Color management in the graphic arts
 

Color management in the graphic arts

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Martin Bailey, CTO of Global Graphics, discusses color management in graphic arts using the Harlequin RIP. ...

Martin Bailey, CTO of Global Graphics, discusses color management in graphic arts using the Harlequin RIP.

Martin represents Global Graphics on a number of industry bodies and standards committees including those developing PDF and XPS: he is the primary UK expert on the committees working on PDF/A, PDF and PDF/VT.

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Color management in the graphic arts Color management in the graphic arts Document Transcript

  • ColoR ManaGeMenT In THeGRaPHIC aRTsHarlequin RIP solutions ®IntroductionColor printing using a printing press has been around for slightly over 100 yearsusing a variety of techniques from stones and grease pencils to films and plates tomore recently using “digital” processes for page design, imaging and plate making. This white paper detailsWe see color because of the RGB sensitive cones in our eyes and sophisticated Global Graphics Colorprocessing in our brains. To observe color there must be a light source, an object Management solutionsthat the light source is illuminating and an observer that is viewing the object. Thecolor of light is defined in terms such as wavelengths and radiation whereby using the Harlequin RIP.different wavelengths produce different colors within the human visual spectrum.The term color management has recently been used to describe the digitaltransforms from different color spaces in the digital revolution. In actuality, any manipulation of the separations to beprinted either through etching of printing stones, changes of exposure times when using process cameras, orchanging calibration curves in a digital system are all forms of color management since they all are intended tochange/improve the color output of the final printed piece. It is beyond the scope of this white paper to go intothe history of how color was produced and enhanced in previous pre-press systems. Rather this white paper willdefine current color management concepts and offerings from Global Graphics with regards to color workflowsthat require color management.Photographic & Printing PrinciplesIn today’s graphic arts market images are still captured on film and scanned but there is a steady growth in digitalcameras as well. This in a way is putting the scanner in the camera. The result in either case is an original scenecaptured in color through a lens, either via the scanner lens when scanning film or a digital camera lens capturedinto a digital file format. In most cases this is a RGB color space. Some scanners can generate a CMYK filehowever this will limit the re-usability in later design work and therefore will not be included in the discussion. Thecaptured digital file is typically viewed on a RGB monitor for editing incorporating enhancements that may includesharpening, color correction and cropping. This would be considered the starting point in a pre-press workflowwhere this original scene capture is going to be used in a future printed piece.Color management if implemented, could already be in use in this scenario by mapping the RGB colors in theoriginal file into the monitor’s color space, or stated another way, the best representation of those original colorswithin the color gamut limitations of the monitor. What this concept suggests is that the scene brightness ordynamic range in the original is often larger than the RGB color space that has been used to render it andtherefore color management must be used to compress the colors in such a way as to keep the visual 1
  • appearance of the original scene on the monitor as close as possible to that original scene. This form of imagerepresentation is known as additive and is emissive since the monitor is emitting light.Graphic arts printing makes use of CMY (K) inks which are cyan, magenta, yellow and black inks. If the CMY inkswere spectrally pure one could print all colors very accurately and black would not be necessary. Primarily due tocosts in their manufacture printing inks are not pure so a combination of CMYK inks will provide the bestopportunity to print the greatest number of colors within a press, ink and paper combinations gamut. CMYKcolors are produced by reflected light. Mixing colors together when printing a color image, the inks togethersubtract wavelengths of light, thus the more ink on the paper the darker the color. This system therefore is asubtractive color model, as opposed to the RGB additive model. In general the number of colors printable in aCMYK print space is considerably smaller than a RGB emissive space so once again the original scene must befurther compressed. Illustration 1 below is an example of a RGB monitor space and a smaller CMYK print spacetypical of magazine printing. Color management can be one way of further compressing the colors in the originalscene in such a way that insures the printed image looks visually correct and is representative of the originalscene. The quality of this process from original scene to printed image on a printing press, in today’s digital world, isreally color engineering. It requires a combination of color software specifications, industry specific standards andtools to build the color components of a pre-press workflow, an area that Global Graphics and the Harlequin RIPhave been at the forefront of since the beginning of printing color digitally. Illustration 1 – RGB Monitor Color Space vs. Coated Paper Color SpaceCompany logos, artwork, and Black and White imagesOften times a design includes a company logo or artwork that has a specific color requirement for example colorsthat are from a proprietary library like Pantone or a combination of CMYK colors that are known to be withinthe press gamut of the printing system and must be printed in such a way as to achieve a specified color. TheHarlequin RIP includes colorimetric lookup tables for almost all of the Pantone colors in named databases andadditional color databases can be added.These databases insure that customer colors are printed accurately but alsothat digital proofs can accurately print these custom colors when proofs are part of the workflow and approval process. 2
