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From: dec...@lt1.cs.rhbnc.ac.uk (Declan White)
Subject: [SAM] Need a better screen squisher.
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Date: Fri, 28 Oct 94 20:06:15 GMT
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The screen compressor in the utils.td0 (or wotever) in the ftp site
has been corrupting sum of my screens, and anyway, I need a squisher
with an independant decompressor, not builtin.

I've heard that Lord Insanity has a compressor like that, and maybe sum
others (including FRED).

Can any1 point me in the right direction, or better still, send me one?

Also.. my copy of EXODUS has gone gaga. Can I get it fixed/replaced?
If not, wots the address of the guy who sells it, so I can buy a new one?

Thanx to any an all...

--
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From: D A Roberts <D.A.Robe...@durham.ac.uk>
Newsgroups: comp.sys.sinclair
Subject: Re: [SAM] Need a better screen squisher.
Date: 8 Nov 1994 16:03:41 GMT
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  I have general purpose Z80 huffman compress/decompression code, however
it does need some optimisation and the dictionary is not as small as it
could be. This is for the spectrum, and supports compression over the 
existing file in memory. (uses the screen memory area as workspace and a 
buffer). This allows you to compress fairly largeish files without 
resorting to opening files to disc.
  Writing an RLE encoder is a fairly easy task, although the one in the
Artist 2 disc supplied by Format is absoultely dire (like it uses zero to
signal compression instead of changing the escape character, and can
easily be fooled with a little work into producing 12k 'compressed'
screens - just alternate 00 and FF in both directions - horizontally and
vertically). 
  Dictionary type compressors are a little bit more difficult to write,
but not much. I haven't got round to this yet, although I did start work
on one about 5 years for Tasword 2. One of the main issues here is that
many of the algorithms are actually patented, although I can't see them
coming after you for royalties on SAM. 
  I don't think lossy compression like JPEG does is that feasible on a 
Z80 based system, simply because of the time the FP calculations take to do.

--
  David Roberts    d.a.robe...@dur.ac.uk

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From: mr...@greenwich.ac.uk (Russell Marks)
Subject: Re: [SAM] Need a better screen squisher.
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Date: Fri, 11 Nov 1994 17:31:15 GMT
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[apologies if there was a blank message before - mailer screwup :-(]

>   I have general purpose Z80 huffman compress/decompression code, [...]

Correct me if I'm wrong, but huffman compression is patented. By two
separate patents, I believe. The decompression isn't. I always thought
this was quite funny (assuming it's true!) because huffman compression
isn't exactly mind-bendingly hard... although nothing's fun in
assembly, of course...

A GIF decoder and encoder shouldn't be hard to do in assembly, and it
would have 3 advantages: standard format, compresses quite well,
algorithms not patented.

>   I don't think lossy compression like JPEG does is that feasible on a 
> Z80 based system, simply because of the time the FP calculations take to do.

I understand the the Independant JPEG Group's jpeg lib v5 uses integer
versions of the fp calculations by default, and the results look the
same. Of course, 24-bit files are still impractical on a medium-size
Z80 box, and they're also probably using 32-bit integers.

-Rus.

-- 
/ : russell marks :: mr...@gre.ac.uk :: speak softly and carry a +6 kitten : \
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From: iia...@iifeak.swan.ac.uk (Alan Cox)
Subject: Re: [SAM] Need a better screen squisher.
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References: <D.A.Roberts@durham.ac.uk> <19012.9411111731@atlas> 
<3a14nh$k4q@lyra.csx.cam.ac.uk>
Date: Wed, 16 Nov 1994 13:34:13 GMT
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>Correct me if I'm wrong, but huffman compression is patented. By two
>separate patents, I believe. The decompression isn't. I always thought

Patents dont apply to software except in the USA and a couple of other
countries where corporations are more powerful than government and the
people.

To my knowledge the only common patented one is LZW (in the USA). Since
the zip/gzip algorithm is free, unpatented and trashes it for performance
its a non-issue anyway.

Alan

-- 
  ..-----------,,----------------------------,,----------------------------,,
 // Alan Cox  //  iia...@www.linux.org.uk   //  GW4PTS@GB7SWN.#45.GBR.EU  //
 ``----------'`----------------------------'`----------------------------''

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