Code

import processing.video.*;
Capture capture;
int avg;
int tolerance = 20; //control how much a pixel must be diffrent from the corresponding pixel of the "samplingImage"
int pixelsNecessary = 100; //this describes the quantity of reactive pixels required to set to activare a sensor
  int rows=20;
  int col=20;
 int rectnumber = rows * col;
int whichMode = 0; //this variables is just to show the functionality and is not necessary

boolean backgroundCaptured = false; //this variable are just to show the functionality and is not necessary

rects[] rects; //this object array contains all the instances of the rectangles defined into the class "rects"

PImage samplingImage; //this is the image that is used check the differences with the live image

void setup() {
 int w=800;
  int h=600;
  size(w, h);


  samplingImage = createImage(w, h, RGB); //create a new blank image just to define its dimensions

  capture = new Capture(this, w, h, 30); //here begins a new sessions of capturing of the images from the webcam

  rectMode(CORNERS);
  rects = new rects[20000]; //define the rects array dimension

  //here you can insert more rectangles (sensors) by specifying x1,y1,x2,y2
  //  rects[0] = new rects(0, 0, 100, 100); //a1
  //  rects[1] = new rects(100, 0, 200, 100); //b1
  //  rects[2] = new rects(200, 0, 300, 100); //c1
  //  rects[3] = new rects(300, 0, 400, 100); //d1
  //  rects[4] = new rects(0, 100, 100, 200); //a2
  //  rects[5] = new rects(100, 100, 200, 200); //b2
  //  rects[6] = new rects(200, 100, 300, 200); //c2
  //  rects[7] = new rects(300, 100, 400, 200); //d2
  //  rects[8] = new rects(0, 200, 100, 300); //a3
  //  rects[9] = new rects(100, 200, 200, 300); //b3
  //  rects[10] = new rects(200, 200, 300, 300); //c3
  //  rects[11] = new rects(300, 200, 400, 300); //d3
  //  rects[12] = new rects(00, 300, 100, 400); //d4
  //  rects[13] = new rects(100, 300, 200, 400); //d4
  //  rects[14] = new rects(200, 300, 300, 400); //d4
  //  rects[15] = new rects(300, 300, 400, 400); //d4

  int x=0;


  int xgap = w/rows;
  int ygap = h/col;
  println("xgap="+xgap+"\nygap="+ygap);
  int x1=0, y1=0, x2=0, y2=0;
  while (x< (rows*col))
  {
    for (int i=0;i<=h-ygap;i+=ygap)
    {
      for (int j=0;j<=w-xgap;j+=xgap)
      {
        rects[x++]=new rects(j, i, j+xgap, i+ygap);
        println(x +"OK");
      }
    }
  }
}


void keyPressed() {

  if ( key == 'q' ) { //if the key is pressed reset a new sampleImage

    setSampleImage();
  }

  //Others keys commands to change the parameters
  if ( key == 's' && tolerance > 0 ) tolerance--;
  if ( key == 'a' && tolerance < 255 ) tolerance++;
  if ( key == 'f' && tolerance > 0 ) pixelsNecessary--;
  if ( key == 'g'  ) pixelsNecessary++;
  if ( key == 'z' ) {
    if (backgroundCaptured==false)backgroundCaptured=true;
    else backgroundCaptured=false;
    whichMode++;
  }
}

void captureEvent(Capture myCapture) {

  capture.read(); //this refresh the image captured by webcam and stored in the capture variable
}


void draw() {

  int pixelX = -1, pixelY = -1;

  if ( whichMode % 2 == 1 ) setSampleImage(); //this is necessary for the motion mode because it needs a new sampleImage every cycle


    loadPixels();
  capture.loadPixels(); //fill an array of pixels from the current captured image

  for (int i=0; i<capture.height*capture.width; i++) { //read every element of the pixels array

    pixelX = i % width;                                                            //get the real pixel X position

    if ( pixelX != width ) pixelY = ( i / width ) +1;
    else pixelY = ( i / width );  //get the real pixel Y position


    if ( abs( hue( capture.pixels[i] ) - hue( samplingImage.pixels[i] ) ) > tolerance && abs( saturation( capture.pixels[i] ) - saturation( samplingImage.pixels[i] ) ) > tolerance && abs( brightness( capture.pixels[i] ) - brightness( samplingImage.pixels[i] ) ) >tolerance ) { //HSB values comparison between the current image and the sample image at the current pixel

