​

function setup() {

  createCanvas(windowWidth, windowHeight);

  xstart = 50;

  ystart = 350;

  x = xstart;

  y = ystart;

  scl = 0.05;

  xspeed = 15*scl;

  yspeed = -1*scl;

}

​

function draw() {

  background(0);

  x += xspeed;

  y += yspeed;

  stroke(255);

  strokeWeight(10);

  line(xstart, ystart, x, y);

  if((x < 0 || x > width) || (y < 0 || y > height)) {

    x = xstart;

    y = ystart;

  }

}