Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!cs.utexas.edu!uwm.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!m.cs.uiuc.edu!brown
From: brown@m.cs.uiuc.edu
Newsgroups: comp.graphics
Subject: Re: VGA Colors
Message-ID: <4400050@m.cs.uiuc.edu>
Date: 9 Nov 89 17:40:24 GMT
References: <824@uwm.edu>
Lines: 30
Nf-ID: #R:uwm.edu:824:m.cs.uiuc.edu:4400050:000:1112
Nf-From: m.cs.uiuc.edu!brown    Nov  9 00:13:00 1989


Written  1:03 pm  Nov  8, 1989 by jrn@csd4.csd.uwm.edu in comp.graphics:
>I changed the color palette to
>all shades of grey.  What I got was 4 sections of colors where each section
>had the same colors as the all other sections.
>
>I did something like this
>      
>      for i:=0 to 255 do
>         SetRGBPalette(i,i,i,i);
>
>I would assume that I should have gotten 256 varying shades of grey.
>But I didn't.  I got 4 sections of 64 shades of grey,  were each section
>had the same 64 colors.

    The problem that you've run into is a limitation of the VGA hardware:
the VGA digital to analog converters are only six bit devices.  While you're
specifying the desired value as an eight bit quantity, only the top six bits
are used--the bottom two are ignored.  Because of this, there are only 64
distinct shades of red, 64 of blue, and 64 of green available (64*64*64 =
262144 or the total number of colors in the VGA palette).  

>How would I get 256 DIFFERENT shades of grey?

    With the VGA, I'm afraid that you can't.  


	William Brown
	brown@cs.uiuc.edu
	University of Illinois at Urbana-Champaign