Introduction to Color Management with PDF files

Transcription

1 ColorLogic GmbH Introduction to Color Management with PDF files Preparation of Media-Neutral PDF Files

2 2 Impressum Copyright 2013 ColorLogic GmbH. All rights reserved. Reproduction of this information - including excerpts thereof - is only permitted with express, written consent. ColorLogic GmbH assumes no liability whatsoever for the accuracy of the information contained herein as regards the details and descriptions of standards and third-party products. Adobe, Acrobat, Photoshop, Postscript and PDF are registered trademarks of Adobe Systems Inc. Windows, Windows XP and Windows 7 are either registered trademarks or trademarks of Microsoft Corporation in the USA and/ or other countries. Apple, Macintosh, Power Macintosh, MacOS, MacOSX and ColorSync are trademarks of Apple Computer Inc. in the USA and/or other countries. PANTONE and other Pantone trademarks are the property of X-Rite Inc. All other trademarks are the property of the respective owners. 2nd Edition, Version 4.0, March 2013

3 Content 3 Introduction to color management with PDF files Color spaces for objects and the complete document...4 Rendering intents for ICCbased color spaces...5 Spot colors, DeviceN and alternate color spaces...5 The different PDF/X versions...6 Converting and optimizing PDF files...8 Color management problems in the PDF workflow Lack of black point compensation Problems when handling black and gray Unsatisfactory quality of ICC-based CMYK-to-CMYK conversions Special features of the overprinting model in PDF files Differences in gamut mapping when combining profiles from different manufacturers The soft proofing problem with Multicolor separations Transparencies...13 Transparencies in the PDF/X workflow and color management...14 Special features of the color management of PDF files with flattened transparencies...14 Special features of the color management of PDF files including transparencies...15 Suggestion for applying SaveInk or TAC reduction to PDF files with transparencies...16 Preparation of media-neutral PDF files Preliminary thoughts on media-neutral PDF generation Generation of media-neutral PDF files and color conversion to CMYK Realization of simple, reliable and efficient production...19 Important and less important colors in the layout...19 Media-neutral production in the CMYK workflow...20 Advantages of the CMYK method...21 Media-neutral production in the RGB workflow...22 Advantages of the RGB method...23 The RGB workflow in Adobe Photoshop, InDesign and ZePrA Selection of the perceptual rendering strategy...24 ColorLogic Black Point Compensation...24 ColorLogic Absolute Compression...24 DeviceLink-based conversion from RGB to CMYK Soft proof and proof of an RGB master image Composition of master documents in InDesign Color conversion of the master documents in ZePrA Switching between true spot colors and their CMYK rendering...30 Printing spot colors as process colors...31

4 4 Introduction to color management with PDF files PDF is the most versatile, but also the most complex, data format in the graphic arts industry. It is a standard format for submitting printing data to printshops, and is also used for many other applications if it is important to view, print and archive complex documents in a fixed layout on different operating systems. The PDF format was originally invented by the Adobe company. In 2001, Adobe decided to include the format in ISO standardization, making PDF an open, vendor-independent format. Depending on application, there are different PDF versions in ISO standardization that only permit certain types of PDF files in order to reduce the complexity and permit more robust use. The most important ISO Standards for PDF are: ISO 32000: PDF 1.7 without restrictions, for the most flexible possible use ISO PDF/X: PDF files for exchanging printing data ISO PDF/A: PDF files for archiving documents The sections below concentrate on PDF files for exchanging printing data, and thus on PDF/X. Color spaces for objects and the complete document Internally, a PDF file consists of numerous individual objects. In the case of printing data, they can be images, vector graphics, and text objects. As regards the color space, various options are available for every single object. This makes PDF a highly complex file format, especially when it comes to color, with considerable potential for errors in the event of incorrect operation. Apart from there being fundamental differences in the nature of the color space, such as Gray, RGB, CMYK, Lab or spot color, individual PDF objects can also have individual ICC profiles. The table below provides an overview of the most important color spaces in PDF files for print production. Without ICC profile With ICC profile Gray DeviceGray ICCbasedGray RGB DeviceRGB ICCbasedRGB CMYK DeviceCMYK ICCbasedCMYK Lab - - Spot color DeviceN -

5 5 For submitting color-managed printing data, it is essential that colors in a PDF file be unequivocally defined with the help of color profiles. The ISO or PDF/X standard introduced an additional Document Color Space, also known as the Output Intent, to supplement the color spaces of individual PDF objects. As a rule, the output intent is an ICC profile that describes the output color space for which the PDF file was created. The output intent is often used incorrectly in practice, in that the profile for the required output intent is entered instead. If, for example, all the CMYK data are in the ISO Coated V2 color space, but the PDF file is to be printed by the newspaper printing process, the output intent still has to be set to ISO Coated V2, rather than ISO Newspaper. Conversion from ISO Coated V2 to ISO Newspaper can be performed with the help of DeviceLink profiles in ColorLogic ZePrA. Rendering intents for ICCbased color spaces If individual PDF objects with embedded profiles are present, a conversion method (rendering intent) is also assigned to each of these individual objects. These are the four rendering intents specified in the ICC standard. The additional Black Point Compensation used specifically in Adobe applications cannot be selected in current ISO-compliant PDF files (status 2011) and workflows. The section on PDF and color management problems on page 8 goes into more detail on this subject. For special applications, where the composition of layout documents and subsequent PDF generation take place under controlled conditions, it may make sense to use other rendering intents than defined in the ICC standard. Additional rendering intents are available in the ZePrA workflow product, for example, and discussed in greater detail on Media-neutral production on page 18. Spot color patches with alternate CMYK colors. Spot colors, DeviceN and alternate color spaces It is possible to print not only with CMYK on the press, but also with spot colors. The corresponding color space description in the PDF format is DeviceN. To simplify the rendering of spot colors with a viewer on a monitor, or on a printer without true spot colors, a DeviceN color space additionally has what is known as an Alternate Color Space. This alternate color space is generally either a CMYK or a Lab color space.

