;
        vcount++;
     }
     if(curr+width < img.width*img.height &&  green(img.pixels[curr + width]) < 128){
        visited[vcount] = curr+width;
        vcount++;
     }
     if(curr-width >= 0 &&  green(img.pixels[curr - width]) < 128){
        visited[vcount] = curr-width;
        vcount++;
     }
  }
  
  
  
  img.updatePixels();
  image(img, 0, 0);
}
void mouseClicked(){
  
  floodFill(mouseX, mouseY);
  
}