Monday, April 6, 2009

GCR Reseparation for ink savings and color stability in offset printing

Special note: This posting on ink savings via reseparation was republished by Print Action magazine in December of 2009 and subsequently became the basis for the IPA Ink Optimization RoundUP technical evaluation of ink optimization software presented at the IPA Technical Conference in June 2010.
Ink storage vats at a magazine printing facility in Australia

Reseparating customer supplied files is rapidly gaing popularity with printers and publishers as a way to lower print manufacturing costs.
Although the application is not limited to specific print market segments - these are the ones that are most quickly adopting this technology:
• Newspaper publishers
• Insert and flyer printers
• Magazine publishers
• Catalogers
• Directory printers

Calculating ROI

Although there are many benefits to reseparating customer supplied files, the most promoted and fairly easy to justify is in regards to reduced ink usage - typically suggested as a savings of around 20% in CMY inks with an increase of about 6% in K ink used while maintaining the same visual appearance in presswork.

Based on that figure, calculating a return on investment seems fairly straight forward. For example - based on the industry average of ink consumption for a sheetfed printer being about 2% of their gross earnings, a $10 million dollar a year printer will spend $200,000 a year on ink. If they reduce their ink usage by 20% they will save about $40,000 a year in ink costs. Theoretically, if the printshop spent $10,000 on a reseparation solution their payback time would be just three months and they will have saved $30,000 in the first year of implementation - a very good investment.

However, the promoted savings are typically based on the ink saving resulting from converting a "UCR" separation into a "heavy GCR" separation (see part 4) and then extending that savings to total annual ink usage - a method which doesn't really reflect the actual print production experience. One reason is that the majority of separations are already done using GCR since it is the default setup with the most popular separation profiles used by Photoshop. For example, this image was separated using Photoshop's defaults:while this:was separated using heavy GCR. In this case the resulting CMY savings is about 15% with an increase in K ink consumption of about 6%.

Another consideration is the amount of CMY used in daily production vs the amount of black only presswork. For example, if your typical color ratio usage is low on CMY vs K as in this example:
you will not benefit as much as if this is your typical CMY to K ink ratio:
So, to get a better idea as to what your ink savings ROI might be, you should also consider taking a representative sampling of materials that you are currently receiving and examine the separation techniques used (part 4 will have some tips for determining separation technique used). You might also look at your current presswork to determine the ratio of CMY ink usage (where the savings should occur) relative to black ink.

Benefits of GCR in the press room

Ink usage

The difference in ink usage of GCR vs a UCR separation, for the same final visual appearance on press, can be substantial. While ink usage, and therefore ink savings, is image content dependent, it is not unusual to see an average reduction of 15-25% in ink usage with GCR separations compared with UCR separations (see art 1 for ROI). Note that the savings occur primarily with the often more expensive chromatic inks.

Color stability
Because a GCR separation uses a non-chromatic color – black – throughout the tonal range and reduces the proportion of C, M, and Y in the mid- and quarter tones, the color in GCR separated images is more stable as solid C, M, and Y ink densities naturally vary through a press run. Note, however, that the added stability means less ability for the press operator to move color if required. For many printers, the increased color stability is a perfect compliment to the industry trend for a “by the numbers” print manufacturing process.

Other benefits
• Reduced make-ready times/faster start-ups/less wastage
• Harmonized separations enhance press form printability
• Reduced fan-out or web growth
• Dramatic improvement of image appearance when slight press misregistration occurs
• Reduced drying times
• Higher printing speeds
• Improved repeatability of print jobs
• Grey balance within images is more stable

Key basic concepts

Converting raster images from an RGB colorspace into a print CMYK colorspace has two significant impacts:

1) Typically a compression and alteration of colors as the image is transformed from the original RGB gamut to the different gamut used for CMYK presswork.
The pixels of an RGB image (left) plotted against the color gamut of a CMYK press (right) shows that much of the original color is outside of press gamut and therefore cannot be accurately reproduced.

2) The on-press printability of the imagery in terms of color stability, press performance/runnability, and ink usage (i.e. cost).
The CMYK image on the left uses more ink than necessary for reproduction and will cause ink drying and offsetting issues. The image on the right does not use enough ink and has compromised color gamut and stability on press.

Converting images from one CMYK separation condition into a different CMYK separation condition by reseparating files is primarily intended to enhance the printability of the imagery while maintaining the appearance of the original CMYK imagery. Put another way, reseparating CMYK files is effectively a way to optimize press forms.

