At 09:30 AM 11/30/99 +0000, Ian A. Nichols wrote:
[snip]
>LZW is indeed a lossless compression scheme, however, I have found that
>with scanned photographic images it doesn't work very well at all.
>There's no quality loss but I've found that the resulting files actually
>get *bigger* as often as they get smaller, and that any reduction you do
>get is not worth the extra processing overhead to decompress the
>"compressed" data.
Yes.
This is because, in terms of the statistical distribution of image data in an
image file, most images appear to be almost random. Since lossless compression
schemes ultimately depend on some data redundancy in the file to effect
compression, these algorithms often fail on image files. Thus was born "lossy"
compression, where the compression algorithm starts to remove information from
the file (rather than just "tokenizing" or otherwise uniquely representing it),
usually in such a way as to have a *minimal* impact on the *human* perception
of the image. Depending on the luck of the draw, some images can withstand a
huge amount of compression with little if any noticeable effect on the
perceived quality of the image. Other images start to noticeably degrade
almost immediately (that is, with very little lossy compression applied).
Still others will compress very little even when cranking up the compression to
the maximum, and there will also be virtually no perceived loss of quality (in
other words, they're "lossy compression-resistant" -- there's a picture from
Branko Turk in the Unofficial Olympus Web Photo Gallery which is exactly like
this, and was a source of real surprise to me when I first worked on it).
That's why it's a crapshoot doing JPEG compression. The best general advice I
was ever given was "Save as lossless, open, rename file, and start cranking up
the lossy compression, viewing the results at each step. At some point, image
degradation will become unacceptable *to you*. This is when you stop."
That also explains my comments about the type of system and output device
you're using to view the images -- different pixel bit depths etc. can alter
your impression of the image's "quality," even using the exact same source file.
Garth
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