Archived posting to the Leica Users Group, 2001/01/15
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At 4:33 AM -0800 1/15/01, Pete Su wrote:
> > >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
Again, the highest level of JPEG compression is not lossy, but a form
of LZW compression. Not much compression, but no degredation either.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
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