Path: utzoo!attcan!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: <22132@ut-emx.UUCP>
Date: 8 Dec 89 21:06:35 GMT
References: <49396@bbn.COM> <22099@ut-emx.UUCP>
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: 70

In article <22099@ut-emx.UUCP> I write:
>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:
>   [ awesome but non-functional algorithm deleted ]
I committed a gross stupidity.  Forgive me, It was late at night and I 
probably cost USENET hundreds if not thousands of dollars.  The correct
algorithm follows: (if you're interested)

row:[2][width+1][3] integer
output:[width] char
red:constant integer=0
green:contant integer=1
blue:constant integer=2
temp:integer
colmap:[16][3] char=((0,0,0),(0,0,255),(0,85,0),(0,85,255),/
                     (0,170,0),(0,170,255),(0,255,0),(0,255,255),/
		     (255,0,0),(255,0,255),(255,85,0),(255,85,255),/
                     (255,170,0),(255,170,255),(255,255,0),(255,255,255))
x:range integer=(0,width-1,1)
tempp:^[]
fixem:[3][2] integer=((128,32),(192,64),(128,128))
read first row from file into row[0][0..width-1][R..B] /* 0..255 Intensity */
while not eof(), do {
	read next row from file into row[1][0..width-1][R..B]
	instance x, do {
		color:range integer=(r,b,1)
		instance color, do {
			temp=row[0][x][color]
			output[x]=(temp .and fixem[color][0])/fixem[color][1]
			temp=temp-colmap[output[x]][color]
			row[0][x+1][color] += (temp+1)/2
        	        row[1][x][color] += temp/2
		}
	}
	write output[0..length] to your output file
	tempp=&row[0]
	&row[0]=&row[1]
	&row[1]=&row[0]
}
/* this is in DRIZZLE, a language I am developing.  It should be fairly self-
explanatory and easy to read.  I hope so.  I just noticed that I forgot to put
statement separators.  Damn.  I welcome any comments on the style of the
language.  I developed this algorithm myself, but I think it's already been 
done by someone else.  Notes: .<boolean op> makes it bitwise, otherwise it's
logical.  range is a type (min,max,delta).  instancing a range evaluates
the do succesively with the variable evaluating as the range supertype.  e.g.
x:range(1,10,1); instance x, do write(_<x,2>_); would write  1 2 3 4 5 6 7 8
 910.  I hope it makes sense to you.  I like it.*/

/* p.s. The color mapping table is now defined in the program.  Values in each
triplet are Red, Green, and Blue succesively, ranging from 0..255.  This
program attempts to keep the average color intensities correct by propogating
the differences between displayed and actual value down and to the right.
One possible Change is to change the percentage of propogation right, down,
and down&right depending on the original color.  This will give you different
patterns for colors with the same brightness.  I gave green two bits since
it is about twice as bright as red, the delta(brightness) should be fairly
consistent. */

<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>
<*>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. <*>*<*>*<*>*<*>*<*>
<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>*<*>