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From: alan@ukpoit.co.uk (Alan Barclay)
Newsgroups: comp.graphics,comp.compression
Subject: Re: gif encoding questions
Message-ID: <1991Apr10.095103.26252@ukpoit.co.uk>
Date: 10 Apr 91 09:51:03 GMT
References: <1991Apr9.142255.186@unhd.unh.edu>
Organization: iT - The Information Technology Business Of The Post Office
Lines: 21

In article <1991Apr9.142255.186@unhd.unh.edu> rg@msel.unh.edu (Roger Gonzalez) writes:
>Why have I seen gray scale (256) GIFs that don't have simple color maps like
>00 00 00 01 01 01 02 02 02 03 03 03 ... FE FE FE FF FF FF
>
>Some of them are really strange...


Basically there are three bytes per colour in the colour maps, therefore
00 00 00 is the black, 01 01 01 is the first shade of grey, 02 ...
FF FF FF is white. If when the colour is used it looks up so 0 is 00 00 00,
1 is 01 01 01 etc. There is no reason why 125 cannot refer to 00 00 00 while
126 refers to 53 53 53. And some scanner software will allocate in this
fashion. In addition the difference between 53 53 53 and 52 53 54 is so
small that it will still look grey, so you might call it a grey scale when
it is really a colour GIF with different near grey colours, chosen for
asethetic reasons.
-- 
  Alan Barclay
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