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Subject: Re: Using colour to indicate response intensity



Hector,

you're right. The complete explanation is here:
http://www.fho-emden.de/~hoffmann/ciexyz29082000.pdf

Short explanation:

>From color matching to XYZ:
In the CIE color matching experiment, RGB values are
found for spectral colors. A matrix multiplication maps
RGB to XYZ. If we draw XYZ in cartesian coordinates
then we have three non-orthogonal base vectors for RGB.
Whatever XYZ means - each triple describes a PHYSICAL
color.
The matrix transform is tricky - Y alone describes the
luminance.

>From XYZ to xy:
XYZ is mapped by a perspective transform onto the plane
X+Y+Z=1 . The center of projection is in the origin.
Plenty colors are mapped to the same point in this plane,
all of them have the same hue and saturation. They differ
only by luminance.
Finally, the plane is viewed in negative Z-direction from
top.
The resulting 2D-diagram is renamed x,y or xyY and called
chromaticity diagram.
Purists are saying that this 'horseshoe-diagram' shouldn't
be shown colorized, because the luminance is missing,
because spectral or highly saturated colors cannot be
shown by monitors and because they cannot be printed.
If we understand the colorization symbolically, then it's
IMO OK - helpful for explanation and navigation.

Best regards --Gernot Hoffmann


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