var backButton = '';
    var navbackButton = 'uebermich';
    var gallerybackButton = '';
    var status = 'leer'

    navigation = new Array('uebermich', 'equipment', 'referenzen', 'gaestebuch', 'impressum', 'kontakt');

    function changeGalleryNav(id,gallery) {
      document.getElementById('btn_nav' + id).src = 'img/btn_active.gif';
      document.getElementById('status_img').src = 'img/status_' + gallery + '.gif';

      gallerybackButton = gallery;
    }

    function clearGalleryNav(id) {
      if (backButton != id) {
        document.getElementById('btn_nav' + id).src = 'img/btn_inactive.gif';
      }
      if (gallerybackButton != status) {
        document.getElementById('status_img').src = 'img/status_' + status + '.gif';
      }
    }

    function showGallery(id,gallery) {

      navbackButton = '';
      for (var j = 0; j < navigation.length; j++) {
        document.getElementById(navigation[j]).style.display = 'none';
        clearMainNav(navigation[j]);
      }

      document.getElementById('container').className = 'container_gallery';
      document.getElementById('gallery').style.display = 'block';
      document.getElementById('gallery_iframe').src = 'galerien/' + gallery + '/index.html';
      document.getElementById('status_img').src = 'img/status_' + gallery + '.gif';

      for (var i = 0; i <= 10; i++) {
        document.getElementById('btn_nav' + (i+1)).src = 'img/btn_inactive.gif';
      }
      document.getElementById('btn_nav' + id).src = 'img/btn_active.gif';

      backButton = id;
      status = gallery;

    }

    function changeMainNav(nav) {
      document.getElementById('nav_' + nav).src = 'img/nav_' + nav + '_a.gif';
    }

    function clearMainNav(nav) {
      if (navbackButton != nav) {
        document.getElementById('nav_' + nav).src = 'img/nav_' + nav + '_i.gif';
      }
    }

    function showMain(nav) {

      for (var j = 0; j < navigation.length; j++) {
        document.getElementById(navigation[j]).style.display = 'none';
        document.getElementById('nav_' + navigation[j]).src = 'img/nav_' + navigation[j] + '_i.gif';
      }

      document.getElementById('gallery').style.display = 'none';
      document.getElementById('container').className = 'container';
      document.getElementById(nav).style.display = 'block';
      document.getElementById('nav_' + nav).src = 'img/nav_' + nav + '_a.gif';
      document.getElementById('status_img').src = 'img/status_leer.gif';

      for (var i = 0; i <= 10; i++) {
        document.getElementById('btn_nav' + (i+1)).src = 'img/btn_inactive.gif';
      }

      navbackButton = nav;
      status = 'leer';
      backButton = '';
    }

    function centerPage() {

      var height = window.innerHeight;
      var containerHeight = 589;

      var margin = (height - containerHeight)/2;

      document.getElementById('container').style.marginTop = margin + 'px';

    }

    function checkForm() {
      if(document.getElementById('vorname').value != '' && document.getElementById('nachname').value != '' && document.getElementById('email').value != '' && document.getElementById('betreff').value != '') {
        return true;
      } else {
        alert('Bitte füllen Sie die gekennzeichneten Felder aus.');
        return false;
      }
    }