Path: utzoo!attcan!uunet!cs.dal.ca!ug.cs.dal.ca!legrady
From: legrady@ug.cs.dal.ca (Tom Legrady)
Newsgroups: comp.graphics
Subject: Re: RGB color mixing/averaging
Keywords: RGB, super-sampling, transparency
Message-ID: <1990Mar15.145623.2096@ug.cs.dal.ca>
Date: 15 Mar 90 14:56:23 GMT
References: <173@yak.COM>
Reply-To: legrady@ug.cs.dal.ca.UUCP (Tom Legrady)
Organization: Math, Stats & CS, Dalhousie University, Halifax, NS, Canada
Lines: 31

In article <173@yak.COM> tpd6908@yak.COM (Tom Dickens) writes:
>I am looking for a simple routine that will take 2 rgb values, a ratio
>for mixing them (ie. 0.5), and will return the resulting rgb value.
>This is desired for determining a pixel color after super-sampling, or when
>combining colors while working with transparent surfaces.
>
>The problem I have is I expect the resulting rgb value to follow 
>'real world' results.  (ie. case1: red+blue=purple, case2: blue+yellow=green)

The problem is not in the algorithmn, but in you selection of
'primary' colours.  Blue paint mixed with yellow paint looks green.
That's subtractive mixing.  You started with white light and reduced
level of non-yellow colours.  Then from that you reduced the non-blue 
colours. Since green is between yellow and green, it passed through
both filters.

When you mix RGB values, as when you point several stage lights at the
same location, you get additive mixing.  You start with Black, add some
Blue, then add some yellow (which is a mixture of red and green) ...
Voila ... _WHITE_!!!  It took me a while to pound the additive rules into
my head:

PRIMARIES			Red		Green		Blue

Red		+	Green	=	Yellow
Green	+	Blue	=	Cyan	(pale blue)
Red		+	Green	=	Magenta	(deep purple)

The sequence of combinations brightes to darkest is

White	Yellow	Cyan	Green	Magenta	Red	Blue	Black.