[cairo] Spot colors (and CMYK)

Kai-Uwe Behrmann ku.b at gmx.de
Tue Feb 16 23:28:25 PST 2010

Am 17.02.10, 00:58 +0100 schrieb ecir hana:
> Now this is interesting. If I go from CMYK A to CMYK B:
> - Kai-Uwe Behrmann, does Oyranos really skips LAB? How does it know
> what value CMYK A (40, 30, 30, 100) should be in CMYK B? Is this what
> Chris Murphy and Jon Cruz call "device-link" ICC and "smaller delta"?
> http://lists.cairographics.org/archives/cairo/2010-February/019130.html
> http://lists.cairographics.org/archives/cairo/2010-February/019128.html
> http://lists.cairographics.org/archives/cairo/2010-February/019101.html

Yes. this is a feature long present in lcms, which is the main CMM in 
Oyranos, and specified in the ICC colour profile standard. They are called 
profiles of class device link (ICC v4.2 8.6 DeviceLink profile).
Their purpose is to have small and fast interchangeable colour transforms.
Oyranos uses this as the colour conversion cache as many other systems do.
See e.g. ColorSync or the ICC branch of Ghostscript.

These profiles are sometimes only in memory like in Oyranos. Some
applications and systems store them on disk. Most often they are not 
visible to end users.

> - Bill Spitzak, are you saying that I always have to go trough 3D
> space? You know that 4D->3D + 3D -> 4D is not the same as straight 4D
> -> 4D. Are you saying that even if I could combine "4D -> 3D" and "3D
> -> 4D" into one operation (for performance reasons?) it still
> effectively remains 3D bounded?

In a plain CMM it is bound. But lcms has code since years to preserve K 
the fourth channels content. This means lcms uses the 3D representation 
only as a measure of correctly going from 4D -> 4D.
In contrast with a 3D cut and forget step, a K preserving feature would 
not be possible.
Such direct from device to device converting technices are not 
standardised in ICC. So one has to search for it elsewhere. But they are 
increasingly common in the print industry. Some workflows completely set 
on such device link or alike technology, e.g. GMG, Alwan and many more.
Open source applications are using 4D->4D device links at a beginning by 
exposing the lcms capabilities.
But do not forget, lcms is as well the colour heart of many propriarity 
systems. This has a great impact.

kind regards
Kai-Uwe Behrmann
developing for colour management 
www.behrmann.name + www.oyranos.org

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