var offset = 0.5;

var speed = 0.1;

var angle = 0;

​

function setup() {

  createCanvas(640, 360);

}

​

function draw() {

  background(255);

  var alpha = baseAlpha + sin(angle) * offset;

  fill("rgba(0, 0, 0, " + alpha + ")");

  noStroke();

  ellipse(width / 2, height / 2, 100, 100);

  angle += speed;

}