The principle of RGB to CMYK separation

In order to go to press, RGB color images must be converted to their process color counterparts; cyan, magenta, and yellow. An achromatic black channel is added because if the color black in presswork is just made from CMY it can often appear “muddy” or "patchy." Also, making dark colors from the three chromatic process colors can lead to a higher than desirable volume of ink on the press sheet. Neutral colors made up of three process colors are also more difficult to maintain consistent on press as solid ink densities normally vary through the run compared with a neutral made primarily of a single black ink. The net effect of introducing black ink in process printing is a reduction of ink usage/costs, stabilization of color (especially gray tones), and and better printability.

The conversion process is done by taking the 3 channel RGB image, passing it through a 3 channel device independent CIEL*a*b* profile connection color space where the RGB is converted to CMY and the black channel added, and finally outputting the result as a 4 channel CMYK image.
The two approaches to replacing chromatic colors with black

The colors in an image can be made up of a variety of different percentage combinations of the process colors while still delivering the same final visual appearance. As a result, there are a several ways to introduce the black component in a color separation.

The first separation method is typically referred to as UCR (Under Color Removal) Click image to enlarge.Whenever large percentages of three process inks are together to form a color, there will be a substantial neutral or grey component. That is to say, past a certain point, adding more of one of the C, M, or Y process colors simply darkens the result. So, instead of simply adding more color, the UCR formula uses black ink to replace a partial amount of the other process colors in the shadow areas and in neutral colors. For example, in a screen tint build of 50% cyan, 40% magenta, and 45% yellow, magenta acts as the greying process. So, the 40% magenta value indicates the largest potential amount of black that can be added in place of a percentage of some of the other colors.

The second separation method is typically referred to as GCR (Gray Component Replacement) Click image to enlarge.GCR is a specialized form of under color removal which involves a more general replacement of chromatic inks - not just the close to neutral ones as with UCR. In a GCR separation, black is substituted for a partial amount of the process colors in all areas where the three chromatic colors are present – even in lighter tones. That added range of color where CMY can be replaced with black is the key to how GCR reduces CMY ink usage.

Both UCR and GCR separation techniques use a defined parameter to limit the maximum amount of ink that will be used in the darkest part of the image. This maximum total dot percentage is referred to as either TIC (Total Ink Coverage) or TAC (Total Area Coverage) and is determined according to the type of presswork and substrate that the images will be used for.

The difference in the range of the black channel makes it fairly easy to determine which method was used to create a particular CMYK separation. With a UCR separation the black printer appears as a “skeleton” image usually missing tones around the 0%-30% range while the CMY looks very colorful. In contrast, the black printer in a GCR separation looks more like a full range greyscale image while the CMY looks washed out.

CMYK to CMYK transform – Reseparating documents for press

Reseparating CMYK makes it possible to optimize CMYK data targeted for one, or more, print conditions to work with the required single CMYK print condition. It further allows the changing of the maximum total ink area coverage as well as the application of GCR separation techniques to maximize ink savings and on-press color stability. Reseparating images also renders all separations in multi-page and multi-file source input projects such as magazines and newspapers to a common, optimized, color separation format which ultimately enhances the print-ability of the job.

The reseparating of documents is typically done using a sophisticated DVL (DeVice-Link) profile which may be embedded within the primary prepress workflow/RIP or housed in a dedicated system linked to the main prepress workflow. Unlike ordinary ICC profiles, DVL profiles do not describe a specific color space, but instead define the conversion from a specific source color space to a destination color space.The basis for creating a DVL profile is always an ordinary ICC profile. The use of a DVL profile allows the integrity of the color separations to be maintained so that pure C, M, Y, and K screen tones remain pure and that solid (100%) C, M, Y, and K values remain solid.

Potential issues with GCR separations in the press room

Maximum GCR separations may create problems on press depending on the type of work that you do. Overall though, the benefits by far exceed any issues encountered that might occur in daily print production.
Here are the major concerns to watch out for:

• A GCR separation makes extensive use of black ink throughout the tonal range. This places a greater emphasis on the integrity of the black ink and press unit, particularly in a black first down ink sequence. If there are issues of poor transfer, trapping, dot gain instability, or ink/water issues with the black printer the impact on presswork will be greatly magnified with a GCR separation. Also, because black is used throughout the tone range, there is a slight increase in the visibility of rosettes in large, flat, screen tint builds. There can also be a greater risk of black ink traveling down to and contaminating the next ink units. Lastly, because of the greater amount of black ink, there is a possibility of pastels and skin tones to have a grainy appearance.

