Path: utzoo!attcan!uunet!cs.utexas.edu!wuarchive!mit-eddie!media-lab!atrp.mit.edu!ralph
From: ralph@atrp.mit.edu (Ralph L. Vinciguerra)
Newsgroups: comp.graphics
Subject: Re: CIE (1931) XYZ vs xyY
Summary: Slightly different coordinate systems.
Keywords: CIE color Macbeth
Message-ID: <3568@media-lab.MEDIA.MIT.EDU>
Date: 4 Oct 90 16:25:51 GMT
References: <1990Oct4.025925.22471@qualcomm.com>
Sender: news@media-lab.MEDIA.MIT.EDU
Reply-To: ralph@atrp.mit.edu (Ralph L. Vinciguerra)
Organization: MIT Amiga Users Group c/o MIT Advanced Television Research Proj.
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xyY and XYZ are two slightly different color coodinate systems.
Fortunately, there's a simple transformation. I'll show it now, but
you ought to check a good book on color like Wyszecki&Stiles "Colour Science"
(John Wiley and Sons, 1967).

Y = Y;  x = X/(X+Y+Z); y = Y/(X+Y+Z)

or backwards:

X = (x/y)*Y;  Y=Y;  Z=((1-x-y)/y)*Y


As for interpretation, XYZ values represent the amounts of three
specially defined hypothetical primaries (not physically realizable).
The Y primary was specifically defined to be roughly the human monochromatic
response, so a color image given in XYZ, can be shown in Black and White
by just mapping Y to intensity.

xyY is a kind of normalized form, where Y is that same monochromatic
intensity and x and y are chormatic coordinates which don't include
the intensity. This is useful when defining a color without knowing
the intensity as just (x,y).

The book I mentioned gives all the background you'll need to understand
color science. There are many other wild and wacky color spaces out there.
Some are non-linear and are much better at modeling human color vision.

I hope I helped....