A Primer on GCR - Part 3

The Why of GCR
The main advantage to GCR is the reduction of ink usage. The amount of CMY we subtract should always be more than the amount of K we add, producing an overall net reduction in ink used. This can be particularly beneficial for media that cannot hold 300 percent ink (the equivalent of a 100 percent process gray solid). Not applying GCR leaves us with less-than-optimal choices: restrict the primaries with overly aggressive limits that can degrade one- and two-channel colors (and the achievable color gamut); or, apply secondary ink limits that make recalibration without reprofiling (the aim of G7-based process control) exceptionally difficult.
 
With GCR applied, however, CMY process gray will be largely substituted with K, so a 100 percent process gray solid should print with far less than 300 percent ink; in fact, it should be less than half that depending on the amount of added C, M or Y. The same will be true of any other three-channel combinations – the CMYK equivalents will print with less ink than the CMY originals. Aside from the cost savings, that can also give us more vibrant color reproduction.
 
We often see a tendency for dark colors to fall toward black more quickly than we would like. Dark blues and violets, and even sometimes very dark reds and greens can turn black due to an oversaturation of ink; excess dot overlap results from high concentrations of ink, absorbing too much light and causing dark colors to lose all perception of color (or appear as a muddled gray). By reducing the amount of ink through K substitution, we maintain the same low L* without the excess dot overlap, allowing the chromatic component to remain discernable – this can result in smoother dark gradients with stronger color and more subtle shading.
 
Another problem with high ink concentrations is loss of detail. Excess ink in very dark areas can be drawn into lighter areas, leading to poorly defined edges with some fine detail areas being completed flooded with unwanted ink. This and the poor reproduction of dark colors are the main visual objections to three-color process inkjet, and are generally described as a lack of contrast and poor shadow detail.
 
Beyond this, and possibly somewhat counter-intuitive, is the impact of K on the achievable color gamut. On most inkjet printers, the K solid density will exceed the CMY process solid density by upwards of 30 percent in some cases. This means that not only can K be used to substitute for the CMY gray component, but when mixed with higher ratios of two other channels (CM, CY or MY), can produce colors not achievable with CMY alone – colors with lower L* values that still retain perceptible chroma. This is the result of having an expanded L* axis; the greater dynamic range causes the entire gamut to increase to as near its maximum volume as possible.
 
One caveat to GCR is its impact on very light colors, and the speckling that can occur if GCR is used to replace all CMY process gray in an image. Depending on the physical resolution of your printer, and the dot size used, the high density of the K ink makes its dots very obvious if printed at very low percentages. This is most obvious when low percentages of K are printed alone on white media, or in conjunction with high concentrations of yellow. For this reason, GCR may not be utilized if the process gray component is below a certain threshold – better to print a 3 percent gray with CMY rather than K alone, since the CMY will print in higher concentrations with less discernable dots.
 
The point at which GCR is applied is primarily resolution dependent, but because GCR will have minimal impact on colors with ink concentrations well below the media saturation point, the general rule is that starting a bit late is much better than starting too early. Once GCR is applied, it can be phased in through the mid-tone range so as to achieve its maximum impact at a point just short of saturation. For example, many inkjet media hit a maximum saturation between 240 and 260 percent ink, so using GCR to keep the maximum ink level around 200 percent should avoid K speckling in highlights while still achieving its maximum impact in darker midtones and shadows.
 
Reiterating the G7 Curves
Printers drift; over the course of a production run color can shift due to ink, media and environmental changes. Every new ink refill, every new roll of media can lead to a color shift. When this occurs, recalibrating back to G7 aims by reiterating the active G7 calibration curves, can reestablish the gray balance on printer. But, how does this related to GCR?
 
By calibrating to G7 aims as an initial step, we establish a neutral gray balance, and we establish the relationship between our K grayscale density and our CMY process grayscale density. If a media or ink change introduces a color shift (a cast), reiterating the G7 calibration curves will remove the cast and return the system to its original neutral balance. That means that, while the ratio of C, M and Y substituted for K may change, the colorimetric equivalence of our process gray to K gray will remain intact. In other words, the reiterated curves will adjust the values of C, M and Y such that they produce the same neutral gray with the same density as when the system was originally characterized, and that will maintain the CMY to K equivalency of our GCR function. This is also what ensures the viability our ICC profile; if our GCR remains valid, so too should our profile.

This article first appeared in the November/December 2014 issue of SGIA Journal.