Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!ut-emx!walt.cc.utexas.edu!aceman
From: aceman@walt.cc.utexas.edu (acy james stapp)
Newsgroups: comp.graphics
Subject: Re: Displaying 256 color GIFs on 16 color device
Message-ID: <22099@ut-emx.UUCP>
Date: 8 Dec 89 05:05:47 GMT
References: <49396@bbn.COM>
Sender: news@ut-emx.UUCP
Reply-To: aceman@walt.cc.utexas.edu (acy james stapp)
Organization: The University of Texas at Austin, Austin, Texas
Lines: 49

In article <49396@bbn.COM> sminkin@BBN.COM () writes:
>I've recently written a program to display GIF files on DEC VT340
>terminals using color sixel.  The 16 color GIF files come out beutifully,
>but I don't really have a scheme for converting 256 colors down to
>16 (which is the limitation of the VT340).  Is there a preferred 

Try this algorithm:
read row(0)
convert row(0) to array[length+1] of signed int; (or less, this is what I use)
until eof() do
    read row(1) from file
    convert row(1) to array of int
    for every element of row(0) do
        newrow(element)=newrow(element)+(red>>7)>>3
        row(0)(element+1)(red)=(red+1)>>1
        row(1)(element)(red)=red>>1
        newrow(element)=newrow(element)+(green>>6)<<1
        row(0)(element+1)(green)=(green+1)>>1
        row(1)(element)(green)=green>>1
        newrow(element)=newrow(element)+(blue>>7)
        row(0)(element+1)(blue)=(blue+1)>>1
        row(1)(element)(blue)=blue>>1
    enddo
    write row(0) to file
    row(0)=row(1) /* HaHaHa to all you C programmers! Try doing this with
                     one statement! */
enddo
>> means shift into nothingness, << means shift into overflowingness

use the following color map to display your final thingythingy
Val	R	G	B
0	0	0	0
1	0	0	255
2	0	85	0
3	0	85	255
4	0	170	0
5	0	170	255
6	0	255	0
7	0	255	255
8-15	255	as above-8

email me for an explanation if you're too stupid to figure it out yourself.

<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>
<*>Acy Stapp   email: aceman@walt.cc.utexas.edu   `NO SMOKING IN CORRIDORS' <*>
<*>"Learn to love your anger now, anger here is all you posess" Duran Duran <*>
<*>Disclaimer: The opinions presented above are true and correct, and if *>*<*>
<*>*<anyone disagrees with them, then that's their problem. <*>*<*>*<*>*<*>*<*>
<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>