• Reseparating images using heavy GCR can introduce subject moiré (black screen angle conflicting with dark pattern in the original) where there was no moiré in the original separation.

• Ink savings can be quite dramatic with furniture and other dark imagery, however, if the GCR program is too aggressive and removes all of the Cyan the furniture can simply go red.

• Heavy GCR separations are problematic if the print buyer is the type that prefers to go to press runs and adjust color rather than match the signed-off proof. Since there is little process color to adjust, solid ink density moves to adjust color will not have much impact.

• Poorly implemented GCR algorithms may remove all of the color near a natural trap line in an image - effectively negating the trap. Then, if the press gets a little out of register the reseparation process has created a press issue.

• There is an increased disconnect between gray balance targets in the color bar (CMY no K) and gray balance within the live image area (minimal CMY heavy K). This may cause issues with some closed loop press control systems.

• In coldset newspaper printing where yellow is first down, there will be less yellow ink laid down and hence, a reduced sealing effect of the paper prior to the other process inks being overprinted. This can lead to less saturated appearing color in image reproduction.

• If there are any concerns expressed by the print buyer about the quality of the color in the final presswork, the fact that the supplied separations were not used but substituted with reseparated images may increase the printer's liability for reworks or charge backs.

Considering the various options and offerings

This post is intended to help you narrow down a list of requirements and features (and understand their impact), and provide questions to consider when evaluating a GCR reseparation solution. In general, there is no right or wrong answer, but rather a weighing of values and impacts to determine a best fit for your shop. As a general rule (which is not always true), the simpler the implementation, the less expensive, flexible, and accurate it is while delivering the least amount of ink savings. The more complex the implementation, the greater the expense, flexibility, and accuracy it has while delivering the most amount of actual ink savings.

1) Device Link Profile creation/validation

• Is the DVL profile created/supplied by the vendor, created by the customer, or supplied by a third party?
In general customer created DVLs require greater customer sophistication while supplied profiles equal greater simplicity.

• What is the DVL profile created from? Generic industry reference characterization data/profile (e.g. SNAP, GRACoL 7, SWOP), or customer specific data?
In general, customer specific data provides the best potential for quality since industry data may not reflect actual shop conditions even though the shop may be running to an industry standard. Industry derived DVL profiles provide a simpler implementation.

• Who supplies the customer specific characterization data - the customer themselves, the vendor or third party measuring customer supplied press sheets, the vendor or third party doing on-site press runs?
In general, customer supplied characterization data is the least reliable, vendor or third party conducting on-site press tests the most reliable.

• How will the press run that will be measured to provide the characterization data, be validated to ensure that it is operating within normal parameters and hence will provide reliable data?
In general, it is worth having the press checked for mechanical and chemical condition by the vendor or third party prior to any characterization run

• How will the DVL/or solution be evaluated in order for customer acceptance and sign-off of delivery of the solution?
In general, there should be specific performance criteria (e.g. maintain single color colors, no contamination of solid colors, etc. (subject will be covered in next post.

• Will validation be based on a proof (pre- and post-reseparation), press run (pre- and post-reseparation), or data (pre- and post-reseparation)?
In general, a soft on screen and hard physical proof (pre- and post-reseparation) are sufficient to show color integrity and ink usage reduction.

• Can the reseparation parameters of the DVL profile be changed by the customer or does this require vendor intervention?
In general, allowing the customer to adjust the parameters themselves makes for a finer tuned process that can be quickly adapted should press conditions change. However, it requires a degree of customer knowledge to be successful. If the vendor needs to be involved, then fine adjustments will take longer and likely be more expensive. However, it does place responsibility on an organization that likely has greater resources and knowledge which, in the end, may lead to a better final result.

2) Reseparation implementation

• Is the GCR reseparation done inside the main prepress workflow or is it run on a dedicated workstation?
In general, GCR reseparation done inside the main prepress workflow provides a simpler, faster process. GCR reseparation done on a dedicated workstation provides a more customizable and updatable solution. Some vendors offer both, or allow the customer to scale from one to the other as circumstances change.

