var W3C = false
var IE = false

useragent()

function useragent() {
	if (document.all) {
  	IE = true
  	return false
	}
	else if (document.getElementById) {
  	W3C = true
  	return false
	}
	else {
  	alert('Ihr Browser wird nicht unterstützt!')
	}
}

function init() {
	zentrieren('screen')
	showLayer('screen')
}
 
function zentrieren(objname) {
	if (IE == true) {
		var winWidth = document.body.clientWidth
		var winHeight = document.body.clientHeight
	}
  else if (W3C == true) {
		var winWidth = window.innerWidth
		var winHeight = window.innerHeight
  }  
  var obj = getLayer(objname)
	  
  var objWidth = obj.offsetWidth
  var objHeight = obj.offsetHeight
	 
  var left = (winWidth - objWidth) / 2
  var top = (winHeight - objHeight) / 2
  
  if (left >= 0 && top >= 0) {
	 obj.style.left = left + "px"
	 obj.style.top = top + "px"
  }
  else if (left < 0) {
   obj.style.left = 0 + "px"
   obj.style.top = top + "px"
  }
  else if (top < 0) {
   obj.style.left = left + "px"
   obj.style.top = 0 + "px"
  }
  else {
	obj.style.left = 0 + "px"
	obj.style.top = 0 + "px"
  }
}

function getLayer(layername) {
  var layer
  if (W3C) {
    layer = eval('document.getElementById("' + layername + '")')
    return layer
  }
  else if (IE) {
    layer = eval('document.all.' + layername)
    return layer
  }
  else {
    return null
  }
}

function showLayer(objname) {
  var layer = getLayer(objname)
  layer.style.visibility = 'visible'
}

function hideLayer(objname) {
  var layer = getLayer(objname)
  layer.style.visibility = 'hidden'
}