Perceived download time (was RE: [thelist] Really dumb questi on...)

Jonathan_A_McPherson at rl.gov Jonathan_A_McPherson at rl.gov
Mon Jul 8 15:17:02 CDT 2002


Hey,

> The whole point (in my mind) of slicing images up is to optimize certain
> parts of the image. Reducing certain parts of your your images by several
> KB's will increase the overall download speed of the entire image.  But
> you're probably right, slicing an image up without optimizing sections of
it
> won't reduce the download time.  But what about the additional
simultaneous
> requests to the Web server.  Couldn't that possibly increase the download
> time?

Slicing images up increases the download time of the image, for a few
reasons. You get additional overhead for:
(a) creating a new connection.
(b) reading the "header" information from each image.
(c) loss of compression efficiency. all the compression algorithms I know of
compress a single image more efficiently than the same image sliced in two.
you MIGHT be able to get better compression by using different algorithms on
different parts of the image, but that's both time-consuming and dangerous
(color management, anyone?).
(d) cpu time to compute layout parameters (if you left them out of your
source) and decompress two images instead of one.

Yes, you get have two connections running at once -- but does this really
reduce download time? If -- and ONLY if -- the user's _total_ connection
speed exceeds the speed at which the Web server is delivering data for any
_single_ connection to the user.

Since most users are on dialup, a single connection to the Web server is
generally sufficient to exhaust their bandwidth. Adding another connection
is likely to simply cause both to run at 50%.

You might increase perceived downloading time with a split image, since it's
likely that one of the pieces will show up relatively quickly -- but this is
what progressive GIF, JPG, and PNG are for. Use them.

Aside from layout advantages, I can't think of very many good reasons to cut
up images.

--
Jonathan McPherson, LMIT/SD&I
Software Engineer & Web Systems Analyst
email / jonathan_a_mcpherson at rl dot gov



More information about the thelist mailing list