[cairo] Spot colors (and CMYK)

James Cloos cloos at jhcloos.com
Thu Jan 21 06:37:27 PST 2010

>>>>> "Bill" == Bill Spitzak <spitzak at gmail.com> writes:

Bill> It sounds like the conversion (ie the CMYK source space) is *fixed*
Bill> however. So there is a defined non-color-management way to convert
Bill> to sRGB, and from there color management can be used to produce
Bill> the actual numbers in the display buffer.

No, it is not fixed.

Adobe just chose one particular colour profile -- aproximating the
profile of a colour laserjet printing to roughly 90gsm 80ish-90ish
bright office copier/laser paper -- as thier (customer's) default
and derived a conversion from that to RGB which they use to allow
thier customers to proof DeviceCMYK on monitors in office conditions.

XPDF and Artifex then chose to attempt compatability with acroread
on that front.¹

That particular transform is just a fallback for the cases where no
specific colour profile was specified.

That is was chosen to appease non-colour-savy users makes it a good
choice for cairo's DefaultCMYK, too.

If a colour profile is specified, then that is used instead of said

Cyan toner is much darker than what you get from a combination of
sRGB (or NTSC or PAL or etc) Blue+Green.  Red+Blue is also a poor
match for Magenta toner, and Yellow toner is /far/ outside of the
(constrained) sRGB gamut.  

1] It is possible Adobe got the idea from some other company in the
   publishing biz, but xpdf's comments mention an attempt at adobe
   compatability, and I'm sure I remember discussion of the same on
   either gs-devel or in the gs bug db.

James Cloos <cloos at jhcloos.com>         OpenPGP: 1024D/ED7DAEA6

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