• Does reseparation occur during the initial PDF refine stage, after the PDF refine stage (perhaps at a dedicated workstation) or during the rendering of the files to plate?
In general, early stage reseparation means that print buyers can see that their files have been altered/optimized if they proof the files using a remote soft proofing solution or if they receive the files back from the printer either during the production process or afterwards. This may, or may not, be a concern. Late reseparation at the render stage means that the altering/optimization would be hidden.

• Can the solution handle vector graphics separately from raster images?

• How does the solution handle documents containing a mix of RGB and CMYK images?

• How does the solution handle documents containing a mix of embedded profiles?

Testing the selected solutions

Once you've narrowed down the solutions that you think will provide the best fit for your business, it's a good idea to run a test to see how closely their promise meets their performance. To perform a test requires running a test form through the solution and examining and comparing the results.

This suggested press test form can be used as a basis to build your own.The following test elements are designed to provide an objective evaluation of the solution's performance.

Targets 1-4 and 18 are designed to test the ability of the solution to maintain the integrity of pure single and two color builds. The elements should be butted together and trapped to see how trapping is affected.
1 - CMY gray balance vignette and step wedge.
2 - CMY brown balance (CMY have the same tone values) vignette and step wedge
3 - C, M, Y, K vignette and step wedges should remain single color with tone values preserved.
4 - B (C/M), R (M/Y), G (C/Y) vignette and step wedges should remain two color only.
18 - Maximum TIC/TAC targets ranging from too low (e.g. 230%) to too high (e.g. 400%) to see how the solution brings TIC/TAC to a common requested value.

The following test elements are for subjective visual evaluation.

7 - Gray balance image separated GCR using PhotoShop defaults.
8 - Gray balance image separated UCR using PhotoShop defaults.
7 and 8 Are intended to test preservation of gray balance through reseparation
9 - Saturated and deep skin tone colors. Should be separated according to your existing standard.
10 - PDF file to test ability of solution to handle mix of images and text.
11 - GCR (medium) separation using black limit 100% and maximum ink 320%
12 - GCR (medium) separation using black limit 90% and maximum ink 280%
13 - UCR separation using black limit 100% and maximum ink 320%
14 - UCR separation using black limit 90% and maximum ink 280%
11 to 14 Are intended to reveal how different types of separations are brought to a common setting through reseparation and whether any color appears distorted as a result.
15 - Deep color with lots of black using UCR with a black limit 100% and maximum ink 320%. This is intended to show any loss of richness or any color shift in deep shadows as well as artifacts in the transition to shadow tones.
16 - High key pastel original, originally separated using UCR with a black limit 100% and maximum ink 320%. This is intended to show how pastels with heavy black GCR performs on press.
17 - Vector (e.g. Illustrator) graphic. To see how, and if vector graphics are affected by reseparation.

If you have any unusual image requirements, such as graphics that overlay spot colors on top of CMYK, or process color images that include a 5th or 6th spot color channel, be sure to include those as well.

Once the test elements are returned by the prospective vendor they should be imposed in line with the original test elements. Add elements:
5 - Gray balance bars which go the width of the press sheet
6 - Your preferred color bar.
The top half contains the test images/files as supplied by you to the vendor. The bottom half are the same images processed by the vendor's GCR reseparation solution.Create two proofs of this imposed form.
Cut the proof in half so that you have two pieces - one that contains the original images and the other containing the processed images. Take the proof of the processed images and cut it into strips through the center of the images. Then lay the strips on top of the proof of the original art.The images should fuse together looking no different where they cross over. Examine the objective elements 1-4 to determine whether they have maintained their integrity and 18 to determine how the different TIC/TACs have been handled.

The next step is to validate the increased color stability on press by going on press with the imposed form. The press operator should be instructed to run to the appropriate solid ink densities (SIDs) trying to even them out across the width of the sheet. The proof should not be available - the press sheet should simply be "run to the numbers."
Once the SIDs have been reached - pull out several sheets for later evaluation. Then raise the magenta SIDs by 10 points (i.e. 1.35 M becomes 1.45 M) and pull some sheets. Then raise the magenta SIDs by a further 10 points (i.e. 1.45 M becomes 1.55 M) and pull another few sheets. Then do the same increase, but with the cyan SIDs. Again, pull sheets at the appropriate increased SIDs.

Because the images are inline with each other, the increase in SIDs gets applied to both conventional and reseparated images. By cutting the form in half and comparing strips of the images, as was done with the proof, you should be able to see the difference in color stability between the processed images and their originals.

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