[cairo] Spot colors (and CMYK)

Chris Murphy lists at colorremedies.com
Tue Feb 16 23:12:02 PST 2010

On Feb 16, 2010, at 11:22 PM, Bill Spitzak wrote:

> ecir hana wrote:
>> - 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?
> My understanding of ICC color profiles is that they define the device by 
> how to convert to a matching L*a*b color. Therefore you can do color 
> conversion by using this information to convert to L*a*b, followed by a 
> white point and viewing intent transformation, followed by another 
> conversion from L*a*b to the output device.
> Most implementations concatenate all these transforms together so there 
> is no intermediate 3D color. However the result cannot be different than 
> doing the transformations one after another, therefore the result cannot 
> convey more than 3D of information.

True for device class profiles, i.e. display device, input device, output device (there are commonalities, but these are different profile classes).

There is yet another profile class that can be used to preserve aspects of 4D color spaces (or more channels even). That is the DeviceLink class. It does not make use of a PCS, so there isn't, in effect, a conversion to LAB or XYZ. So such profiles can be built to do things like preserving 100%K only (e.g. for black text).

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

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