let DIM;

function setup() {

  DIM = 1000;

  canvas = createCanvas(DIM, DIM);

}

​

let boxw;

function setupEnv() {

  boxw = width * 0.01;

}

​

function draw() {

  background(220);

  let ctr = 0;

  for (let y = 0; y < height; y += boxw) {

    for (let x = 0; x < width; x += boxw) {

      if (ctr % 2 == 0) {

        fill(0);

      } else {

        fill(220);

      }

      rect(x, y, boxw, boxw);

      ctr++;

    }

  }

}

​

function windowResized() {

  let w, h;

  if ((windowWidth / windowHeight) < (width / height)) {

    w = windowWidth;

    h = w * height / width;

  } else {

    h = windowHeight;

    w = h * width / height;

  }

  canvas.position((windowWidth - w)/2, (windowHeight - h) / 2);

  resizeCanvas(w, h);

  setupEnv();

}

​