consultants to magazine publishers

Gray Component Replacement

by Alex Brown


Get a few production people together and you’ll hear gray component replacement (GCR) called everything from a breakthrough to smoke and mirrors. The former opinion takes note of the clear value of the principle, while the latter reflects the wide range of results achieved without standardized specifications. This article will acquaint you with the technical basis for GCR and offer some guidelines for applying the technique, whether on your own in PhotoShop, or through instructions to your color separator.

In a color reproduction made with GCR, the neutral color element of each tone is made with black ink instead of a neutral mix of process colors. In theory, this should yield purer hues because darker tones are produced with a neutral black instead of a trace color. Further, since ink is a wet film applied at great speed in printing, anything we do to limit the problems inherent in trapping translucent inks will improve reproduction. We even cut costs: GCR uses inexpensive black into to replace pricier process inks, and GCR uses less ink all told.

GCR also theoretically offers more stable conditions for color adjustment. When we eliminate the process yellow as a gray component in a blue tone, all the adjustments made to yellow on the printing form have no effect on this hue. At an extreme, GCR can reduce three-color overprints in every area except area except the browns that come from a full cyan, yellow, and magenta mix. Although there are few tones in an image that can be broken down without some use of the process inks, we’ve eliminated some and reduced the trace amounts in all the others. Practically speaking, we should achieve easier color adjustment on press since we’ll tend to jockey black and two colors, not three, for major corrections.

If GCR earned its pie-in-the-sky reputation for any good reason, it’s because early proponents promised greatly reduced ink consumption and lightning-fast makereadies. In practice, GCR is not quite this godsend. GCR’s big benefits to the printer don’t arise in a magazine form mixing the publisher’s GCR images with ads that don’t use the technique. However, we have learned that combining chromatic (conventional) and achromatic (GCR) separations poses no problem of its own.

The process color inks are more expensive than black ink, but don’t expect use of GCR to lower your ink bill drastically, if at all. Printers don’t have much savings to report, for two reasons. First, GCR can be applied at any intensity, and when used cautiously it will not convert much process color to black. Second, some printers feel that the greater color stability of GCR inspires pressmen to increase ink levels overall.

For a catalog that uses GCR in all images, GCR can simplify makeready, but the typical magazine signature will pose most of the same challenges it always has. Nevertheless, some printers report improvements in makeready time when GCR is used in editorial work alone. Although some printers haven’t found any particular speed boost, GCR advocates mutter that these unconverted souls simply haven’t been exposed to proper GCR. As we’ll see, it’s not simple to define what’s proper, and GCR zealots are to be forgiven for assuming that unimpressed printers have been working with unimpressive GCR.

During the pressrun, we can expect color to be more stable. Not only have we reduced the constituent colors making up a hue, we’ve curtailed the potential for variations in color from dot gain. However, as you’ll see in a moment, dot gain remains a serious problem for GCR color.

Replacing the gray component of a tone with black has some further advantages, chiefly of interest to your printer. In theory, GCR’s thinner ink film provides better results on lighter paper weights. Along the same lines, faster drying times are theoretically possible with less ink to dry. Finally, whenever we can make a pesky brown tone in two colors plus black instead of three process inks, we take a step toward reducing moiré. Moiré is most evident with a great concentration of large dots of similar size, which is just about the definition of a picture of a mahogany wardrobe or a dark leather sofa.

Overall, GCR is not a way to reduce costs but a way to improve quality. Still, quality benefits are not automatic because use of the process is not firmly under control. There are two reasons why: the variations in the amount of GCR adjustment among separators, and the lack of specific proofing controls.

When you apply GCR in PhotoShop, you need to decide just how much of a good thing you’d like. A newsprint product might use 90% GCR, while a magazine on gloss coated grade 5 stock would tend toward as much as 70% GCR. These percentages mean just what you’d assume: we’re replacing 70% of the gray component with black.

Under the benign printing conditions of most magazines, GCR should be limited to avoid allowing too little color definition in shadow areas. But it is here that the trial and error must really begin. Many printers advise their customers that using anything less than 65% GCR represents virtually no change from the chromatic process, and the real dividends only begin with 70% GCR or better. Each publisher’s range of subject matter must play a part in determining the extent of GCR adjustment, and there’s no substitute for on-press testing when determining a GCR specification.

Testing’s fine, but why can’t a proof help us evaluate the process before we commit to paper? Only a set of proofs or a test sheet with multiple images lets us compare the results of different specifications side by side. Within the magazine, we can only print or regular subjects and note whether or not they look good; we can’t compare a range of GCR settings. A set of proofs of different GCR values can also include test targets of three color overprints. These targets show gray balance and overall color, and can demonstrate whether or not black is contributing what it should.

Although test targets help us see that proofs were processed properly with GCR in effect, we’ll still have trouble anticipating the printed effects of different amounts of GCR. Call it mysterious, but we can obtain similar printed results from a host of different GCR specs. More important, two proofs that look identical may represent film or image data that prints quite differently when dot gain and particular on-press color corrections come into play. In short, we lack a proofing system that accounts directly for GCR, or allows us to evaluate it completely before committing ink to paper.

