Dave asked:
>... can I assume that TIF files with LZW compression don't compromise the
>image at all? I.e. like JPG compression...
-------------------------------------------------
LZW (Lempel-Ziv-Wegman) is a "lossless" compression algorithm. When
expanded, the resulting file will be identical to the original file
before compression. LZW (or something close to it) is what is used in
ZIP files since, when it comes to code and data, you can't afford to
lose even a single bit.
JPEG (Joint Photographic Expert's Group) is a "lossy" algorithm. When
the compression is done some data may be discarded as unimportant based
upon the quality level or degree of desired compression.
Assume that the image contains a string of 100 blue pixels with
intensity 100, 1 blue pixel with intensity 101 followed by 50 more blue
pixels with intensity 100. The inner workings of LZW is a bit too
complicated to describe here but, in general, a compression algorithm
will be able to compress this data by noting that there are patterns
contained in the data and then stores the patterns instead of the
original data.
A "lossless" algorithm will have to record the data above in a manner
similar to the way I've described it... 100 of X, 1 of Y and 50 more of
X. Given sufficient latitude on the quality side a "lossy" algorithm
might say; "That one Y looks pretty insiginificant, probably can't even
notice it, maybe the monitor can't even accurately reproduce it, guess
I'll record it as 151 of X."
We lost a little bit of quality in the process but reduced the amount of
data stored from three patterns to one.
My apologies to LZW and JPEG since this is a pretty simple explanation
but I think it's sufficient to get the idea across.
Chuck Norcutt
Woburn, Massachusetts, USA
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