The Ins and Outs of GRACoL 7 and G7
by CHROMiX President Steve Upton
The Importance of Whites and Grays
Perhaps I've spent too much time gazing at color in three dimensions, but I continue to think of color and color concepts in relation to the 3D color wheel from early school days, and in relation to the 3D color gamuts of more recent ICC profiles and the devices they represent. One of the first things I notice about a 3D gamut is the central peak, which represents the paper white in a print profile and, to me, resembles the peak of a tent.
If the white point of a printer's paper represents the peak of a tent, then the neutrals - grays flowing from black up to paper white - are the tent pole holding the whole color structure up. It is interesting to find that human color perception follows this analogy closely. The eye adapts quickly to the most dominant white in a scene - paper, in the case of printing - and the other colors "fall out" relative to this white. Only slightly less important than white, however is gray. The neutrals in a scene are also an important perceptual reference that our visual system uses to help 'position' the relative saturation of other colors.
Indeed, basic image correction techniques involve first adjusting where the white and black points of an image should lie (the footing and angle of the tent pole), and then click-balancing a known-gray element in the image. This pulls the tent pole tight and true, and is often 95% of the correction any image requires. I never cease to get gasps of amazement from the digitally inexperienced when I perform these three simple operations on a seemingly dead image, and it springs to life with a new depth and breadth of color, and, in many cases, it's all the correction the image needs.
Our pro photographer customers, aware of this important color relationship, often apply this principle as a stress test to a color system by first printing a grayscale image. They correctly realize that if the color printer is incapable of handling the first three important parts of image reproduction, then they needn't bother testing its color output. Why bother venturing into a tent that appears ready to topple over at any moment?
Gray Balancing Presses
So what does this have to do with press output and printing standards? Well, we gray balance cameras, we gray balance monitors, and we gray balance images. Should it not follow that we could gray balance presses? Naturally. Does this mean it hasn't been done up to now? No, gray balancing presses has been done for many years - but differing methods have been used, and some print standards have been based on systems that were not necessarily gray balanced.
So what's different today? In a word, CTP.
Computer-to-plate imaging, the ability to create a printing plate directly from a digital file, has opened up new frontiers in print control. Once printers realized that their new plate making system could be curved to reduce or even eliminate dot gain on press, all sorts of different printing methods ensued. Unfortunately, it was not clear which was the best technique for plate control. Should printers follow the dot gain that their older film-to-plate systems produced? It certainly helped when reprinting older jobs. Should they reduce dot gain to zero? Should they shoot for somewhere in the middle?
When plate curving is combined with modern ICC profile-based color management, the decision is confused further. Profiles can compensate for a wide range of printing conditions so, in some ways, the curving decision is irrelevant once a profile is made for the press. But what if you don't want to profile each and every press condition? What if you receive files that have already been separated to CMYK? Then profiles may not be the solution to regulating press behavior. What do you do?
Wise people who have been considering this problem for some time now have observed that having presses run in their natural state makes for a more stable printing system and happier press operators. It would follow that whatever press condition is selected for a standard should follow the natural behavior of a well maintained press.
Printing experts realize that with the power and flexibility of CTP, we now have the ability to gray balance a press. Wouldn't it be nice to have a standard AND a methodology for press-friendly printing curves that were gray balanced? It would also make sense that the mid-tone gray level of both the black and combined CMY curves were measured for density and gray. This doesn't mean that monitoring solid ink density and TVI (dot gain) numbers should stop. They are important indicators of press behavior, and many diagnostic tools and techniques are based on using them as indicators. What should happen is a reprioritization of measurements' relative importance on press.
In simpler wording, if gray balance is so important in every other stage of imaging, and in color perception, it should be one of the most important things to aim for, and monitor, on press.
There is one massive side benefit of using this technique - press to press consistency. It turns out that gray balance and consistent tone curving is so important to image perception that a print job separated for one printing condition (say SWOP) will look VERY SIMILAR when printed using a different printing system (such as sheet fed GRACoL) when both presses have been gray balanced and toned using the same method. This is huge. I cannot tell you how many times I've heard printers say that profiles and color management can't help them with one of their everyday problems - that of CMYK being created before the paper, press, and even printing company has been chosen. What can we do to help them get their different presses looking as similar as possible so that they can survive the last minute decisions their clients continue to throw at them? The answer lies in gray balance and toning to a standard. (This does not remove the need to use profiles if printing conditions vary in certain important ways, such as ink colors)
This brings us to the topic of standard vs specification vs methodology.
