Message-ID: <19971017045203.14573.qmail@hotmail.com>
From: "John Patton" <gimli21 AT hotmail DOT com>
To: djgpp AT delorie DOT com
Cc: bporter AT rabble DOT uow DOT edu DOT au
Subject: Re: ALLEGRO - 256 shades of grey?? How?
Content-Type: text/plain
Date: Thu, 16 Oct 1997 21:52:02 PDT
Precedence: bulk



>Subject: Re: ALLEGRO - 256 shades of grey?? How?
>To: pscargill AT cix DOT compulink DOT co DOT uk (Peter Scargill)
>Date: Fri, 17 Oct 1997 13:47:28 +1000 (EST)
>Cc: djgpp AT delorie DOT com (DJGPP)
>From: Brett Porter <bporter AT rabble DOT uow DOT edu DOT au>
>
>> 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!!!
>> 
>In a 256-colour mode it is only possible to have 64 shades of grey, 
because
>you can go from 0,0,0 to 63,63,63. I don't know how GRX does it, maybe 
they
>just repeat them 4 times and claim that they are gray?
>
>Why do you want gray anyway, and isn't 64 shades enough?
>
>Brett
>
>-- 
>"Who here believes in telekenesis? Raise MY hand!"
>--
>Brett Porter
>bporter AT rabble DOT uow DOT edu DOT au
>

Can't you create a palette containing 256 shades of grey and set the 
palette with allegro's set pallete function?  I haven't tried, but I 
believe this should work.  
i.e., a palette consisting of values like:
0,0,0
1,1,1
2,2,2
...
255,255,255
and then set it.  I believe the 6 bits per color is only applicable when 
converting a palette value to it's equivalent DAC register value.  I may 
not have the details right, but I believe the DAC is limited to a 6-bit 
digital to analog conversion per color(2^6 = 64).

Any corrections are welcome.

Good luck,

John

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