csu@alembic.acs.com (Dave Mack) (06/01/91)
In article <1991May30.203723.21550@ns.network.com> logajan@ns.network.com (John Logajan) writes: >I'm reporting this without understanding it, but, Don >Lancaster claims that JPEG (which he says uses a >DCT -- discrete cosine transform ) is obsolete because >the new "wavelet" methods of compression make it slow, >much less compressive, and less fidelity in comparison. I can only base my comments on very limited experimentation with the Yale wavelet package, of which my understanding is at best poor, but it appears that wavelet image compression is drastically slower than JPEG, the fidelity is dreadful, and the compression is not nearly as good. On a Sun-4/110: (times are for compression only) c.pgm 266014 bytes 256x256 grayscale, 120/256 grayshades, ASCII format c.gif 46180 bytes (ppmtogif - 6.5 seconds real time) c.jpeg 7249 bytes (ppmtojpeg - 9 seconds real time) c.c2p 21859 bytes (c2p/std filters/20% threshhold ~6 minutes real time) c.c2a 1429977 bytes (c2a/std filters/20% threshhold ~12 minutes realtime - decompression failed with a segment fault) c2p - Yale compress with 2-dimensional periodic wavelet packets c2a - Yale compress with 2-dimensional aperiodic wavelet packets The times associated with these compressors are approximate because they require interactive input (unless you know how to build their parameter files - I don't.) The decompressed c2p image is recognizeable, but looks smeared, as if seen through a camera lens covered with water droplets. We can safely assume that the output of c2a is incorrect for some reason (just as well!) given the core dump. This is probably my fault - perhaps selecting a margin-width of zero is bad. The decompressed JPEG image is visually indistinguishable from the input image. Decompression took 2.7 seconds. The fact that this was a grayscale image aids JPEG, which tends to retain image structure while distorting color values. None of this should be taken as condemnation of either wavelet compression in general or the Yale package in particular. The Yale package has quite a few adjustable parameters, none of which are clearly documented in the HELP file, which is the only documentation I have for it. It is possible that I managed to pick really bad parameter values (likely even.) I would be delighted to repeat this experiment with a more intelligent choice of filter, filter range, threshhold, et al, if anyone has any recommendations. The Yale wavelet package is available for anonymous ftp from yale.edu. I believe that the directory is /wavelet/binaries; you may have to hunt around. It contains the binaries for Sun 3/4 and a couple of other architectures. Source code may or may not be available if you sign a nondisclosure agreement with Yale - see the README file in /wavelet. The PD JPEG implementation is still under development and will probably be ready for release sometime this summer. Contact jpeg-request@think.com if you are interested in being added to the PD JPEG mailing list. -- Dave Mack