ex13.pde

float X;

void setup() {
  size(200, 200);
  colorMode(RGB, 100);
  background(99);
  X = 0;
}

void draw() {
  noStroke();
  fill(99, 30);
  rect(0, 0, width, height);
  
  X = X + 1.0;
  
  noStroke();
  fill(0);
  rect(X, 100, 10, 10);
}