      for ( int q = 0; q < rectnumber; q++) { //loop trough the rects array

        if (rects[q]!=null) {

          if ( pixelX > rects[q].x1 && pixelX < rects[q].x2 && pixelY > rects[q].y1 && pixelY < rects[q].y2 )
         //check if there is a sensors in this part of the image
            rects[q].amount++; //increase the number of pixels activated for this sensor}
        }
      }
    }
  }


  image( capture, 0, 0 ); //show the live webcam image




  if (!backgroundCaptured) { //this is just for this demostration. you nees just what is in the else parenthesis
    fill( 0, 0, 0, 100 );
    rect(0, 0, width, height-30);
    fill(255);
    textAlign(CENTER);
    text( "Go out from the screen and press 'q' key\n in order to capture the background.\n\n Drag the rectangles (sensors) to change the sensors positions.\n\nPress 'a'/'s' keys to decrease/increase the sensors tolerance.\n\nPress 'f'/'g' keys to decrease/increase the number of pixels\n necessary to activate a sensor.\n\n Press 'z' to switch to the motion sensor mode. ", width/2, 100 );
    textAlign(LEFT);
  }
  else
  {
int sum=0; int count = 0;
    for ( int q = 0 ; q < rectnumber ; q++) { //loop trough the rects array
      int x1 =0, y1=0, x2=0, y2=0, amount = 0;
      if (rects[q]!=null)
      {
        x1 = rects[q].x1;
        y1 = rects[q].y1;
        x2 = rects[q].x2;
        y2 = rects[q].y2;
        amount = rects[q].amount;
      }

      if ( amount > pixelsNecessary || amount == ((x2-x1)*(y2-y1)) )
      {
       sum=sum + rects[q].x1;
       count= count + 1;
        fill( 0, 255, 0 );
        if ( rects[q] != null )
          rects[q].draw(); //draw this rectangle object (go to the class to learn more)

      }
      fill(0, 0, 0, 100);
    }
    if(count!=0)
     println(sum/count);
  }

  //these are for this demo as well
  fill( 0, 0, 0, 200 );
  stroke( 0 );
  rect( 0, height - 30, width, height );
  fill( 255 );
  text( "Tolerance (a/s): " + (255-tolerance), 10, height - 10 );
  text( "Pixels required (f/g): " + pixelsNecessary, 150, height - 10 );

  if (whichMode%2==1) text( "Mode (z): Motion Sensor", 320, height - 10 );
  else  text( "Mode (z): Presence Sensor", 320, height - 10 );
}


class rects { //rectangle (sensors) class

    int x1, x2, y1, y2, amount ; //coordinateso of the corner-corner rectangle and the amount of activated pixels

  rects( int get_x1, int get_y1, int get_x2, int get_y2 ) { //lets create a new rectangles with the passed parameters

    x1 = get_x1;
    y1 = get_y1;
    x2 = get_x2;
    y2 = get_y2;
  }

  public void draw() { //rectangle draw function

    //    noFill();

    //if ( amount > pixelsNecessary || amount == ((x2-x1)*(y2-y1)) ) fill( 255, 0, 0 ); //if the amount is higher than ne pixelsNecessary variable or if it is equal to the number of maximum pixels of a rectangle draw it of red

//    if ( mousePressed && mouseX > x1 && mouseX < x2 && mouseY > y1 && mouseY < y2 ) { //this is just for this demo in order to let you move the sensors
//
//      x1 += mouseX - pmouseX ;
//      x2 += mouseX - pmouseX ;
//      y1 += mouseY - pmouseY ;
//      y2 += mouseY - pmouseY ;
//    }

    //draw the rectangle and the amount value text
    stroke( 0, 255, 0 );
    rect( x1, y1, x2, y2 );
    fill( 255 );
    //textAlign( CENTER );
    //text( amount, x1 + ( x2 - x1 ) / 2, y1 + ( y2 - y1 ) / 2 + 5 );
    amount = 0;
    //textAlign( LEFT );
  }
}



void setSampleImage() {

  backgroundCaptured=true; //this is for the demo

  loadPixels();
  samplingImage.loadPixels(); //fill an array of pixels from the sampling image

  for ( int i = 0 ; i < capture.height * capture.width ; i++ ) samplingImage.pixels[i] = capture.pixels[i]; //change every pixel with the current pixel of the live image

  samplingImage.updatePixels(); //update the image
}