var opacity = 0;
var speed = 50;

var nogeentimer;

function start_fading() {
  clearTimeout(fadetimer);
  nogeentimer = setInterval( do_fading, speed);
}

function do_fading() {
  opacity = opacity + speed / 9000;
  if ( opacity > 1 ) {
    opacity = 1;
  }
  document.getElementById('fade_layer_1').style.opacity = opacity;
  document.getElementById('fade_layer_1').style.filter  = 'alpha(opacity=' + ( opacity * 100 ) + ')';
  document.getElementById('fade_layer_2').style.opacity = opacity;
  document.getElementById('fade_layer_2').style.filter  = 'alpha(opacity=' + ( opacity * 100 ) + ')';
  document.getElementById('fade_layer_3').style.opacity = opacity;
  document.getElementById('fade_layer_3').style.filter  = 'alpha(opacity=' + ( opacity * 100 ) + ')';
}