Even if we do arrive at standards for GCR percentages, there will still be three problems that limit the quality of GCR reproduction. First, our reliance on black ink brings two nasty side effects: black shows the greatest dot gain, and black must remain as even in density as possible to keep text a uniform black color. When black is used to correct color, it can, at an extreme, vary enough to undermine consistent type density. However, even though a great deal of black is used in GCR, it should behave as a neutral tone, and not be used for excessive color correction.

The dot gain issue is more serious. PhotoShop retouchers must be alert to the need to compensate for dot gain. Simply increasing the amount of black will not do the job properly—it must be increased with compensation for gain built in.

The second troubling matter is the problem, perceived or real, in reproduction of certain light tones with GCR. In the abstract, a pastel tone might be reduced to two colors plus black, without an adequate trace of the third process color. This can lead to weak color, with less brilliance than the chromatic method produces. Caucasian flesh tones are particularly susceptible to deficiencies here. According to one printer, this concern may be exaggerated, but the basic issue still requires consideration. Reducing the gray component equally in every tone may not yield good results, and some adjustment for the relative saturation of a color may be necessary. In another printer’s opinion, you may see no advantages from GCR in light tones, but you won’t see any additional problems either.

Finally, we face the need to develop tone curves unique to the GCR method. A tone curve places the shadow tone, quarter tone, and midtone at particular densities in order to compensate for the inherent compression of the original’s tonal range in printing. Since the black film or image data will now shoulder a very different burden, this tone curve must reflect the reality of dot gain, the new contribution of black, and the need to position highlights and shadows for a balance of detail and contrast.

There’s one last potential problem, and a good solution for it. You may want to add one more task as well, a further adjustment called undercolor addition, or UCA. In straight GCR, the density in dark areas may be artificially reduced, yielding weak shadows and reduced color intensity. Some compensation for this lack of gloss is required, and properly implemented program of UCA should correct the deficiency.

As you can see, using GCR is not a simple matter. The key points for successful implementation: special care in making and interpreting proofs, compensating for dot gain, and monitoring the potential problems in shadow detail and light colors. Many images identified as GCR are in fact little more than lightly adjusted chromatic separations. Confusion and disappointment are the result; this is an in-for-a-dime-in-for-a-dollar process. Make sure you're using the technique with enough intensity to make it worthwhile. Overcautious GCR gains little, while a full and proper dose permits superior reproduction.

GCR and color reproduction

The reproduction of continuous tone originals through the halftone process relies on the subtractive colors cyan, magenta, and yellow, each of which represents an equal amount of two primary, additive colors. Cyan, for instance, is blue plus green.

Subtractive color principles determine how we’ll break down and manipulate the color spectrum. For example, magenta serves as red plus blue and as the opposite of green. A green filter produces the magenta component of a separation, and every trace of green in the original will be held back by the filter, while red and blue will be recorded in proportion to their intensity. We can use magenta dots to represent the areas that are red and blue, and we can also rely on the inherent overlap from the remaining process colors. Cyan, for example, has blue in common with magenta, and green in common with yellow. Thus, when magenta and cyan are combined in equal strength as printing dots, they produce blue, the color they share.

Using the process colors to reconstitute the primary colors is fine in theory, but some physical limitations restrict our success. The two-color overprint of magenta and cyan makes a blue, but it is not the pure primary color blue. Printing ink is a translucent film loaded with pigment. When you look at a printing dot, you’re seeing light reflected through the ink by the paper and directed back to your eye. Ink pigment is not in fact pure enough to serve the ideal of a spectrum perfectly divided into subtractive color thirds, and paper itself adds some color and breaks up the accurate reflection of light rays by the variations in its surface.

Added to these limitations are the special problems that result when we make up a range of colors by overprinting two or more inks. A three-color overprint of 95% magenta, 80% cyan, and 20% yellow is a blue. That trace of yellow is intended to provide a necessary darkness and body to the color. The two predominant process colors determine the hue, while the least dominant color adds detail to any three-color overprint. Yellow, in this example, is the gray component of a blue tone.

The gray component of an overprint absorbs some of the light of the intended hue. In the blue we made, yellow as a gray component absorbs some blue light and darkens the color. Seen at a distance under regular viewing conditions, the yellow dot in the blue actually looks black. We need a certain amount of this effect to add dept that a slight darkening produces, but we’re also well on the road to dirtying the hue. GCR is based on using a different method to darken hues to their required depth. Instead of using a trace of a complimentary process color, we’ll use black ink.

The conventional separation process produces what’s called chromatic printing, while a GCR system is called achromatic. It’s important to understand the chromatic principle well, because the GCR approach is a subtle addition to the technique and not an independent system of its own. For example, even when GCR is used, all the two-color overprints behave just as they would in chromatic printing, with yellow and magenta combining to make red, cyan and magenta combining to make blue, and yellow and cyan combining to make green. Most important, we can apply the GCR approach in different degrees to keep as much of the chromatic technique as we like. The amount of GCR correction, described as a percentage, is up to the color separator or retoucher, working under standards the production manager and printer agree upon.