var Backdrop = {
    init: function(num,image,speed,container) {
        container = document.getElementById(container);
        var images = new Array();
        for (var i = 0;i < num; i++) {
            images[i] = document.createElement('img');
            images[i].src = image;
            images[i].style.top = Math.floor(Math.random()*801) + 'px';
            images[i].style.left = Math.floor(Math.random()*1901) + 'px';
            container.appendChild(images[i]);
            Backdrop.animate(images[i],speed);
        }
    },
    animate: function(img,speed) {
        currentPos = parseInt(img.style.left);
        if (currentPos >= 0) {
            newPos = currentPos - speed;
            img.style.left = newPos + 'px';
        } else {
            img.style.left = '1901px';
            img.style.top = Math.floor(Math.random()*801) + 'px';
        }
        setTimeout(function(){Backdrop.animate(img,speed)},50);
    }
};
window.onload = function () {
    Backdrop.init(80,'images/starbright.png',1,'header');
    Backdrop.init(30,'images/starmed.png',3,'header');
    Backdrop.init(20,'images/stardark.png',5,'header');
}