[meteorite-list] Mystery

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:07:05 2004
Message-ID: <3DBF748D.97BFEB68_at_bhil.com>

Hi, Michael, List,

    The GIF image format, devised by Compuserve many years ago, is limited to
images with no more than 256 colors (or 8-bits-per-pixel), usually
palette-based, while the JPEG format can handle much greater bit-depths than
GIF. Most often, JPEG images are 24-bits-per-pixel.
    That factor alone (8 vs. 24 bits) would make a GIF image 1/3 the size of a
"true-color" image of the same size. Further, JPEG's have variable data
compression schemes in which compression efficiency trades off with data
fidelity; that is, a highly compressed JPEG loses image data and gets "fuzzy,"
while a sharp detailed JPEG image has relatively poor compression.
    GIF format uses a complicated, even sophisticated, compression algorithm
that preserves the image perfectly and achieves good data compression. Its
fatal limitation to 256 colors, however, makes it unsuitable for high-end
graphics. Saving a 24-bit image as a GIF is accomplished by "dithering" 24-bit
pixels into cell arrays using a standardized 256-color spectrum palette. If you
look closely at a GIF conversion, you will see it break up into "dots" like a
Georges Seurat painting.
    The smaller size of the GIF file is usual and normal. The slow loading
simply means the the "GIF engine" (or decompression code) of your browser is
much less efficient than its "JPEG engine." Netscape's GIF decoder and older
Windows GIF decoders are, well, they stink, being about 8 to 40 times slower
than, for example, the code used on Atari computers 12 to 14 years ago.
Afterall, a Wintel computer doesn't need decent code; it just needs a higher
clock speed, right?
    However, Photoshop is correct to offer the GIF alternative for websites.
Since the image file is going to be smaller than a JPEG, the file will download
much faster, a boon to those with slow modems, but speed in decoding the file
depends on the browser and the clock speed of the computer running the browser
program. If the user has a 100 mhz Pentium I, it's going to take a tad longer
than a 2.0 ghz Pentium IV (like 20 times longer)!
    Looked at others' responses, I can only add: if Photoshop is saving a
16-bit image in something it calls a "GIF" file, that file is a bastard file
format of their own contrivance and not a true GIF format (neither 87a nor 89a)
which does not have a high-color or true-color option. That would explain why
it was bloated and slow, like Photoshop itself. To obtain a true GIF file,
which will be smaller, you do not need to reduce the image to 64 or 32 colors;
there is no data size saving in creating 5-bit or 6-bit image formats in GIF.
Reduce to 8-bit (256 colors); that's all that's necessary.
    And, JPEG was not designed for the internet; the format was proposed by an
industry working group in 1986 or 1987, long before the internet was even a
gleam in Al Gore's eye, even before little Billy Gates invented Monopoly.


Sterling K. Webb
(in a former life in a galaxy far away,
 a software development programmer
 for Atari computers, specializing
 in, by an odd coincidence, graphics
 image formats and image processing)
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Michael L Blood wrote:

> Hi all,
> I have a serious question:
> Well, this was the deal - I put up an image on my web site and
> decided to try a Photoshop feature that is SUPPOSED to make images
> much "smaller" in terms of data - and, therefore, download time. In
> Photoshop, it is under save this image as...."for the internet" and it
> makes it a "gif" instead of a JPG, and the "size" is listed at about
> 25% of a JPG, even though the dimensions are the same.
> Funny thing is, when Jim Hartman checked it out for me, he said
> the GIF took about 4 times as long than the JPG! (With cable, everything
> is "instant download," so, I can't tell)
> You all can check for yourself, if you like by going to my site,
> scrolling down to DHURMSALA. Try clicking on the JPG and then on
> the gif and see how long each one takes to come up.
> My question is: IF the gif is supposed to be using only 1/4 the
> memory of the JPG of the same image of the same dimensions, then
> WHY is it taking LONGER to download than the JPG???
> Anybody know?
> Thanks, Michael
Received on Wed 30 Oct 2002 12:56:30 AM PST