video.js

var capture;
var stepSize = 5;

function setup() {
  createCanvas(400, 300);
  capture = createCapture(VIDEO);
  capture.size(400, 300);
  capture.hide();
  rectMode(CENTER);
  noStroke();
}

function draw() {
  background(50);
  fill(255);
  capture.loadPixels();
  for (var cy = 0; cy < capture.height; cy += stepSize) {
    for (var cx = 0; cx < capture.width; cx += stepSize) {
      var offset = ((cy*capture.width)+cx)*4;
      var xpos = (cx / capture.width) * width;
      var ypos = (cy / capture.height) * height;
      var pixelBrightness = capture.pixels[offset+1]/255; //TECHNICALLY the green brightness
      rect(width - xpos, ypos, stepSize, stepSize*pixelBrightness);
    }
  }
}