I have a problem to draw the brushes strokes. Here is my code. Are there some parts missing so it doesn;t work??

void setup() { 
  size(800,600);
  smooth();
}
void draw() {
  background(0);
  fill(0, 10);
  noStroke();
  rect(0, 0, width, height);

  if (mousePressed == true)
  {
    int r = mouseY / 15;
    int c = mouseX / 15;

    fill(255);
    rect(c*15+1, r*15+1, 14, 14);
  }
}