Don't cache changed image.src?

06-28-2003

Hi.

I have a webcam page (video surveillance) that refreshes 9 images
once a second (1000ms rate). It uses javascript to change the
image.src

To trick the caching mechanism of the browser and force a fresh
download from the IP camera, a random tag is appended to the URL.

Unfortunately MSIE6 (WinXP) caches each and every frame ON DISK.
That is, while the page is displayed, the harddrive stores 9*20kb
or ~200kb per second. And worse, MSIE lets the temp folder grow
very big, resulting in reduced system performance (especially
when booting up after a work-day of video surveillance).

What can I do about it? The life-time of an image is 1 second,
it will never be used again. I don't want the browser to store
it on the harddrive.

I tried this on my page:

<HEAD> <META http-equiv="Pragma" content="no-cache"> </HEAD>

but it seems to work only for the HTML portion of the page, not
for the image.src loading.

I tried to locate a RAM drive where I can point the MSIE6 temp
folder to, so that at least it won't cache on the harddrive,
but couldn't find any solution (for WinXP) either. And also, I
went for HTML/JavaScript to NOT have to install anything on the
client machine

Can you point me to a solution for this problem?

Marc

06-28-2003

On 28 Jun 2003 10:21:06 -0700, (jetmarc) wrote:

i.
>I tried this on my page:
>
><HEAD> <META http-equiv="Pragma" content="no-cache"> </HEAD>
>
>but it seems to work only for the HTML portion of the page, not
>for the image.src loading.

Of course, why would it work anywhere else?

>Can you point me to a solution for this problem?

If you'd read the FAQ, you'd've seen the link to
http://www.mnot.net/cache_docs/

It will not, and cannot be solved with javascript.

Jim.
--
comp.lang.javascript FAQ - http://jibbering.com/faq/

06-28-2003

On 28 Jun 2003 15:46:51 -0700, (jetmarc) wrote:

>> If you'd read the FAQ, you'd've seen the link to
>> http://www.mnot.net/cache_docs/
>>
>> It will not, and cannot be solved with javascript.
>
>Thanks for your follow-up.
>
>Actually I have seen that page before, when researching. The problem
>is that I don't have any influence on the HTTP headers, because the
>image URL is on a physically separate web cam (Axis 2100 IP network
>camera).

Then get some influence, simply proxy it through a real server, and
next time choose a tool that allows you to do your job.

>I think, actually the MSIE browser does honor the no-cache pragma in
>that it reloads the HTML page _AND_ the initial dummy images from
>scratch each time I open the page.

It's unlikely that a web cam server is sending cacheable headers,
therefore IE won't cache the image between page reloads on its default
setting - it wll within instance.

> I conclude that maybe Axis as a manufacturer of
>web cams themselves did not find a way to circumvent MSIE's behaviour
>of producing a local FILE

IE does not _always_ save a local file.

>Basically, I'm looking for a client-side solution.

There is no client-side solution...

>I can't change
>the cameras behaviour unless I re-engineer their embedded firmware,
>or cache all cam images on a linux server

No, it's simple, put Apache as a proxy between the web and the camera,
you can then control the headers and everything else in Apache.

Jim.
--
comp.lang.javascript FAQ - http://jibbering.com/faq/

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