function centerHorizontal(objID) {
  // if image in layer exists, take size of image:

  baseLeft = 0;
  var layerWidth = document.getElementById(objID).offsetWidth;
  var areaWidth = screen.width;
  var leftCorner = Math.max(baseLeft,baseLeft + (areaWidth - layerWidth)/2);
  document.getElementById(objID).style.left = leftCorner;
  document.getElementById(objID).style.visibility = "visible";
}