The JPEG format is actually a combination of
several elements, and it's important that you understand what it does before you
use it. The JPEG algorithm reduces the amount of space a compressed image will
occupy on disk by selectively removing details from the image. Pictures with
fewer details compress more effectively.
At the default value of 75,
relatively little picture degradation will take place but a significant amount
of compression will be obtained. At lower values you'll experience still better
compression, but with a marked loss of image quality.
Once an image has been
converted to JPEG, its lost details are lost for good. Note also that repeatedly
reading and writing the same image in the JPG format will successively degrade
it – it's a very bad idea to convert from JPG to JPG, as your pictures will
get a bit uglier each time you do so.
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NOTE: All color JPEG files are 24-bit images, having
sixteen-million colors. Because some systems can only display 256 colors
directly, JPEG files can be read as 256-colour images. This actually
means that they'll be dithered when they're read.
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GFX-View
includes support for progressive JPEG. A progressive JPEG file will initially
appear blurred if it's part of a web page or other on-line service, and will
resolve as more of the file is transmitted. GFX-View will read progressive JPEG
file but it will not display them progressively – that is, if you view
progressive JPEG file, it will appear as a single sharp image. Note
that many applications with JPEG support will not read progressive JPEG files.