[cairo] Spot colors (and CMYK)

Kai-Uwe Behrmann ku.b at gmx.de
Tue Feb 16 13:32:10 PST 2010

Am 16.02.10, 12:08 -0800 schrieb Bill Spitzak:
> Kai-Uwe Behrmann wrote:
>> There is no one colour space. sRGB is one of many.
> However we all agree that unclamped sRGB values can be converted to all other 
> 3-dimensional color spaces, including XYZ and Lab. So it is sufficient for 
> all 3D spaces.

The CMS Oyranos uses end to end colour transforms bypassing the 
3-dimansional PCS during colour conversion. For applications it is 
possible to specify a colour trasnform by their own. This means DeviceN to 
DeviceN is completely valid for input and output at the same time. A 
user can work in Cmyk, while seeing the print simulation on screen and 
then print the same colours to the intented printer or massage the output 
with through additional Cmyk to Cmyk conversions.

All CMS work like this. Needless to say that sRGB is out of scope for Cmyk 

>> All colour management experts will agree that it is not possible to spcify 
>> (CMY) K behaviour with RGB or whatever tripples.
> Obviously mathematically a 4D space cannot map to a 3D space without multiple 
> values mapping to the same one. However from all the discussion here it 
> appears that ALL color management is done by passing through an intermediate 
> 3D space, so this loss of data is apparently not a real problem.

In theory, but there are shortcuts and typical this step is bypassed if 
nedded. Cmyk as input should most often not be converted to a intermediate 
3 channel representation.

> Therefore the 4D space is only useful for a device-specific api. All device

Agreed. PDF is such API. If cairo would support DeviceN there would be 
users for that.

> independent interpretation is done by reducing to 3D, therefore the 
> application can do the 3D reduction itself and use the 3D sRGB api.

Applications should rework cairos backend output? Are you talking about 
the same cairo: one drawing API ==> multiple outputs?

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

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