Path: utzoo!utgpu!water!watmath!watdcsu!mgardi
From: mgardi@watdcsu.waterloo.edu (M.Gardi - ICR)
Newsgroups: comp.binaries.ibm.pc
Subject: Re: GIF pictures
Message-ID: <4577@watdcsu.waterloo.edu>
Date: 29 Mar 88 00:14:23 GMT
References: <2851@omepd> <6870002@hpcvca.HP.COM>
Reply-To: mgardi@watdcsu.waterloo.edu (M.Gardi - ICR)
Organization: U. of Waterloo, Ontario
Lines: 23

In article <6870002@hpcvca.HP.COM> scott@hpcvca.HP.COM (Scott Linn) writes:
>Regarding these GIF pictures...
>
>Does anyone have a "good" algorithm (or at least a start on one) that will
>convert a 256-color palette down to two (black/white)?  I know that this
>has been done (for the Mac GIF reader), but I have been unable to find any
>sources.
>
>Thanks,
>
>Scott Linn
>HP - Northwest IC Division

I have written a GIF decoder for the monochrome ST which basically calculates
a 'darkness' factor for each of the colours in the palette (R + G + B) / 3
Each time I want to plot a pixel I throw a 256-sided die.  If the number
is greater than the 'darkness', I plot a white pixel, if it is less, I
plot a dark pixel.  The resulting image is generally pretty good, though
it is no substitute for colour.  I will probably post my GIF decoder
sometime in the future (as soon as I make it fast).

David Rowley
...watmath!watdcsu!mgardi