JPG's are different than gifs. The compression is more complicated, and works better
for complex images, like photographs. The format is "lossy", meaning that some of the image
data is thrown away, but even so the quality is usually excellent if the source image is
clear.
There are 100 levels of jpg compression, though most graphics software will give you
choices for high, medium and low file sizes. These photos were all taken with a digital
camera at 72dpi, then adjusted and compressed in Adobe Photoshop. Photoshop 5.0 gives
you the coice of 10 different compression levels, and either "Standard" or "Progressive".
Progressive gives you slightly better compression rates.
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This is saved at the highest quality, and is 127KB, which would take over a minute
to download at normal speeds. |
The same image saved at a medium level. 33KB, aproximately 15-20 seconds to download.
The image quality is still excellent, but we're starting to see some compression
artifacts. |
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This is saved at a low file size. 23KB, aproximately 10 seconds to download. The
compression causes lots of quality degredation at this point.
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And finally, here is the same image that I've tweaked a little.. it's now down to 10.5Kb.
Not bad, hunh? How'd I do it? Well, there's this little program called WebJPG, by
Vimas that costs $30 (and well worth every penny).
It can fuction as a stand alone, or as an export filter for PhotoShop and gives you much
better control over the compression.
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So how does JPG compression work? I have no idea, and feel no burning need
to find out either. It's enough that we to know how to use them, and how to format a
picture to get the most out of them.
Applying a single pixel blur to a photo before compressing with JPG can result in
considerable savings in file size. It degrades the quality a little, but the difference
in barely noticable and the reduction significant. Your visitors will thank you for keeping
the images small.
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