Date   : Mon, 01 Oct 2012 18:20:37 +0100
From   : jgh@... (J.G.Harston)
Subject: Manual GIF creation

Jules Richardson wrote:
> J.G.Harston wrote:
>> Can anybody help. What is wrong with China.gif that makes it not a 
>> GIF?
...
> The LZW stream isn't right, either; you seem to have 128 lines of 
> image
> data in there (1743 data blocks of 47 bytes each, less one byte for 
> the
> stop code), rather than 512.

That's probably me missing an extra loop to double the vertical pixels
to get the aspect ratio correct. The first version of BBCtoBMP 
generated
some very nice letterboxes ;)

> staggered, but that's due to the order in which GIF decoders spit out 
> line
> data and the way that 640 doesn't divide nicely by 47 :-)

The GIF documentation is /very/ shonkey about code size bytes and data
blocks, it just talks about "emitting bitwise data stream". I've been
working backwards from the quilt demo and translating the documentation
into what the documentation /should/ say.

I've just got home from getting very soggy delivery telephone books, so
a cup of tea and a kip, and then I'll attack it again.

-- 
J.G.Harston - jgh@...      - mdfs.net