Path: utzoo!attcan!uunet!husc6!bloom-beacon!apple!claris!ames!oliveb!sun!vector!poynton
From: poynton%vector@Sun.COM (Charles Poynton)
Newsgroups: comp.graphics
Subject: Luminance from RGB (Was "intensity" from RGB)
Summary: ((R+G+B)/3.0) is for insects.  Humans use .299*R+.587*G+.114*B.
Message-ID: <76353@sun.uucp>
Date: 6 Nov 88 10:17:54 GMT
Sender: news@sun.uucp
Lines: 59

In Comp.windows.x article <8811011523.AA02242@LYRE.MIT.EDU>, Ralph R.
Swick <swick@ATHENA.MIT.EDU> comments:

> When converting RGB values to monochrome, the sample server(s) compute
> an intensity value as (.39R + .5G + .11B) ...

(.39R + .5G + .11B) is apparently incorrect.  This set could be a
typographical error (from .29R + .6G + .11B ?), a liberal approximation,
or perhaps an unusual phosphor set.  Could someone enlighten me on this?

In followup article <8811042303.AA21505@dawn.steinmetz.GE.COM>, Dick
St.Peters <stpeters@dawn.UUCP> makes the statement:

> I'd like to suggest that (.39R + .5G + .11B) is not a good choice for
> "intensity" in the realm of computer graphics.  ...
>
> A better choice in computer graphics is to equally weight the colors:
> ((R+G+B)/3.0).  Let white be white.

Equal weighting of the primaries is NOT the right thing to do, unless the
viewers of your images are members of some species that has uniform
response across the visible spectrum, unlike homo sapiens.

Humans see 1 watt of green light energy as being somewhat brighter than 1
watt of red, and very much brighter than 1 watt of blue.  The science of
colourimetry began to flourish in 1931, when the CIE standardized a
statistical entity called the "standard observer".  This includes a
standard spectral luminance response defined numerically as a function of
wavelength.  It is from this data that the factors which are used in
colour television are derived:  .587 for green, .299 for red, and 
.114 for blue.

The particular factors depend on the wavelengths or chromaticities
that you call red, green, and blue:  there is wide disparity in these
choices.  For computer graphics and television, the luminance factors
depend on the chromaticity coordinates of the phosphors of your CRT.
There are compromises in the choice of phosphor primaries, but it turns
out that the NTSC did a spectacularly good job of selecting primaries.
The luminance coefficients 0.299 for red, 0.114 for blue, and 0.587 for
green are unquestionably the best values to use, unless you know your
phoshphors intimately.

The second article continues, 
> 						The formula is from
> the (1954) NTSC standard for compatible color TV, and it has built
> into it a lot of compromises to accommodate old technology and
> problems inherent in the analog transmission of composite color
> television.

Contrary to this assertion, the ONLY compromise in NTSC which impacts the
luminance equation is the choice of reference phosphor chromaticities, and
a choice of phosphors MUST be made for any system which transmits colour
in RGB.  Just because it's old (1954) doesn't mean we should throw it
away.


Charles Poynton						"No quote.
poynton@sun.com						 No disclaimer."
(415)336-7846