createCanvas(400, 400);

  pixelDensity(1);

}

​

function draw() {

  background(220);

  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();

}