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From: gary@uoregon.UUCP
Newsgroups: comp.graphics
Subject: Re: Simple RGB to Gray scale question
Message-ID: <454@uoregon.UUCP>
Date: Fri, 27-Feb-87 16:02:07 EST
Article-I.D.: uoregon.454
Posted: Fri Feb 27 16:02:07 1987
Date-Received: Sun, 1-Mar-87 15:43:36 EST
References: <718@hp-sdd.HP.COM>
Reply-To: gary@fog.UUCP (Gary Meyer)
Organization: University of Oregon, Computer Science, Eugene OR
Lines: 28

In article <718@hp-sdd.HP.COM> nick@hp-sdd.HP.COM (Nick Flor) writes:

>
>How does one convert a pixel defined by an R, G, B value into
>a single Gray level?
>

What you want to compute here is the luminance of the light that is
being emitted from your color television monitor.  This will depend on
the chromaticities of the monitor phosphors, the chromaticities of the
white point that the monitor has been balanced to, and the non-linear
relationship between voltage applied to the CRT guns and light emitted
from the display (gamma correction).  Using the colorimetric properties
of the NTSC color television system with an assumed gamma of 2.2, this
works out to

     Y = 0.30 ( R ** 2.2 ) + 0.59 ( G ** 2.2 ) + 0.11 ( B ** 2.2 )

where R, G, and B correspond to the values stored in image memory and
are assumed to lie on the range 0.0 to 1.0.  If you are content to map
this back onto the neutral axis of the monitor gamut, then the values
to be reloaded into image memory are

     R = G = B = Y ** ( 1 / 2.2 )

It is easy to work this out for the colorimetric properties of your
own system, but the above expressions are fine for most practical
applications.