Pre-caching images on IE

Selim Tuvi Tue, 14 Dec 1999 19:51:18 -0800
I am having trouble in successfully caching (or pre-caching) an array of image objects 
in Internet Explorer. I am loading a series of images for animation purposes. The 
images are generated directly through the TestImage servlet by specifying 
"res.setContentType("image/gif");" On the onload event of the HTML frame I am loading 
the images to an array of image objects. This works perfectly in Netscape, so there 
are no server roundtrips. But if I use IE it accesses the server constantly and reruns 
the doGet() method of the servlet. Is there a way to prevent this from happening ? Or 
am I doomed to the browser's mercy ?

FYI, previously the servlet was creating a series of GIF files on the server and the 
script was reading them into the image objects. In that scenario IE was able to cache 
the images successfully. My goal is to accomplish the same when the image.src is a 
servlet URL.

I am including the frame source for your perusal.

Thanks
-Selim Tuvi
 Rockwell Science Center
 Palo Alto, CA


<HTML>
<HEAD>
<META http-equiv="pragma" content="no-cache">
<META http-equiv="expires" content="10-JAN-1980">
<style type="text/css">
  #encap {position:relative; visibility:visible}
  #img1 {position:absolute; visibility:visible; z-index:2}
</style>
<SCRIPT LANGUAGE="JavaScript">
var isNav, isIE
if (parseInt(navigator.appVersion) >= 4) {
  if (navigator.appName == "Netscape") {
    isNav = true
  } else {
    isIE = true
  }
}
var images = new Array(23)
var currentImage = 0
var animation_on = false
var time_index
function preCache() {
  for (i= 0 ; i < images.length ; i++) {
    images[i] = new Image(320,360)
    images[i].src 
="/servlet/awareServlet.TestImage?timeIndex="+(images.length-i-1)+"&x=1434&y=1087"
  }
}

function loadImage(imgNum) {
  if (isIE)
    document.images[0].src = images[imgNum].src;
  else
    document.encap.document.img1.document.images[0].src = images[imgNum].src;
}

function nextImage() {
  if (animation_on) {
    if (currentImage < images.length - 1) {
      currentImage++;
    } else {
      currentImage = 0;
    }
    loadImage(currentImage);
  }
}

function stop() {
  animation_on = false;
}

function goBack() {
  parent.document.location.href = '/servlet/awareServlet.WxBrowse';

}

</SCRIPT>
</HEAD>
<BODY bgcolor="#C0C0C0" onLoad="preCache(); loadImage(0)">
<form>
<table border=3 cellspacing=0 cellpadding=0>
  <tr>
    <td>
    <div id="encap">
      <span id="img1">
        <img width=320 height=360 VSPACE=0 HSPACE=0 ALIGN=middle 
onError="setTimeout('nextImage()',100)" onLoad="setTimeout('nextImage()',100)">
      </span>
      <img id="bground" src="/servlet/awareServlet.OverlayImage?x=1434&y=1087" 
width=320 height=360>
    </div>
    </td>
  </tr>
  </table>

<table>
<tr><td width=100></td><td>
<input type="button" onClick="animation_on = true; nextImage()" value=">>">
<input type="button" onClick="stop()" value="Stop">
</td></tr></table>
<br><br><br>
<input type="button" onClick="goBack()" name="back" value="Back">
</form>
</BODY>
</HTML>

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Pre-caching images on IE

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