function correctPNG() {
		var arVersion = navigator.appVersion.split("MSIE");
		var version = parseFloat(arVersion[1]);
		if ((version >= 5.5) && (document.body.filters)) {
			for ( var j = 0; j < document.images.length; j++) {
				var img = document.images[j];
				var imgName = img.src.toUpperCase();
				if (imgName.substring(imgName.length - 3, imgName.length) == "PNG") {
					var imgID = (img.id) ? "id='" + img.id + "' " : "";
					var imgClass = (img.className) ? "class='" + img.className
							+ "' " : "";
					var imgTitle = (img.title) ? "title='" + img.title + "' "
							: "title='" + img.alt + "' ";
					var imgStyle = "display:inline-block;" + img.style.cssText;
					if (img.align == "left")
						imgStyle = "float:left;" + imgStyle;
					if (img.align == "right")
						imgStyle = "float:right;" + imgStyle;
					if (img.parentElement.href)
						imgStyle = "cursor:hand;" + imgStyle;
					var strNewHTML = "<span "
							+ imgID
							+ imgClass
							+ imgTitle
							+ " style=\""
							+ "width:"
							+ img.width
							+ "px; height:"
							+ img.height
							+ "px;"
							+ imgStyle
							+ ";"
							+ "filter:progid:DXImageTransform.Microsoft.AlphaImageLoader"
							+ "(src=\'" + img.src
							+ "\', sizingMethod='scale');\"></span>";
					img.outerHTML = strNewHTML;
					j = j - 1;
				}
			}
		}
	}
	window.attachEvent("onload", correctPNG);

	function changeValidateCode(obj) {
		//document.write("nimei");
		//获取当前的时间作为参数，无具体意义  
		var timenow = new Date().getTime();
		//每次请求需要一个不同的参数，否则可能会返回同样的验证码  
		//这和浏览器的缓存机制有关系，也可以把页面设置为不缓存，这样就不用这个参数了。  
		obj.src = "makeCheckCodeImage.do?d=" + timenow;
	}

	function validate() {
		
		var usernameValue=document.getElementById("usernameId").value;
		var passwordValue=document.getElementById("passwordId").value;
		var imageValue=document.getElementById("imageId").value;


		if(usernameValue.length==0)
		{
			alert("username should not be blank!");
			return false;
		}
		
		if(passwordValue.length==0)
		{
			alert("password should not be blank!");
			return false;
		}
		
		if(imageValue.length==0)
		{
			alert("check code should not be blank!");
			return false;
		}

		return true;
	}