​

function setup() {

  createCanvas(400, 400);

  noStroke();

}

​

function draw() {

  background(255);

  fill(226,22,80, 50);

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

  fill(226,22,80, alpha);

  let side = map(alpha, 0, 255, 100, 20);

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

  alpha = alpha - 3;

  if (alpha < 0) {

    alpha = 255;

  }

}