Embedding Iteration.

Embedding "for" structures allows repetition in two dimensions.

 
float box_size = 11; 
float box_space = 12; 
int margin = 7; 
 
size(200, 200); 
background(0); 
noStroke(); 
 
// Draw gray boxes 
 
for (int i = margin; i < height-margin; i += box_space){
  if(box_size > 0){
    for(int j = margin; j < width-margin; j+= box_space){
      fill(255-box_size*10);
      rect(j, i, box_size, box_size);
    }
    box_size = box_size - 0.6;
  }
}