G7 Myths

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This reserved article originally appeared in CHROMiX ColorNews Issue 41 on March 11th, 2010.

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colornews(at)chromix.com to subscribe to the ColorNews newsletter.

- an article by Steve Upton



It seemed like a good time to return to our "Color Myths" series. G7 is a great calibration method for presses and other printing systems and is right up the alley of us color management types. Nonetheless, I hear different things about G7 that are unclear or simply untrue. So, please bear with me as I clear up a few misunderstandings about the G7 method:


Contents

G7 is only used in the US

Not true, G7 is in use on every continent where printing is done. In addition to the great reception it's had in North America we're seeing rapid adoption in Asia and other parts of the world.

G7 is expensive to implement

Not true. Training is a good idea and software can save a lot of work but, strictly speaking, G7 can be implemented in a printing facility for next to no capital expenditure and the primary cost would be the press run(s) that are required to sample your press.

G7 is not in compliance with international print standards

While G7 uses a different method to tone-curve a press and TVI doesn't include gray balancing steps, a G7-calibrated press can certainly operate within published ISO tolerances.

"Near Neutral Calibration" is another name for G7

Actually, no. Near neutral calibration is just that; the gray balancing of a system. G7 has a very specific tone curve shape that ensures that G7 calibrated systems, of varying types and technologies, look very similar in their highlight grays. This "shared appearance" method is a core benefit of G7 that other calibration methods don't share. G7 is the first device-independent definition of constant grayscale appearance.

Converting through ICC profiles (especially as a device link) is an effective replacement for G7 calibration

A custom profile for a press, used in conjunction with a reference profile like GRACoL can make a press look a lot like the reference profile's printing condition. There are significant differences though. A G7-calibrated press has tone curve irregularities removed and is easier to control AND profile. A press that has smooth tone curves and full gray balance is easier to monitor and the natural variations that occur during printing don't create unpredictable or rapidly varying prints. If your press was not curved & gray balanced then what dot gain would you look for in the mid tones? What patch would you use to determine if the press was gray balanced?

G7 is primarily useful for commercial offset printing

Not so! G7 is indeed useful for commercial printing but it really shines when used for a print process like flexo or screen where smooth, predictable tone curves and gray balance are tough to achieve by other means. G7 is making great inroads into non-offset printing technologies and several new reference printing color sets are in development that are based on G7.

G7 has undergone changes since it was introduced and is not ready for prime time

Some small modifications have been made to G7 since it's initial introduction but they are evolutionary tweaks and adjustments to the process. G7 has been and will continue to be in it's prime for users to adopt today.

G7's scaling is adaptive and so checking for G7 compliance is difficult or impossible

One of the more powerful functions of G7 is it's ability to scale to the maximum density of your printing system. This does mean that there are not necessarily "fixed" colors for a typical G7-calibrated system. A system that can perform G7 curve calculations (like Curve2) can easily determine if when measurements from a printed sheet are in conformance with G7 specifications. Speaking of G7 specifications, CHROMiX is part of the group at IDEAlliance currently working on G7 compliance testing methods and tolerances. Expect to see news on this front in the near future.

G7 and GRACoL are pretty much the same thing

Definitely not. G7 is a calibration method while GRACoL is a print specification. What's the difference? Well, G7 is the calibration upon which the GRACoL press runs were done. But G7 is also the calibration on which the SWOP #3 and SWOP #5 press runs were done. GRACoL is a specific case of G7 calibration and other printing conditions for ISO colored inks on #1 paper. For more, see the next note.

G7 Proofs are what you aim for on your proofing system

Not really, no. Any proof made to hit GRACoL #1, SWOP #3 or SWOP #5 could be argued to be a G7 proof. They will have the correct tone curves and gray balance of G7 (because G7 was used as the basis calibration of the GRACoL and SWOP runs). In reality the G7 calibration process is too broad a definition to proof to. You will want to create GRACoL or SWOP proofs on your proofer - proofs whose colors match the original patch colors of the GRACoL / SWOP colors.

The TVI method of calibration is pretty much the same as G7 when people are printing properly

IF you are printing using ISO standard inks AND your system is TVI calibrated and is able to achieve gray balance then they are similar, but not the same. The TVI method ensures that each printing channel has the correct tone curve / dot gain to match idealized tone curve shapes. G7 uses a different tone curve shape that keeps the highlight portion of the curve very stable from process to process - independent of the maximum density available. Also, G7 ENSURES gray balance explicitly rather than relying on the ink colors and dot gains achieving it implicitly. It also raises the importance of tracking gray during production. In short G7 is more likely to achieve similar, gray balanced appearance between different paper types, on different presses and even on different press types (web vs sheetfed, for instance).

G7 is finger printing your press

"Finger Printing" your press involves many different measurements and the recording of all of the important consumables, settings and variables that went into its setup and use. G7 takes a quick snapshot of the color of a current printing condition. Finger Printing is a valuable process that is much more comprehensive than printing a G7 P2P target and generating curves. G7 is not intended to replace finger printing in any way.


Thanks for reading,

Steve Upton



List of all color management myths


Don't forget, you can discuss this article and anything else from this newsletter in ColorForums.com:

http://www.colorforums.com/viewforum.php?f=19

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