good ole image preloader...

[bedges]

okay, the scenario:

i have a header image which changes randomly across all pages in the
site. that works fine. i also have an image preloader within the random
header picker which theoretically loads the header image before the
rest of the graphical content on the page. however, what appears to be
happening, judging by the status bar messages, is that the banner
always loads last.

here's the random header code -

function random_header()
{
no=Math.random()*14;
no=Math.round(no);
no = no + 1;
bname = "banners/banner_" + no + ".jpg";
preload=new Image();
preload.src=bname;
document.getElementById('rndimg').src=bname;
}

banners are all named sequentially, 'rndimg' is the blank header image
having its source changed.

any help is appreciated. thanks meantime

 

[RobG]

okay, the scenario:

i have a header image which changes randomly across all pages in the
site. that works fine. i also have an image preloader within the random
header picker which theoretically loads the header image before the
rest of the graphical content on the page. however, what appears to be
happening, judging by the status bar messages, is that the banner
always loads last.

Why not randomise the banner on the server? Why rely on unreliable client
scripting when there seems to be zero benefit?

here's the random header code -

function random_header()
{
no=Math.random()*14;
no=Math.round(no);
no = no + 1;
bname = "banners/banner_" + no + ".jpg";

Keep variables local if they don't need to be global. If your image names
are in the range 1 to 15 inclusive:

var bname = 'banners/banner_'
+ (Math.floor(Math.random()*15) + 1)
+ '.jpg';

Or using bitwise OR instead of Math.floor:

var bname = 'banners/banner_'
+ ((Math.random()*15 | 0) + 1)
+ '.jpg';

preload=new Image();
preload.src=bname;
document.getElementById('rndimg').src=bname;

Here you immediately load the image into the banner, you've given 'preload'
less than a millisecond to load and cache the image file. Also, it seems
you are running this onload so the image doesn't start loading until the
entire rest of the page has finished, making it more like a post-load
function. The function does not start downloading the image until it is
executed.

You are probably better to use a default image if scripting is not
available and replace the src with the random src using script:


<img src="noScriptDefault.gif" alt="" name="bannerImg">

<script type="text/javascript">
if (document.images && document.images['bannerImg'])
{
document.images.bannerImg.src = 'banners/banner_'
+ ((Math.random()*15 | 0) + 1)
+ '.jpg';
}
</script>


Presumably the banner is right at the top, so it should load very near the
start anyway so any kind of preload is (almost) pointless. If you have
lots of script and CSS, then maybe to a preload right after the title
element will have a small benefit.

But the effect of multiple simultaneous connections will dilute the value
of pre-loading anyway. It was mostly useful for getting small button
images to load before big banners, but here you want to load the banner
first, which happens anyway.

Test and see.

 

[bedges]

unfortunately server-side randomising isn't an option in this case.
however i used a modified version of your code snippet in the
individual pages after the image appears and it seems to work a treat.

many thanks for the help.