[cairo] Spot colors (and CMYK)

Chris Murphy lists at colorremedies.com
Tue Feb 23 08:19:50 PST 2010

On Feb 22, 2010, at 10:53 PM, Bill Spitzak wrote:

> Jon Cruz wrote:
>> Cairo needs to emit proper CMYK+spot details to the PDF.
>> Cairo *should* at the same time be able to 'preview' to an RGB display.
>> If that preview does not have ANYTHING to do with the spot color specified, then most even semi-profesionals will laugh their heads off at Open Source and dismiss such software as mere toys.
> This sounds completely impossible. It sounds like you want composites in Cairo where some of the input is a spot color to exactly reproduce whatever effects the overprinting and mixing of the ink does on the printer.

A DeviceN (multichannel) profile with a CMS will do this for you. I don't know if lcms supports 5+ channel profiles, I presume it does but don't know what the upper limit is.

For solid spots, non-interactive (not part of color separations) they don't need a profile, just an XYZ or LAB value to define their color.

> I do not believe sufficient information is provided in cms descriptions for Cairo to produce a correct simulation color for any mixes. I also do not believe that anything, including OSX, does anything remotely resembling this.

It honors what's in the PDF. If the PDF spot colors contain LAB alternates, then it uses them. If the PDF spot colors use CMYK alternates then it uses those and assumes a default CMYK source profile if one isn't provided in the PDF.

> The only plausable way I see for the display to be "accurate" is if it uses cms to reproduce the resulting composite on the printer. However this is certainly not what is wanted for spot colors, even if 100% spot color is detected and produces the desired ink, the antialiased pixels around the edge would be replaced with some simulation with the CMYK inks.

I don't know any product that behaves this way. Display anti-aliasing is done either by the viewing application (e.g. Acrobat) or as an OS service (e.g. OS X  provides this capability to Preview), and in any case it's done in RGB. If the spot alternate definition is LAB, it's converted to RGB for antialiasing. It's a question whether this is done in some intermediate RGB space, or in DisplayRGB.

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

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