[cairo] Spot colors (and CMYK)

Jon Cruz jon at joncruz.org
Wed Feb 24 01:22:49 PST 2010

On Feb 23, 2010, at 11:16 AM, Bill Spitzak wrote:

> If I print a spot color with 50% opacity over an existing device-space rgb, the result will be (r+R)/2, (g+G)/2, and (b+B)/2, where rgb are the existing device-space color there, and RGB is the (correct) translation of that spot color to device space.
> Now the backend can reproduce this on the output, but it is unlikely to magically produce 50% of that spot color as part of it. This is because the exact same result can be arrived at by mixing normal colors and the user certainly does not intend for the spot color to be used then!
> If you can explain how this works I would appreciate it, but it appears mathematically impossible unless a channel is allocated for each spot color

I don't think you've been getting the print world application I'm trying to be able to support.

The main point is that in *practice* graphic designers do not overlap CMYK with spot color. They trap them to avoid such issues.

So in the print world the situation you seem so worried about is one that is normally avoided. Just as is 100%C, 100%Y, 100%M 100%K is avoided as in the real world that will usually oversaturate the printing medium with ink.

Oh, and again keep in mind that normal offset printing is incapable of reproducing many spot colors. So often "mixing normal colors" fails to reproduce the targeted value.

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