[cairo] Spot colors (and CMYK)

Arjen Nienhuis a.g.nienhuis at gmail.com
Mon Jan 25 05:42:35 PST 2010

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.
- 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.
- My customers are happy.

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.

> If Cairo is responsible for/capable of interpreting PDF, then the color management functionality belongs in Cairo. The flavors of PDF used for printing: PDF/X-1a, PDF/X-3, PDF/X-4, etc. all have an output intent specified in the PDF. By default, any application should use this output intent as the assumed source profile for any CMYK content in the PDF, or it's simply being displayed wrong.
> The way Acrobat works on both platforms, by default - it always honors embedded ICC profiles in the PDF, there is no way for them to not be used. Embedded profiles are the source space for an object, and the destination profile is the display profile. Further, objects that are untagged have assumed source profiles, which is configurable.

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