Date: Fri, 17 Oct 1997 12:45:11 -0400 (EDT)
Message-Id: <199710171645.MAA26299@delorie.com>
To: DJGPP mailing list <djgpp AT delorie DOT com>
From: Alexander Bokovoy <bokovoy AT bspu DOT ac DOT by>
Subject: Re: ALLEGRO - 256 shades of grey?? How?
In-Reply-To: <6269en$a32$2@news.ox.ac.uk>
References: <EI61FB DOT 4Bn AT cix DOT compulink DOT co DOT uk> <6269en$a32$2 AT news DOT ox DOT ac DOT uk>
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On 16 Oct 1997 23:53:59 GMT mert0407 AT sable DOT ox DOT ac DOT uk (George Foot) wrote:

> 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.

 Neither GRX nor Allegro is lying. 
 Because standard VGA DAC specification is limited by 6 bits, 
 VESA standard defines extended DAC register capacity. Peter use
 800x600x256 mode which is not VGA mode but SVGA and exists only on
 SuperVGA compliant videocards. Because old VGA cards are rare and we
 all use SVGA-compatible cards this strange behavior of Allegro may be
 explained by different implementation schemes in GRX and Allegro
 palette manipulation routines (correct me if I wrong).

Alexander Bokovoy, <bokovoy AT bspu DOT ac DOT by>
---== The Soft Age coming soon ==---