var drawing, scaleX, scaleY, z;

function setup() {

  createCanvas(256, 256, P2D);

  pixelDensity(1);

  drawing = createGraphics(width, height);

  scaleX = width;

  scaleY = height;

  z = 0;

}

​

function mouseMoved() {

  scaleX = max(1, mouseX);

  scaleY = max(1, mouseY);

}

​

function draw() {

  background(220);

  drawing.loadPixels();

  for(var y = 0; y < height; y++) {

    for(var x = 0; x < width; x++) {

      var shade = floor(map(noise(x / scaleX, y / scaleY, z), 0, 1, 0, 255));

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

      drawing.pixels[index] = shade;

      drawing.pixels[index+1] = shade;

      drawing.pixels[index+2] = shade;

      drawing.pixels[index+3] = 255;

    }

  }

  drawing.updatePixels();

  z = (z + .1) % 1;

  image(drawing, 0, 0);

}