let video;

function setup() {

  createCanvas(windowWidth, windowHeight);

  video = createCapture(VIDEO);

  video.hide();

}

​

function draw() {

  background('black');

  image(video, 0, 0, width, width*640/480);

  x++;

  x%=width;

  strokeWeight(5);

  stroke('white');

  line(x, height/2, 0, height/2);

}