[cairo] Spot colors (and CMYK)

[Chris Murphy]

On Jan 25, 2010, at 8:42 AM, Arjen Nienhuis wrote:

> On Mon, Jan 25, 2010 at 2:19 AM, Chris Murphy <lists at colorremedies.com> wrote:
>> With respect to Cairo allowing applications to do soft proofing, this really requires ICC based color management. Soft porofing is a rather specific thing that also implies rather specific ambient and print viewing conditions, in addition to getting other things correct. But certainly we also need to take into account the specific CMYK source space (and optional spots), and convert those to display RGB, or we simply do not have soft proofing. A simple CMYK to RGB conversion is completely inadequate for soft proofing. So if soft proofing is a goal, something is going to need to use ICC profiles.
> 
> If you stay in sRGB that is actually not needed:
> 
> - My data is sRGB.
> - Cairo is sRGB.
> - My preview is sRGB.
> - The web is sRGB.
> - On my PC Ubuntu leaves the sRGB colors the same for my cheap sRGB-ish monitor.

The display market has diverged from sRGB, and continues to do so. There is a great degree of likelihood that this display in fact does not correlate to sRGB, especially its blue primary, and that on-screen previews do not constitute a soft proof. 

Further the displayed results are questionable because colors do exist in sRGB that can be displayed, but not printed, which makes the soft proof invalid. The whole point of a soft proof is that it shows you how a document will print.

> - My macbook converts the sRGB preview to the LCD-profile.
> - My RIP uses color profiles to convert sRGB to 4 inks for the press.
> - My other RIP uses color profiles to convert sRGB to 8 inks for the
> large format printer.

All of these use some kind of color management. The first one for sure is ICC-based, the other two could be either ICC-based or proprietary.

> - My customers are happy.

The emotional state or opinions of customers, while important for a business to secure, has no bearing whatsoever on whether soft proofing is occurring or whether it is correctly simulating the output condition.


> This leads to better results than the 'official' way:
> 
> - Insert RGB-jpegs and text into Indesign template.
> - Create ISOCoated-v2.icc (that's like SWOP) 4 color PDF-file from InDesign.
> - Use acrobat to open the PDF-file and convert the CMYK back to RGB
> for proofing.
> - My RIP does something(?) to convert the ISOCoated PDF to 4 inks for printing.
> - My other RIP does something(?) to convert the ISOCoated PDF to 8
> inks for printing.
> - My customers are still happy.

On-the-fly conversion to a CMYK space appropriate for the output conditions is necessary for soft proofing. It's certainly not the case you can get better results by avoiding this conversion, if the goal is soft proofing.


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