GRACoL color conversion formula

[lnivin]

Is there a formula that decides how GRACoL separates and image? I've tried to get a feel for it by processing several images and I do notice that it usually takes some of the color and moves it to the Black separation.

But what is it looking at and how does it determine what to do?

 

[benstarr]

The separations that are created using a GRACoL profile are dependent on the settings used to create the profile. Different GRACoL profiles created created with different black generation settings will produce different separations. The GRACoL profiles distributed by IDEAlliance have some GCR built in and the ones from Adobe have a slightly different level of GCR. There are also many custom GRACoL profiles out there that would have different separation settings. There can be many different separations methods used for GRACoL CMYK separations and they can all work and be valid GRACoL profiles because GRACoL describes the color space not the separations.

If you have a tool like ColorThink Pro you can examine the profiles and see some info about how the separations are generated, or you can play around with coveting images to GRACoL and see how they separate out.

Hope this helps.

 

[cosmo]

Its all done through the profile, what rip are you using? Rip the same job with Gracol and then with say SWOP and have a look at the difference. Like benstarr said, there might be different GCR or UCR settings in the rip that can affect the black. I use XMF and if we get supplied files with 400% coverage the rip settings i have will tame it back down to a 320% coverage, its a bonus when you have idiot designers thinking black is all CMYK at 100%.

 

[kennycu]

Is there a formula that decides how GRACoL separates and image? I've tried to get a feel for it by processing several images and I do notice that it usually takes some of the color and moves it to the Black separation.

But what is it looking at and how does it determine what to do?

Your files don't convert directly to Gracol. There may be some "usuals" but it doesn't work that way.

The first "it" in your question refers to Gracol profile. The second "it" refers to your CMM or engine that does the calculations.

When conversion occurs, your embedded profile is converted to a Profile Connection Space, usually Lab. Then that (Lab) data gets converted to your target color space. SWOP and GraCol are indeed CMYK color separations. They are intended for different printing conditions. That's why you have different values.

So to answer your question...

Gracol is looking at the Lab values in the Profile Connection Space and the CMM determines the calculations.