Xref: utzoo comp.windows.x:35752 comp.graphics:17522
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!charyb!dan
From: dan@kfw.COM (Dan Mick)
Newsgroups: comp.windows.x,comp.graphics
Subject: Re: changing color intensity in X
Message-ID: <1991Apr25.214357.7963@kfw.COM>
Date: 25 Apr 91 21:43:57 GMT
References: <1991Apr20.204325.2078@eng.ufl.edu> <728@modulex.dk>
Reply-To: dan@kfw.com (Dan Mick)
Organization: KFW Corporation, Newbury Park, CA
Lines: 16

In article <728@modulex.dk> ssn@modulex.dk (Svend Skafte Nielsen) writes:
>
>   Intensity (also called lightness or luminance) is usually calculated
>   as
>
>         I = (R+G+B)/3
>
>   where R, G and B are the resp. red, green and blue components of the
>   color (e.g. from the XColor structure). 

I'd take exception to the "usually".  Most RGB-to-grayscale convertors I've
seen use a formula much more like the one in ppmtopgm, part of the PBMPLUS
package:

(From the manpage):
"The quantization formula used is .299 r + .587 g + .114 b."