Path: utzoo!utgpu!watserv1!watcgl!imax!dave
From: dave@imax.com (Dave Martindale)
Newsgroups: comp.graphics
Subject: Re: RGB color mixing/averaging
Keywords: RGB, super-sampling, transparency
Message-ID: <1990Mar18.143554.2918@imax.com>
Date: 18 Mar 90 14:35:54 GMT
References: <173@yak.COM> <2201@osc.COM>
Reply-To: dave@imax.com (Dave Martindale)
Organization: Imax Systems Corporation, Oakville Ontario
Lines: 34

In article <2201@osc.COM> jgk@osc.COM (Joe Keane) writes:
>
>Also, despite what some
>people might say, mixing red and green light gives you only passable yellow
>light, which is why you can't get a really good yellow on an RGB monitor.

Depends on the red and green light you start with.  The eye has only
three colour receptors.  Spectrally pure yellow light produces responses
in these three receptors in a certain ratio; any other combination of
colours that produces the same ratio will also be perceived as yellow.

The CIE chromaticity diagram is a way of displaying perceived colour.
It also shows you what colours you can obtain by mixing 2 or 3 other
colours.  Spectrally pure colours are on the curved line around
the edge of the chart.  Now, the area between red and green that
contains yellow is almost a straight line.  This indicates that, given
almost any spectrally pure red, and a spectrally pure green that is
somewhat on the yellowish side, you can find a ratio to mix them that
gives extremely good yellow.

So, you can get good yellow from red and green light - but the light
would have to be generated by lasers, or by separating white light
with a prism or grating.  The red and green phosphors used in CRTs
are nowhere near spectrally pure, and in fact were chosen more for
efficiency (brightness) rather than colour.  That's why you can't get
a good yellow, and the best green is still yellowish, and there are
no pure reds.

On the other hand, there are some colours that you can't get by
mixing lights - pure cyan (blue-green) for example.  That portion
of the CIE diagram is continuously curved, so any pure cyan-like colour
lies outside a straight line joining anything you could call pure green
to any pure blue, thus you can't obtain pure cyan even by mixing
single-frequency green and blue.