function getBoolIsInViewportById(strId)
{
  return getBoolIsInViewportByObj(document.getElementById(strId));
}

function getBoolIsInViewportByObj(oElement)
{
  var oRect=oElement.getBoundingClientRect();
  var oViewport={
        left:0,
        top:0,
        right:(window.innerWidth || document.documentElement.clientWidth),
        bottom:(window.innerHeight || document.documentElement.clientHeight)};
  
  // 面積為零，直接跳出
  if(oRect.left==oRect.right || oRect.top==oRect.bottom || oViewport.left==oViewport.right || oViewport.top==oViewport.bottom) { return false; }
  
  if(oRect.left>oViewport.right || oViewport.left>oRect.right) { return false; }
  if(oRect.bottom<oViewport.top || oViewport.bottom<oRect.top) { return false; }
 
  return true;       
}