[cairo] Spot colors (and CMYK)
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"?
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.
developing for colour management
www.behrmann.name + www.oyranos.org
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