function toggle3D() {
  var threedElements=jQuery(".threed");
  var nonthreedElements=jQuery(".nonthreed");
  if(threedElements.size() > 0) {
    threedElements.addClass("nonthreed");
    threedElements.removeClass("threed");
  } else if(nonthreedElements.size() > 0) {
    nonthreedElements.addClass("threed");
    nonthreedElements.removeClass("nonthreed");
  }
}