Message-ID:	<19971017150215.21713.qmail@hotmail.com>
From:	"John Patton" <gimli21 AT hotmail DOT com>
To:	djgpp AT delorie DOT com
Subject:	Re: ALLEGRO - 256 shades of grey?? How?
Date:	Fri, 17 Oct 1997 08:02:13 PDT

>From: mert0407 AT sable DOT ox DOT ac DOT uk (George Foot)
>Subject: Re: ALLEGRO - 256 shades of grey?? How?
>Date: 16 Oct 1997 23:53:59 GMT
>To: djgpp AT delorie DOT com
>
>On Thu, 16 Oct 1997 22:35:35 GMT in comp.os.msdos.djgpp Peter Scargill
>(pscargill AT cix DOT compulink DOT co DOT uk) wrote: 
>
>: In 800*600*256 mode, GRX20 lets you set up 256 shades of gray. 
ALLEGRO in 
>: the same mode only lets you set up 64 (then you get them repeated 3 
times 
>: as the routine uses 6 bits per colour).
>
>: How do I get the full 256 shades of grey. HELP!!!
>
>I expect GRX is lying; the 6 bit limit is imposed by the VGA hardware, 
not by
>Allegro. What GRX does is probably to map strengths 0-3 to strength 0,
>strengths 4-7 to strength 1, ...., strngths 252-255 to strength 63. 
Simply
>put, dividing the [0,255] strengths by four to give [0,63]. For 
example:
>
>PALETTE grey_pal;
>int i;
>
>for (i = 0; i < 256; i++)
> grey_pal[i].r = grey_pal[i].g = grey_pal[i].b = i/4;
>
>If you really need 256 distinct colours, you'll have to sacrifice some 
hue
>accuracy and use slightly off-grey colours. If you don't mind this, try
>something like:
>
>for (i = 0; i < 256; i++) {
> grey_pal[i].r = grey_pal[i].g = grey_pal[i].b = i/4;
> if (i&1) grey_pal[i].r++;
> if (i&2) grey_pal[i].g++;
>}
>
>The hue inaccuracy will be most noticeable on dark colours.
>
>-- 
>george DOT foot AT merton DOT ox DOT ac DOT uk
>


So, what you're saying is, I'm limited to 64 shades of gray by the way 
Allegro handles the color hardware?  I thought it was possible to have 
256 different colors by updating the color DACs with the appropriate 
color values?  I also thought you could choose between a total of 
262,000+ (IOW 2^18) different colors in 256-color mode, you were only 
able to use 256 distinct colors because the 8-bit value is an offset 
into the VGA palette array, of which I thought held a value 
corresponding to the desired color.  I may be wrong, however, I know 
I've loaded a 256-color palatte of the same color (i.e. red or 
something...) and it appeared to have all of those colors -- not with 
allegro, but when I was doing asm in BC++ a while ago.  Shawn, any 
answers on this with your library?  I'll experiment with this, I 
guess...

One last thought:  Does this mean Allegro loads a palette into the DACs 
that connot be changed?


-John

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