createCanvas(400, 400);

  generate();

}

​

function generate() {

  background(0);

}

​

function mouseDragged() {

  noFill();

  stroke(255,255,255,64);

  var t = millis()/100

  var diam = 20 + 10*sin(t);

  ellipse(mouseX, mouseY, diam, diam/2);

}