[cairo] Spot colors (and CMYK)

[Chris Murphy]

On Jan 21, 2010, at 9:37 AM, James Cloos wrote:
> 
> 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.

Which I will characterize as almost completely useless because anyone who has printed a reference document to a dozen color laser printers can attest to the fact they all behave very differently. Even if the reference file is CMYK, even if the OS and driver are set to a pass through "raw" mode to eliminate proprietary color management from being a factor in the differences. The primaries are all very different.

> 
> 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.

Right it's a failsafe. Or actually it's closer to a fail WTF, as in you get something, and it will probably be at least readable, but may not at all have the intended color. It's just designed to avoid mapping one (or multiple) spot channels to black, or giving them patterns, which very well might make them completely unreadable.

I would consider a good fail safe for this purpose would ensure separation between spot channels, even if a simple algorithm would map them to the same value. Think of a document with two spot colors where the conversion to RGB and then to grayscale for printing on a black and white device, produces completely illegible output - as in, text is obliterated. I wouldn't call that a great failsafe. But that's maybe a bit out of scope.


Chris Murphy
Color Remedies (TM)
New York, NY
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Co-author "Real World Color Management, 2nd Ed"