Archived posting to the Leica Users Group, 2001/01/15

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Subject: Re: [Leica] Re: Digital Image File Formats
From: Pete Su <psu_13@yahoo.com>
Date: Mon, 15 Jan 2001 04:33:36 -0800 (PST)

> >For instance, I have been told that JPEG images, if uncompressed for
> >editing and then recompressed, will lose information, but I've also heard
> >that there is such a thing as a JPEG algorithm that doesn't lose
> >information when uncompressed/recompressed.

Compression algorithms compress files by constructing what is essentially a big
translation table between data in the file and a "code" that can represent that
data in a smaller form. A simple example of a compression code for text is to
say "OK, whenever the some letter occurs more than twice in a row, instead of
writing out (say) AAA, write out the number of times the letter occurs, then
the letter. So AAA becomes 3A. This code will obtain modest compression ratios,
although it's pretty good for images, since images tend to contain long runs of
similar data.

JPEG does something like this, but it uses a code that is even more compact at
the cost of throwing away some information that is needed to completely
reconstruct the data. This is called "lossy" compression. JPEG is also tuned
specifically for image data, which tends to have it's own characteristics, as
compared to say, text.

There are various other standard compression algorithms (like the one used to
make .zip files) that are not lossy, but they also do not obtain the same
compression ratios.

HTH,
Pete


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Replies: Reply from "Henning J. Wulff" <henningw@archiphoto.com> (Re: [Leica] Re: Digital Image File Formats)