var bg = (function(){

  var bg = {}
  bg.el = document.getElementById("bg")
  
  bg.change = function(url){
    var img = new Image ()
    img.src = url
    oktween.add({
      obj: bg.el.style,
      from: { opacity: 1 },
      to: { opacity: 0 },
      easing: "circ_in",
      duration: 500,
      finished: function(){
        bg.el.style.backgroundImage = "url(" + url + ")"
      }
    }).then({
      delay: 500,
      obj: bg.el.style,
      from: { opacity: 0 },
      to: { opacity: 1 },
      easing: "circ_in",
      duration: 500,
    })
    
  }
  
})()