On April 1, 2004 11:45, Chris Barker wrote:
> When you use this format, you have the choice of qualities or
> 'lossless'. I should have expected that to fulfill its description.
> Are you telling me that this dialogue box is an out and out liar? ;-)
The, JPEG 2000 file format includes a lossless option., in which residual
error from the main image compression result is also stored. Of course, if
your program is not doing it correctly, then its dialog box may indeed be
lying. Its easy to check though: Compress your image file with Jpeg 2000 then
expand in your image processing program, subtract expanded image from the
original, and you will get the error image. This will appear black until you
expand its (hopefully minuscule) tonal range to full scale. Lossless will
stay black, as there is no tonal range.
Try this with the same image compressed to the same compressed file size,
using jpeg and jpeg2000. You should see that for most photographic images
jpeg2000 is indeed much better, and the error is generally well below the
noise inherent in the photographic image (although it is not uniformly
distributed,) so that going "lossless" is practically speaking, a meaningless
waste of storage and processing. Some artifacts in digicam images with Bayer
mask sensors will greatly exceed jpeg2000 artifacts at reasonable compression
ratios.
see
http://www.jpeg.org/faq.phtml?action=show_answer&question_id=q3d5bc0701c9b6
--
Parzival Herzog
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