[cairo] Spot colors (and CMYK)

Bill Spitzak spitzak at gmail.com
Tue Feb 16 22:22:28 PST 2010

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.

I tried to make an analogy: if you convert dollars to pounds by 
multplying by 1.6, and you convert pounds to euros by mulitplying by 
1.4, then you can multiply these two factors together and get a more 
efficient method of converting dollars to euros. At no point is the 
value in pounds now. But this does not mean that you have invented some 
new type of money that cannot be specified as a number of pounds.

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