void setup() {
  size(300,300);
  background(0);
  fill(255);
  noStroke();
  smooth();
}

boolean firstpress = false;
float xnoise = 0.0;
float ynoise = 0.0;
float inc = .04;

void draw () {
  for (int y = 0; y < height; y++) {
    for (int x = 0; x < width; x++) {
      float gray = noise(xnoise, ynoise) * 255;
      stroke(gray);
      point(x,y);
      xnoise = xnoise + inc;
    }
    xnoise = 0;
    ynoise = ynoise + inc;
  }

}

void mousePressed () {
  if (firstpress == false) {
    noLoop();
    firstpress = true;
  } 
  else {
    loop();
    firstpress = false;
  }
}