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Posting-Version: version B 2.10.2 9/18/84; site watmath.UUCP
Path: utzoo!watmath!jsgray
From: jsgray@watmath.UUCP (Jan Gray)
Newsgroups: net.graphics
Subject: Re: HELP!! SUN specific graphics question.
Message-ID: <11195@watmath.UUCP>
Date: Tue, 29-Jan-85 23:52:53 EST
Article-I.D.: watmath.11195
Posted: Tue Jan 29 23:52:53 1985
Date-Received: Wed, 30-Jan-85 06:11:14 EST
References: <225@eneevax.UUCP>
Reply-To: jsgray@watmath.UUCP (Jan Gray)
Distribution: net
Organization: U of Waterloo, Ontario
Lines: 53
Keywords: colour map

> From eneevax!phaedrus 24 Jan 85 19:15:43 GMT
> ..........  And if that isn't possible could someone explain to me how
> the following routine (which sets up a colormap) works?  I don't quite
> understand what all the shifting and the oring does.
> 
> int mak_col_map ()
> {
>     int     index;
>     unsigned char   red[256],
>                     blue[256],
>                     green[256];
>     int     r,
>             b,
>             g;
> 
>     for (b = 0; b < 4; b++) {
> 	for (g = 0; g < 8; g++) {
> 	    for (r = 0; r < 8; r++) {
> 		index = (b << 6) | (g << 3) | (r);
> 		blue[index] = b * 84;
> 		red[index] = r * 36;
> 		green[index] = g * 36;
> 	    }
> 	}
>     }
>     pr_putcolormap (col_mon, 0, 256, red, green, blue);
> }

It looks like the program is building separate red, green and blue look
up tables, each of which has 256 entries and is 8 bits deep.  I expect that
you have an 8 bit frame buffer, and you set up the internal data paths
so that those eight bits index a red, green and blue intensity.

The program you include sets up the colour map so that a colour
contains two bits of blue, three bits of red, and three bits of green
intensity.  You therefore can select colours with three levels of blue
(plus black) and seven levels of red and green (plus black).

(84 == 255 / 3, and 36 == 255 / 7)

Colour 0   is 00 000 000	== no blue, no green, no red
Colour 1   is 00 000 001	== no blue, no green, 1/7 red	(36/255)
Colour 2   is 00 000 010	== no blue, no green, 2/7 red	(72/255)
...
Colour 255 is 11 111 111	== full blue, full green, full red


I expect this is a standard trick for 8 bit deep frame buffers with
separate rgb lookup tables.

I expect too much,

Jan Gray (jsgray@watmath.UUCP)   University of Waterloo   (519) 885-1211 x3870