function wheel(event){
  var delta = 0;

  // ie event handler
  if (!event) {
    event = window.event;
  }

  // ie Opera
  if (event.wheelDelta) {
    delta = event.wheelDelta/120;

    // kpera korrektur
    if (window.opera){
      delta = -delta;
    }

  // mozilla
  } else if (event.detail) {
    delta = -event.detail/3;
  }

  // wenn gescrollt wird tue dies
  if (delta){
    setZoom(delta);
  }

  // mozilla scrolling verhindern
  if (event.preventDefault){
    event.preventDefault();
  }

  // ie opera scrolling verhindern
  event.returnValue = false;
}

function dowheel(){

  // scroll listener hinzufügen
  if (window.addEventListener){
    // mozilla
    window.addEventListener('DOMMouseScroll', wheel, false);
  }

  // ie opera
  window.onmousewheel = document.onmousewheel = wheel;
}

function stopwheel(){
  // scroll listener hinzufügen
  if (window.removeEventListener){
    // mozilla
    window.removeEventListener('DOMMouseScroll', wheel, false);
  }

  // ie opera
  window.onmousewheel = document.onmousewheel = '';
}