overlay_scroll

window.addEventListener('scroll', function(){
   if (vOverlay) {
      vOverlay.style.top = window.pageYOffset + "px";
      vOverlay.style.left = window.pageXOffset + "px";
   }
}, false);


window.addEventListener('resize', function(){
   if (vOverlay) {
      vOverlay.style.top = window.pageYOffset + "px";
      vOverlay.style.left = window.pageXOffset + "px";
      vOverlay.style.width = window.innerWidth + "px";
      vOverlay.style.height = window.innerHeight + "px";
   }
}, false);


/*  Usage:

 LargeImg.addEventListener('load', function() {
    // resize if taller
    if (this.height > window.innerHeight) {
      this.ratio = window.innerHeight / this.height;
      this.height  = this.height * this.ratio;
      this.width - this.width * this.ratio;
    }
    // if wider
    if (this.width > window.innerWidth) {
      this.ratio = window.innerWidth / this.width;
      this.height  = this.height * this.ratio;
      this.width - this.width * this.ratio;
    }
    
    centerImage(this);
    vOverlay.appendChild(LargeImg);
    
 }); // image has loaded

*/
function centerImage( img ) {
   var diffX = (window.innerWidth - img.width) / 2;
   var diffY = (window.innerHeight - img.height) / 2;
   img.style.top = diffY + "px";
   img.style.left = diffX + "px";
   return img;
}