// Sets the background gray value based on the distance 

// of the mouse from the center of the screen

void draw() {

  noStroke();

  float d = dist(width/2, height/2, mouseX, mouseY);

  float maxDist = dist(0, 0, width/2, height/2);

  float gray = map(d, 0, maxDist, 0, 255);

  fill(gray);

  rect(0, 0, width, height);

}