​

function setup() {

  createCanvas(400, 400);

  background(220);

  px = -1;

  py = -1;

}

​

function draw() {

  // background(220);

}

​

function mousePressed() {

  if (px == -1 || py == -1) {

    px = mouseX;

    py = mouseY;

  } else {

    line(px, py, mouseX, mouseY)

    px = mouseX;

    py = mouseY;

  }

}