Standard vs Specification vs Methodology
Standard - a standard is a set of measurement aim points and tolerances to be used as an aim point and a means of exchanging data. Things become standards when standards bodies such as the ISO accept them and publish their numbers and methods. Two specific standards that affect printing in gray balance and toning are ISO-12647-2 and ISO-2846-1. I'll discuss these more later.
Specification - a specification is a body of numbers and methods that is put forth as a working technique and may be in line for submission to a standards body for acceptance as a standard. It often doesn't have the strict tolerances of a standard. Specifications we're familiar with in the print world include SWOP and GRACoL. Sometimes specifications are considered an implementation of a standard, where other implementations that use different techniques but also adhere to the standard may also exist.
Methodology - a methodology is, quite simply, a way of doing things. In the case of gray balancing and toning a press, the methodology I'm talking about in this article is called G7 (I bet you were wondering when I would finally get around to it).
So, G7 is NOT GRACoL, and GRACoL is NOT a standard. BUT, both G7 and GRACoL operate within the published tolerances of several ISO printing standards. The G7 gray-balance and toning technique can be applied to GRACoL, SWOP and other printing conditions. As it was formalized at the same time as GRACoL 7, it is often talked about in the same context and is sometimes confused with it. GRACoL 7 is due to be available electronically early 4th quarter of this year, with printed publication in January 2007. G7 is available NOW from the GRACoL website. (Sure it adds to the confusion but it's also the best place to put it for now.)
Make sense? I hope so. If not, please refer to the list of references I have gathered at the end of this article for further reading and reference.
So how does this all affect things? And how (in case you're curious) is CHROMiX involved?
Well, first, GRACoL 7 includes a set of press measurement data to be used to create profiles for sheetfed printing, which will be a considerable improvement over what's available today. Probably the most widely used sheedfed profiles are available in Photoshop and other Adobe software, and are actually based on the Matchprint proofing system rather than an actual press run. Good profiles based on actual press behavior are long overdue. CHROMiX will be creating a full set of profiles based on the beta data soon so stay tuned...
As the data are finalized, we will recreate the profiles from the final release data and update the profiles on our website. The profiles contain our new serial number and versioning tags, and you will be able to tell when yours need to be updated using upcoming software from CHROMiX... stay tuned on that one.
This data is also useful as a reference for evaluating the performance of proofing systems. For instance, it can be combined with profiles - or actual measurements - from your proofing system to illustrate how close your proofing system is to the standard and where its problems or deficiencies may lie. ColorThink Pro can be used for this purpose, and its ColorSmarts Guide includes a technique for just such a comparison. (See ColorSmarts Guide - Comparing Device Gamuts)
Second, G7 really is as good as it sounds. There's an entire document describing the step-by step procedure for gray balancing your press or proofing system available on the GRACoL site. It describes a method using measurements and graph paper that determines the best curves for your CTP system. Which brings us to...
Third. CHROMiX has been chosen by IDEAlliance, the umbrella organization for both GRACoL and SWOP and Don Hutcheson, the source of the current G7 technique and mathematics, to write a software package to streamline and simplify the application of the G7 method. This package is available this week and is called IDEALink Curve.
IDEAlliance is the reseller for the software.
And we are also selling it with our G7 bundle specials mentioned above.
- (Editor's note: The original IDEAlink Curve has been updated with new versions over the years.
See below for more recent versions:)
- (Editor's note: The original IDEAlink Curve has been updated with new versions over the years.
Once again, I have produced a fairly lengthy article, but I felt it was necessary to cover the benefits, strategy and state of the new print capabilities that are emerging today. This is an exciting time for printing in the US, and other markets, as we color geeks finally have a standard and method that works well with color management which also actually creates many benefits for the press room.
The truth is, there's more to cover on G7 and GRACoL, but it will have to wait for a future article. If you want to discuss it, feel free to pose questions and offer opinions in the Print & Press forum on ColorForums.com.
Thanks for reading,
G7 Process: http://www.gracol.org