void setup ()
{
  size (200, 200);
  background (51);
  noStroke();
  rectMode(CENTER);
  noLoop();
}

  void draw()
{
  draw_rect(100, 100, 70, 70);
}

  void draw_rect (int xloc, int yloc, int size, int num)
{
  float grayvalues = 255/num;
  float steps = size/num;
  for (int i=0; i<num; i++) {
  fill (i*grayvalues) ;
  rect (xloc, yloc, size-i*steps, size-i*steps) ;
}
}