  • Black & White images are often found in design layouts and with the Harlequin RIP these can be handleddifferently than color work. In other words, should one have multiple black & white images within a layout, fromdifferent sources, a black and white profile could be installed in the RIP and then applied to all black & whiteimages. This would insure the tonality of these black & white images are consistent throughout the publication.Device Dependence vs. Device IndependenceBoth RGB and CMYK color spaces are known as color models that are device dependent. Thus if you view thesame RGB image on two different monitors they will look different. This is because the monitors likely have slightlydifferent white and black points and therefore the internal gamuts are different as well. This can be corrected usingcolor management. In the CMYK print space if the same CMYK values are sent to two different printers theprinted pieces would likely look different because the inks are different. The solid ink densities at the time ofprinting would also likely be different, another reason the printed pieces would look different. Thus the CMYKvalues in the file are device dependent. They would produce predictable color on a single device but the CMYKfile would produce different colors on a different device. In some workflows this would be okay and nothing morewould be needed but in many modern digital workflows often times the final output device is not known at thedesign stage. This again can be handled with color management if engineered properly.Color Management elementsThe elements, the color bits if you will, that comprise a color management system for the graphic arts contains ata basic level a source/input profile, a CMM and a destination/output profile. The input profile defines the colors inthe source or input space in such a way as to align RGB or CMYK values to LAB (a uniform intermediary colorspace). The processing engine is the intermediary place to process/convert the profiles, and is called a ColorManagement Module (CMM). The CMM contains the Profile Connection Space (PCS) which is a conversion colorspace. The destination or output profile takes the LAB values that the CMM has identified in the source image andprofile, looks them up in the table of LAB to CMYK values in the output profile, which then gives the appropriateCMYK values to print a specific color on the printer or press sheet (via the plate and press) as needed.The power in this solution and working model is a designer working in RGB does not have to know where theirwork is going to be printed and still obtain a quality print within the gamut limitations of the output device. Obviouslya pre-press system could be optimized if the output destination is known along with the ability to produce coloraccurate digital proofs but the principle is that once a color space is converted to LAB via a profile and CMM, thedestination for output can change and an acceptable print can be produced on a variety of output devices. This isknown as device independent color and used in some graphic arts workflows. The profiles mentioned can be ICCprofiles based on the ICC specification (more information can be found at www.color.org) or proprietary formatsthat are capable of providing additional parameters unavailable within the ICC profile specification.Defining Workflows - late Binding/early BindingThe term Color Management has gained in popularity when talking about software, hardware and measurementdevices used in colorimetry and likely has somewhat different definitions. Part of the challenge in succeeding atprinting in the digital age is to understand the elements that must be controlled and “engineered” to insurepredictable and color accurate prints that meets the requirements of one’s clients. Marketing people have alsorecognized the inherent advantages and often tout that their products(s) in the workflow chain, be it software orhardware is color management capable. So, if I am a designer do I turn on color management in image 3 View slide
  • manipulation software, then my page layout application then my RIP? Isn’t this going to insure that my colorelements are color managed properly? Well, the answer depends, and the secret is understanding what it takes tobuild a color managed workflow, the elements involved, and when color management would best be applied.The Harlequin RIP is fully capable of performing all of the necessary color management because it can make useof ICC profiles and Harlequin HQ profiles (which contain extra color information unavailable in ICC profiles), toconvert between color spaces as needed on the way to the intended output device. The Harlequin RIP CMMwould be considered late-binding as all of the color management decisions are performed at the last stage of theworkflow, that of converting the page files or PDL into a raster form, appropriate for the intended output device.Thus a designer who receives digital images from a digital camera in aRGB color space can place those images on a page layout and providedthe workflow is designed properly and the correct profiles are either "Color management requiresattached to the images or installed and made available in the RIP, all RGBimages can be handled and optimized properly at the RIP. CMYK image a combination of colordata can be also color managed accurately at the RIP, again with eitherproperly attached profiles or profiles that are in the RIP. PDF/X files are software specifications,also color managed correctly in the Harlequin RIP as the profiles industry specific standardsselected for output and identified in the PDF/X format are honored.There are options to use additional profiles to produce color accurate and tools to build the colorproofs to a variety of output devices. Yet some other examples of thecolor management capabilities of the Harlequin RIP with a properly components of aengineered workflow includes the ability to accurately color manage pre-press workflow".mixed color space files where perhaps the pictorial images are in RGBand the advertisements are in CMYK. Perhaps a last minute press changeis required. Again if the appropriate profiles have been installed anemulation workflow can be color managed to redirect/repurpose the data for a different press condition keepingthe colors accurate. Finally, the graphic arts standards bodies continue to develop standards that define differentprinting conditions for different industry segments. Global Graphics is an active participant in these trend settingindustry standard groups. The Harlequin RIP continues to be engineered to insure these standards areimplemented properly thereby insuring that the color meets a given standard and client’s expectation. Recently forexample we obtained SWOP® certification on an ink