Compressing alt.sex.pictures files

[Achille Hui]

peter at sugar.hackercorp.com (Peter da Silva) writes:

>In article <1990Aug28.192024.22435 at zorch.SF-Bay.ORG> xanthian at zorch.SF-Bay.ORG (Kent Paul Dolan) writes:
>> Hmmm.  Good idea.  Here's one suggestion for an experiment.  Somewhere on
>> the net, a month or two back, there was a suggestion that a good way to
>> compress realistic images was to XOR scan lines after the first with their
>> predecessor line, then LZW compress the output.
>I believe that XORing alternate scan-lines and run-length-encoding the result
>would do a better job. I've heard of this algorithm being pretty effective
>in past discussions. Should one do this with the image bitplane-mapped or
>pixel-mapped?
>-- 
>Peter da Silva.   `-_-'
><peter at sugar.hackercorp.com>.

I have tried several things on several images:

	lighthouse, oldmill, lush (1024x768x ~ 256 )    each ~ 600k
	babe24 ( ~ 512 x ~ 800 x ~ 256 )		     ~ 400k

avaliable from ticsys.tamu.edu
1) just compress the raw image extracted from the gif files
2) XOR neighbouring scan lines in raw image and then compress it.
3) subtract neighbouring ......
4) build a color index table for each scan line

and it turns out the size of the resulting files ..
1)	~ 90% of original gif
2,3)	~ 110%
4)	~ 100% if we exclude the color index table for the lines!

						-eillihca at fizzle.stanford.edu