Grading Compression Algorithms II
Degradation : Many of the approximation algorithms introduce some degradation in the data in the compression process. The amount of this degradation is often controllable, and the better algorithms offer a tradeoff between degradation and ˇ lesize. Smaller ˇ lesmean more degradation and vice versa.
In many cases, the degradation can also be characterized and maybe even
measured. The JPEG image compression algorithm wlll tend to replace
areas of similar colors with large blocks.
Patent Status : Some compression algorithms are patented. Some of these patents
have some legal strength. Others are weakened by challenges to their applicability. The legal status of many of these algorithms has tripped up others. Microsoft, for instance, lost a large lawsuit ˇ ledby a small company that contended that Microsoft infringed on its patents. The best solution formany developers is to license a library of compression routines, because the authors of the compression routines have already done much of the patent evaluation. They should be paying the patent holders for you.
have some legal strength. Others are weakened by challenges to their applicability. The legal status of many of these algorithms has tripped up others. Microsoft, for instance, lost a large lawsuit ˇ ledby a small company that contended that Microsoft infringed on its patents. The best solution formany developers is to license a library of compression routines, because the authors of the compression routines have already done much of the patent evaluation. They should be paying the patent holders for you.
Compression Speed : There are wide differences in this value. Some programs spend a fairly large amount of time creating complicated statistical proˇ les of the data. Others make a quick pass over the data. If the algorithm is going to remain competitive, then the extra compression time must result in smaller ˇ les. In many cases, developers don't care about compression speed, but they do care about decompression speed. Movies, photos, and audio ˇ les are usually compressed once by the artist and then decompressed many times by the different viewers. Developers don't care about spending a few extra hours or even days, if the result is substantially smaller.
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