Path: utzoo!utgpu!cunews!bnrgate!bigsur!bnr-rsc!bcarh188!lalibert
From: lalibert@bcarh188.bnr.ca (Luc Laliberte)
Newsgroups: comp.arch
Subject: Re: Resolution, etc.
Summary: 24bit color in 6bits with dithering
Keywords: colors, resolution, dithering
Message-ID: <3728@bnr-rsc.UUCP>
Date: 21 Nov 90 15:06:06 GMT
References: <240@csinc.UUCP> <1990Nov15.052925.1265@imax.com> <2928@crdos1.crd.ge.COM> <1990Nov16.190248.20437@ux1.cso.uiuc.edu> <4023@osc.COM>
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Reply-To: lalibert@bcarh188.UUCP ()
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Organization: BNR Ottawa, Canada
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Back before I upgraded from EGA to VGA I wrote a GIF viewer in C to
display large (up to 800x600) GIFs in dithered form.  (Other programs
had restricted themselves to 320 pixels wide.)  I got exceptionally
(near VGA) quality with a two pass algorithm.
The process goes like this: reduce 8 bits of color to 13 2x2 bit pattern
blocks.  This reduces the number of colors from 16million to 64 (2 bits
per color).  Count the number of times each color is used, and then
perform a priority best fit to reduce that to 16 colors.  Then redecode
the GIF, substituting the best fit 16 colors for the 64 possible.
The program FastGIF does a similar operation, CSHOW uses a single pass
method that is vastly inferior.

What does this mean?  It is possible to represent 16million colors
adequetly with only 16 colors (out of a possible 64).  Therefore, it is
conceivable to display 24bit color only using 6bit color and dithering.
(The best fit portion is removed and it can be done in a single pass.)
However, your resolution drops by a factor of 2 in both directions.
Consider that the best resolution of common displays is 1024x768, this
yeilds a dithered display of 640x384, far below common 800x600 8bit from
18bit displays.

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