<< ctxmodel.net

2008-09-17 11:20:41 chornobyl       >
Now that what i call REAL compression
not some computer synthetic stuff :)

2009-07-13 22:32:58 Yuri+G.         > This is a CatPressor >)

2009-12-30 17:37:43 Chris           >
Hmm, is that a good book ?
Can anyone suggest good one for ppm,cm compressors
any learning resources ?
thx

2010-01-04 09:41:12 Shelwien        >
Its not bad as an overview, but there're only like 30 pages
about PPM, and its PPM from 20 years ago.
And well, there's not that much written about modern algorithms
anywhere, and its not quite easy to explain them in words anyway.
There're sources though, and some articles.

2010-01-12 14:52:19 Chris           >
Thanks Shelvien
Could you point me to some sources or articles
that can help me get started.
I have no idea where to begin context mixing.
my knowledge of ppm is very new.

thanks and here is my mail
seenu4all @ gmail DOT com

2010-01-12 17:14:40 Shelwien        >
Unfortunately that email doesn't seem to work,
don't know what I'm doing wrong.
My email is shelwien@gmail.com , if you need it

>> Could you point me to some sources or articles
>> that can help me get started.

We have a forum on data compression,
and even an IRC channel: http://encode.dreamhosters.com/

>> I have no idea where to begin context mixing.
>> my knowledge of ppm is very new.

1) It might be a good idea to start with PPMd:

http://ctxmodel.net/files/PPMd/

PPMd is featured as "text compression method" is most
popular archivers now, and there's my rewrite of PPMd source
which is hopefully easier to read, and also Shkarin's
article describing the algorithm.

The ppmonstr, which is ppmd with some additional logic,
actually still keeps a world record on large text compression
(see http://cs.fit.edu/~mmahoney/compression/text.html)
"durilca" there = ppmonstr with text filters

2) As to specifically CM, you can consider Mahoney's stuff:
http://mattmahoney.net/dc/
(there're good algorithm descriptions in the sources)

or toffer's m1:
http://ctxmodel.net/files/m1/

or some of my coders:

BWT+CM:
http://ctxmodel.net/files/mix_test/BWTmix_v1.rar

direct CM with logistic mixing:
http://ctxmodel.net/files/mix_test/mix_test_vC.rar

hashed CM with linear mixing:
http://ctxmodel.net/files/MIX/mix_v3.rar

old bytewise CM with unary coding and secondary estimation
http://ctxmodel.net/files/ASH/ash04.rar

even older and very simple bytewise CM:
http://compression.ru/sh/ppmy_3c.rar

or a new prefix tree template with some trivial model plugged in:
http://ctxmodel.net/files/tree_v1.rar

3) As to good texts on modern CM, I'm not aware of any, except for
Mahoney's algorithm descriptions, and various posts on the forum.

2010-01-13 05:19:49 Chris           >
Thanks Shelwien , this will help me a lot to start understanding
PPM and CM.
Strangely my gmail id dosnt seem to work, i have added your gmail on my alternate pmcontext AT gmail DOT com

Thanks again :D

2010-01-13 05:33:45 Chris           >
I have experience with LZ77, huffman , arith, symbol ranking(matt) , BWT (i dont like bwt that much).

2010-01-13 23:55:39 Shelwien        >
BWT is useful for testing of CM components (counters, mixers, etc)
It allows to produce reasonable compression without dynamic allocation and complex data structures.
As to "arith"... what do you think about my rangecoding article
here?

2010-01-14 15:29:18 pmcontext       >
BWT compression is good without complicated coding
but my dislike for it is due to the sorting i guess.
the simplicity of schindlers range coder is very
attractive , I duno much about combinatorics but the binary
coder implemented in fpaq0 is clear for me to understand.

2010-01-14 18:42:56 Shelwien        >
Usual BWT compression (like bzip) is only good comparing to LZ,
so advanced CM postcoders still improve it a lot.
As to rangecoders, Matt's coder is really simple, but it could
become really troublesome if you'd try to do something unusual
with it, including speed optimizations.
For example, its not compatible with precise probabilities
(>=16 bits) and bytewise coding and 16-bit i/o, and its also
very hard to unroll its renormalization loop - it would need
5 iterations, comparing to 1-2 in my implementation.

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2015-01-10 17:10:58                 >