[argyllcms] Re: Average bug/Density curves/ xicclu -g predictability issue

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 23 Jan 2011 16:33:08 +0100

Am 23.01.2011 15:00, schrieb Elena [service address]:
> But I'm not sure however if dropping K is a good choice or just a Pyrrhic 
> victory!
> After all the inks I'm using are highly metameric: the prints look very 
> different
> when viewed in sunlight, dayshadow, fluorescent light or incandescent light.
> Using K gives dramatic improvements in metamerism.

I just wanted to say that it might be a trade-off. Of course there are
also a couple of good reasons for making use of black ink (usually
deeper blacks, better color constancy for gray tones under different
light sources, ink cost,...).

> Oh yes, of course, surely :) Just it's not the way I like to work to send RGB 
> or CMY values to the driver. I like the versatility of working in CMYK - for 
> example in documents editors when you are mixing maybe photos and text and 
> graphics, and you think in CMYK terms of course, not RGB. You there will 
> choose maybe clean K for text, C for blue shapes, etc.

But you could do it at least for testing/evaluation purposes, can't you?

> So I'm currently stuck with this idea, that colprof should calculate a K 
> curve based on the given parameters, but then it should really KEEP that K 
> shape thru all tints as a main reference rule for separation, not for 
> neutrals only, being at most allowed to drift just SLIGHTLY from it if it 
> thinks it's good to, but always keeping it as a main reference, to detect 
> whether severe discontinuities will be introduced or not.

A too simple UCR (e.g. as defined by the PostScript Language Reference
Manual for PostScript's DeviceRGB to DeviceCMYK conversion) IMO tends to
(potentially significantly) shrink the gamut in dark saturated regions
(i.e. dark, saturated colors can no longer be reproduced using this
separation, although CMYK combinations would exist which could reproduce
these colors - see e.g. figure 9 (a) in the paper referenced below). So
I've doubts whether a simple 1D rule which is fixed for all colors is
really sufficient.

An interesing paper dealing basically with the RGB/CMY -> CMYK
separation issue is "Modeling a CMYK Printer as an RGB Printer" by James
Z. Chang and John C. Dalrymple.

Regards,
Gerhard


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