Compressing pictures (LONG)

[Kent Paul Dolan]

I don't at all see the advantage of differences over XOR.  The color values
exist in three space, and a one bit shift in the color that happens to map
to the most significant bits of your pixel when considered for differencing
is going to map far away in difference space from the origin.  XOR has the
"advantage" that it treats all bits as of equal significance, probably a
"better" thing to do for minimizing the impact of noisy image data on the
compression efficiency.

Is there something (quite likely) that I'm not seeing here?  The previously
menitoned color table ordering problem (again, trying to linearize three
space data) only further complicates the problem of making differencing work
well.

(It may not do a lot for the XOR solution either, on first guess.)

(A solution, perhaps not the best, to the color table problem is to make
 keys from the eight bit per gun true color data that is the color table's
 output value (say, for a high quality system), then interleave the color
 bits like:

         r0g0b0...r7g7b7

 with r0 the red m.s.b., etc., and then sort these keys and use that as
 the new order for the color table.  This way the subtle changes are down
 in the r7g7b7 bits, and sort close together, while the blatant changes
 are in the r0g0b0 bits, and sort far apart.  This will help a lot, though
 the choice of red for most significant bit is still arbitrary.)

Kent, the man from xanth.
<xanthian at Zorch.SF-Bay.ORG> <xanthian at well.sf.ca.us>