var rad = 150;

var xpos, ypos;



var xspeed = 13.0;

var yspeed = 13.0;



var xdirection = 1;

var ydirection = 1;



function setup() {

  createCanvas(windowWidth,

               windowHeight);

  noStroke();

  ellipseMode(RADIUS);



  xpos = width / 2;

  ypos = height / 2;

}



function draw() {

  background(0, 0, 0);

  fill(255, 255, 255);



  xpos = xpos + xspeed * xdirection;

  ypos = ypos + yspeed * ydirection;



  if (xpos > width - rad || xpos < rad) {

    xdirection *= -1;

  }



  if (ypos > height - rad || ypos < rad) {

    ydirection *= -1;

  }



  ellipse(xpos, ypos, rad, rad);

}