proofing device to demonstrate our capability in this area.Building WorkflowsThe key to predictable color is the ability to configure the pre-press workflow such that files prepared for printingwhen arriving at the RIP will be color managed appropriately. With the advent of the digital revolution this processhas caused a great deal of confusion. The Harlequin RIP has been developed to handle a variety of situations byproviding flexibility in setup of color managed workflows. One avenue is via a JDF front end which containsinstructions on how files are to be managed be it fonts, screening, or color management. The Harlequin RIP is JDFcompliant and remains current with the JDF specifications as it evolves. PDF/X file formats developed to improvefile submission accuracy to printers can have a profile and specific color requirements. The Harlequin RIPrecognizes PDF/X file formats, provides pertinent information as to file contents and RIP configuration insuring thatcolor management is honored and handled correctly.For many types of printing a digital color proof is required to insure that the designer, customer and printer are allworking towards the same end point. This requires considerable engineering to insure that the digital proofs are 4 View slide
  • representative of future printing. A source/input profile must be created for the press and a destination/outputprofile must be created for the proofing device. For each combination of paper, ink, screening and resolution, theprinting characteristics of the printing device changes and therefore the color will change as well. Once the colorcharacterizations are built into profiles for each printing condition they can be installed into the Harlequin RIP andused in a variety of Harlequin ColorPro™ setups to meet customer requirements. Illustration 2 below highlightssome workflow options using profiles and a variety of input color spaces.Illustration 2 – Color Spaces and Color Conversion RoutesHarlequin setGoldPro™Part of the success of developing color managed workflows is the building of profiles that capture the colorspace/gamut of a device accurately and then have these profiles available to the RIP Harlequin SetGoldPro uses .patented gray balance techniques along with sophisticated algorithms to build profiles that are optimized for theHarlequin RIP In fact Harlequin SetGoldPro provides the ability to work with a specific RIP so that when the data .has been imported and a profile generated, the resultant profile is placed in the appropriate location within RIP andalso made available in Harlequin ColorPro within the RIP for immediate use. This insures both installation and coloraccuracy as the profiles have been optimized for the Harlequin RIP and press or proof condition being characterized.Harlequin ColorProColorPro is the GUI interface to the Harlequin RIP kernel where all color managed workflow decisions are setup.This window is feature rich and has evolved to be useable by novice users as well as for operators needing todesign complex color managed workflows. It should be mentioned again that setting up a color managedworkflow at the RIP stage is known as late-binding and offers very powerful control over the color managementof jobs when the source is known. It is also very controllable even when jobs files from unknown sources (a newcustomer for example) need to be accurately output in preparation for printing or plate making. In addition to thefeature set found in the Harlequin ColorPro window many additional features can be initiated via PostScript® 5
  • ColorPro GUIcommands to even further control how page elements within a file are color managed. The Harlequin ColorProwindow was designed to allow for most operations but not so complicated as to have limited usability. For moreinformation please refer to the Harlequin ColorPro User’s Guide for the Harlequin RIP.ConclusionColor Management is a combination of color engineering and correct workflow setup. This includes the necessarytransforms to move between different color spaces, applications to build those transforms into profiles, and adecision on where the color transforms take place. If one needs color accurate proofs the obvious location is atthe RIP. If one is receiving job files that are not tagged for specific printing application these files will needconversion. Again the Harlequin RIP is ideally suited for this purpose as all of the color bits can be loaded andconfigured to correct color issues within files (wrong or no profile attached to the submitted images). If one needsto change the printing condition the job files will need conversion. This example would be through emulation asseen in illustration 2 and is a way one can repurpose data or optimize data prepared for one press that at the lastminute needs to be printed on a different press.Using the Harlequin RIP and color tools optimized for the RIP insures that any workflow challenges can beaccurately configured at the RIP prior to printing, plating or proofing. Keeping these color decisions available to thelatest possible stage helps limit mistakes and provides a greatest flexibility to optimizing jobs that may have beensetup for one print condition but now need to run in a different set of conditions.one TECHNOLOGY TO SUPPORT one SCALABLE ARCHITECTURE one SOURCE September 2007 Global Graphics Software Inc. Global Graphics Software Ltd. Global Graphics KK 31 Nagog Park, Suite 315, Acton 2nd Floor, Building 2030 704 AIOS Toranomon Bldg. MA 01720, USA Cambourne Business Park 1-6-12 Nishishimbashi, Minato-ku, Tel: +1-978-849-0011 Cambourne, Cambridge Tokyo 105-0003 Fax: +1-978-849-0012 CB23 6DW UK Japan Tel: +44 (0)1954 283100 Tel: +81-3-6273-3740www.globalgraphics.com Fax: +44 (0)1954 283101 Fax: +81-3-6273-3741the smarter alternative, Harlequin, the Harlequin logo, Eclipse Release, Genesis Release, SetGold and ColorPro are trademarks of Global Graphics Software Limited which may be registered in certain jurisdictions. GlobalGraphics is a trademarkof Global Graphics S.A. which may be registered in certain jurisdictions. Adobe and PostScript are a trademarks of Adobe Systems, Inc., which may be registered in certain jurisdictions. All otherbrand and product names are trademarks or registered trademarks of their respective owners. All specifications subject to change without notice. Global Graphics makes no warranty and accepts no liability for any loss 6or damage arising from the useof information or particulars in this document. Copyright © 2006 Global Graphics Software Limited. All rights reserved.