[cairo] Spot colors (and CMYK)

[Francois Robert]

Bill Spitzak said:
[...]
> It sounds like the conversion (ie the CMYK source space) is *fixed
however.

Hmmmm. *Fixed* in what sense ? A fixed _algorithm_ maybe. But definitely
a parametrizable one: A CMYK space needs to be chararacterized and
that's what ICC profiles are here for. In other words, there may not be
a single CMYK space, depending on your printers, your inks, your paper,
your viewing conditions etc...

[...]
> Are you claiming that the XYZ or CMYK is an "exact result of the spot
color" ?
Depends. When the ink used is a 'normal' substractive colorant (not
metallic, not transparent etc...) then some XYZ exact equivalent can be
found.

> If that was true, then we can just forget about spot colors entirely !
Yes in a non-printing context where colors (and not inks) matters. But
because the PS and PDF back-ends are addressing the printing side of
things, I am not sure this is a viable course. Unless professional DTP
remains out of Cairo scope ?
Remember, spots are not used *only* for out-of-gamut rendering, but also
for economical and production reasons (BTW not unlike the K of CMYK,
where in theory one could get it as 100% C+ 100% M+ 100% Y when using a
over-simplistic formula and perfect dyes / paper...)
So spot support means not only some approximation / rendering /
equivalence / conversion, but also means allowing to round-trip the fact
that a separate dye is being used for certain parts of a drawing.

> However I'm pretty certain that is false, a spot color is in effect
another
> dimension in the color space and cannot be accurately described by XYZ
or sRGB or CMYK.
For usual inks, spot colors can be described in XYZ. Not the other two.

> Therefore the CMKY is an approximation. 
Absolutely.

Lets convert it to another approximation by using that DeviceCMYK you
described to get sRGB and then use that in the api.