UCR using ICC color Profiles

[ddonevski]

Oh... if so, it makes sense

 

[ddonevski]

I have been trying out ICC Profiles based on Different paper types and Different levels of GCR.

So your profiles are different profiles made by some profile creation software?

 

[mastegman]

The links to the articles posted by Stephen give plenty of historical background to the use of UCR/GCR but another paper by two Swedish researchers indicates that there is very little difference in the implementation these days (see link below). As the articles report these separation techniques have been around for a long time. In the pre-desktop age GCR was known by a host of other names e.g. Crosfiled called it Polychromatic Colour Replacement - PCR (or something like that). 'Ink saving' algorithms used in today's RIPs might use similar principles but you would have to ask an engineer about that as they are working with a greater number of inks.

I would add that the promise of saving ink is qualified by the fact that it is really only significant when you are using a lot of it in the first place. In other words, if your publication is a long run with lots of dark (low key) saturated images then saving ink might be significant cost saver however, if it is a catalogue of weddings dresses with little else then savings will be marginal. The shrinking run-length of todays publications is also a factor to be considered. These techniques were developed in an age when printing was done on a truly industrial scale and many publications had run-lengths in the millions.

'Advantages' can also be 'disadvantages'. For example, having less ink might reduce problems associated with high TAC and colour stability but it also gives you less room to compensate on the press by playing around with the amount of ink flowing onto the substrate. Of course, in this day and age of colour managed workflows age old practices like this have been eliminated ;-)

If I rummage through my archive of presentations I used as a teacher for many years I might find one that lists ALL of the advantages as well as disadvantages. For now, it's going on the 'to-do' list.

 

[kyle]

...The thing is, we only need three variables (CIE L*a*b* or XYZ) to specify the color, but we have four inks at our disposal. The presence of K, in addition to the ability to render dark colors not obtainable by CMY alone, also provides the ability to replace some of the CMY combinations (gray component present in them) with K....

Great premiere post, ddonevski. I'd say that sums it up very well. Welcome to the forum!

 

[gordo]

How does this contradict my statement? We agree that they (UCR and GCR) differ in how far from absolute neutrals they introduce Black. UCR operates in near neutrals and is therefore a subset of GCR. GCR takes it (in addition to near-neutrals) a little further and applies black farther from near-neutrals, or as You stated, "It applies wherever the 3 chromatic (CMY) colors print together.".

I can't see any disagreement between our statements.

I did not intend to contradict your statement. My post was to clarify and add to it. Apologies if that wasn't clear.

 

[DeltaE]

ddonevski
I have read your post, that is a very interesting topic for me and please explain more about the dynamic ink saving software:
"...Static ink saving software: Static ink saver typically uses a pre-calculated device link (input CMYK to output CMYK) and applies it consistently, regardless of image content.

Dynamic ink saving software: Dynamic ink saver analyzes the local activity for each pixel to determine the amount of GCR. Image areas with high local activity mask possible noise and therefore allow greater amounts of GCR. Smooth image areas are prone to the appearance of visible artifacts and therefore allow smaller amounts of GCR...."

If I right understood, the dynamic ink saving software will put high amount of GCR in high ink CMY area and smaller amount of GCR in lower ink CMY area, am I right? Is it the way that Alwan Color Hub works?
I have tried by putting high GCR in destination CMYK colorspace in "convert to profile" function of Photoshop. In this function, I can adjust the curve that I will not use GCR in <20% dot area, and set high GCR from 20% to 70% dot area.
So could you please tell me how advantage of the dynamic ink saving software (I suppose Alwan Color Hub) compared to the way I did in Photoshop?

Thank you!
Regards,
DeltaE

 

[kkvimal79]

I'm afraid you are misinformed. A UCR separation is much, much less stable on press than a GCR separation.

How?

 

[gordo]

Because UCR separation only replaces CMY with K in neutrals whereas GCR replaces CMY with K wherever the three chromatic colors appear together. Hence GCR more broadly replaces CMY with K and that results in more stable color.

 

[ddonevski]

@DeltaE
Sorry for the late response, I haven't visited the forum for quite a while.
Yes, that's how dynamic software works generally. It is more conservative in low area coverage regions and applies lower replacements with K there to avoid graininess. In places with low area coverage (light tones), sizes of CMYK dots matter. If the CMY dots are small, and K dots are relatively large, it will result in grainy appearance because large K dots will be in high contrast with white paper surface. Therefore, seperations (UCR or GCR amounts) should be well balanced in low area coverage in a way that K dots aren't too large and CMY are large enough to sufficiently "close" the paper surface with respect to K. On high area coverage (dark tones) paper surface is pretty much covered so the amount of K (more aggressive UCR or GCR) is not an issue.
I don't know what any commercial software does exactly because the implemented methods are their intellectual property, not open source. But generally, dynamic ink savers take the strategy that I described and much more than that. There are whole studies dedicated to finding the right amounts of GCR based on psychovisual experiments such as "Black is Green" by L. Shapira and B. Oicherman. They take into account additional factors (such as greater sensitivity to skin tones) which are implemented in dynamic ink savers' algorithms.
Anyway, what You do in Photoshop does make sense... apply more aggressive GCR in darker areas, but You do it intuitively and probably sub-optimally. Good dynamic ink saver uses advanced algorithms to calculate the optimal amount of UCR or GCR for every pixel, and takes into account several factors mentioned earlier.
Aside from that, there is an issue with the accuracy of Photoshop separations. When one uses a dedicated ICC profile with fixed UCR or GCR amount, accuracy should be pretty good because that profile was created from measured chart containing several hundreds or thousands of color patches. The profile creation software then fits a mathematical model (a function) to the data and that model is used to calculate all of the in-between points not contained in measured data.. remember that profile tables have many more entries (points) than charts. Now when one uses Photoshop's separation, it does not take several hundred or thousand color patches. It uses just a few entries such as paper white, 100% C, M, Y, K and secondary colors. So the model which Photoshop uses is fit to much less data and consequently far less accurate than one used in profile creation software.

 

[DeltaE]

ddonevski Thank you for answer!