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In article <4566@m1.cs.man.ac.uk> lilleyc@cs.man.ac.uk (Chris Lilley) writes:

[CIE stuff deleted]

>Anyhow, this raises several questions which I had hoped someone here would
>know the answer to:

>A) The interpreter must store the CIE chromacity (xy) or tristimulus
>(X,Y,Z) values for its inks internally.

Well, sort of, but not really.  The color machinery converts CIE XYZ to
DeviceCMYK (or DeviceWhatever color space) by use of interpolated
three-or-four-dimensional tables (usually).  The tables are created
empirically.  (sometimes)

All of this is implementation-specific.  You can't rely on any particular
behavior across all devices.  

>*So* how do you get the interpreter to divulge these values?

Check out the colorrendering commands.  There are tables, internal to the
interpreter, which are used to convert XYZ -> DeviceCMYK.  

currentcolorrendering returns a dictionary.  Among other things, there's
something called "/RenderTable" which contains the actual tables as
PostScript arrays. See p. 302 of the level-2 Red Book.



>
>B) what happens if you specify a colour that is outside the printer gamut?
>

Again, implemenatation-specific.  Presently, the machinery picks the closest
in-gamut color preserving first hue, then lightness, and lastly
chromaticity.  This behavior might be subject to change.


>C) what happens if you specify a coordinate that isn't even a colour? Like
>X=0.7 Y=0.1 Z=0.2, which gives x=0.7, y=0.1 and is outside the flat edge at
>the bottom of the 'tongue'. You will need to look at a diagram to udrestand
>this, and I'm not going to attempt one in ascii!
>

You get the out-of-gamut behavior, if I understand you correctly.  Those are
really still colors, just outside of human perception.  Since all printers I
know of print only a subset of the full perceiveable gamut, you'll always
get something on the paper, unless the point gets "clipped" to white.

We've been considering a new line of color printers which feature all the
colors you can't see.  :-)

>
>PS Manufacturers of colour level2 PS devices are especially welcome to
>reply! :) 

Well, at the moment, we're the only one.  We've been using the CIE stuff
quite heavily, and it is really useful, provided you understand it.

KevinD
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