var by;

var diam;

var vx;

var vy;

var radius;

​

function setup() {

  createCanvas(500, 500);

  bx = width/2;

  by = height/2;

  diam = 50;

  radius = diam/2;

  vx = 4.0;

  vy = 2.0;

}

​

function draw() {

  background(255, 200, 200, 10);

  fill(0);

  ellipse(bx, by, diam, diam);

  bx = bx + vx;

  by = by + vy;

  if (bx < radius || bx > width - radius) {

    vx = -vx;

  }

  if (by < radius || by > height - radius) {

    vy = -vy;

  }

}