if(window.addEventListener) {
    // Non-IE
    window.addEventListener("resize", set_bg, false);
} else {
    // IE
    window.attachEvent("onresize", set_bg);
}


minHeight = 680;

function set_bg(){
    if(self.innerWidth) {
          frameWidth = self.innerWidth;
          frameHeight = self.innerHeight;
    }
    else if(document.documentElement && document.documentElement.clientWidth){
          frameWidth = document.documentElement.clientWidth;
          frameHeight = document.documentElement.clientHeight;
    }
    else if(document.body){
          frameWidth = document.body.clientWidth;
          frameHeight = document.body.clientHeight;
    }


    if(frameHeight < minHeight){
        document.getElementById('main_bg').style.height = minHeight + "px";
    }
    else{
        document.getElementById('main_bg').style.height = "100%";
    }
    
}