createCanvas(320, 240);

  pixelDensity(1);

}

​

function draw() {

  background(51);

  loadPixels();

  for(let y = 0; y < height; y++){

    for(let x = 0; x < width; x++){

      let index = (x + y * width) * 4;

      pixels[index + 0] = y;

      pixels[index + 1] = 0;

      pixels[index + 2] = 0;

      pixels[index + 3] = 255;

    }

  }

  updatePixels();

}