6 6 The DeviceN color space additionally includes a formula (Tint Transform) that enables a viewer to convert from DeviceN to the alternate color space. In the simplest case, the DeviceN color space has one channel (separation or spot color), the alternate color space being created with a CMYK color, and the formula is a multiplication of the color value. For instance, if an alternate color for 100% of the spot color is defined as C100 M50 Y0 K0, a 50% spot color for output without true spot colors would be converted to C50 M25 Y0 K0, which would hardly correspond to the real color appearance of the spot color. Most current DTP applications have problems with the true-color rendering of spot color gradations, and with the overprinting of spot colors on each other or in conjunction with process colors. To get a reasonably serviceable rendering of spot colors on the monitor and the proof printer, it is advisable to use Lab color definitions as the alternate color space. In this respect, see also the section on The soft proofing problem in the manual "Multicolor Separations". The different PDF/X versions The ISO PDF/X standard consists of various sub-standards, the most important of which are outlined below. PDF/X-5pg PDF/X-5g PDF/X-5n PDF/X-4p PDF/X-4 PDF/X-3 PDF/X-1a CMYK spot colors (device dependent) Media independent colors - Transparencies, level, JPEG2000, OpenType - - Refence for output profile N-Color (e.g. Hexachrome) External graphic elements (images, adds)

7 7 On the basis of PDF/X, the Ghent Working Group ( elaborates concrete workflow recommendations and practical aids, where the demands on a printable PDF/X file are described in even more detail than in the ISO standard. PDF/X-1a: PDF files without transparencies and without objects with ICCbased color spaces. The device-specific colors must correspond to the color space of the ultimate print. For instance, if CMYK is used on the press, DeviceRGB is not permissible as the color space. The document color space absolutely must be described with an output intent. If, for example, a CMYK profile is present as the output intent, this means that all DeviceCMYK objects must be in this printing color space. In a PDF/X-1a, spot colors on the press are described by the DeviceN color space. PDF/X-3: Same as PDF/X-1a, but with the additional option that individual objects can be present in ICCbased color spaces, e.g. including RGB and CMYK objects with embedded ICC profiles. Transparencies are not permitted. PDF/X-4: Same as PDF/X-3, but with the additional option of transporting transparencies and levels in the PDF file. PDF/X-5: Same as PDF/X-4, but with the additional option of incorporating external objects. In the case of PDF/X-5n, for example, this makes it possible to link a Multicolor ICC profile as the output intent. Color management of PDF files: normalizing The color management of PDF files is only partly described in the framework of ISO standardization. If PDF/X-3 or PDF/X-4 files are involved, the specification is that the embedded ICC profile converts all ICCbased PDF objects to the document color space (output intent) by means of the embedded rendering intent. This is often also referred to as normalizing, since the colors of all objects in the PDF file are based on a consistent "normal" (document color space/output intent) after this step. If just an ordinary PDF file is available for normalizing, rather than a PDF/X file, this PDF file usually contains no indication of the document color space (output intent). For normalizing, you then have to decide on a document color space to which any ICCbased PDF objects present are converted. If a PDF file contains objects in the ICCbasedCMYK color space, normalizing according to ISO / PDF/X leads to compulsory ICC conversion from the ICC profile of the individual PDF object to the document color space. This can result in undesirable artifacts that are discussed in more detail in the section on Color management problems in the PDF workflow.

8 8 Converting and optimizing PDF files Once a PDF file has been normalized, you can subsequently perform optional color conversion or optimization. You should convert it if, for example, the delivered document color space (output intent) does not match the ultimate printing process. If you work in accordance with international printing standards, it is advisable to carry out conversion using DeviceLink profiles that represent conversions between standard printing color spaces. Correspondingly optimized and tested DeviceLink profiles for standard printing color spaces are available from ColorLogic (see our manual "Principles of Device- Link Technology"). When converting from CMYK to CMYK, you should always fall back on DeviceLink profiles in order to avoid the problems explained in the next section. When optimizing, the document color space already corresponds to the subsequent printing process. However, the color composition of the printing data is further optimized in accordance with the printing process. A less intensive form of optimization would be, for example, to limit the maximum total amount of color while retaining the existing black composition. A more intensive form of optimization would be to consistently replace CMY color components by black to save ink. Again, it is advisable to use DeviceLink profiles for optimizing printing data. ColorLogic offers corresponding preconfigured DeviceLink profiles for various printing standards. Color management problems in the PDF workflow When it comes to the color management of PDF files, there are seven problem areas that it is important for you to know about: 1. The lack of black point compensation 2. Problems when handling black and gray 3. Unsatisfactory quality of ICC-based CMYK-to-CMYK conversions 4. Special features of the overprinting model in PDF files 5. Differences in gamut mapping when combining profiles from different manufacturers 6. The lack of true-color rendering of spot color components and combinations of spot colors with each other, and in combination with process colors 7. Transparencies can lead to problems during color conversion

9 9 Black point compensation in CS Color Settings. 1. Lack of black point compensation In Adobe CS applications, such as InDesign, color conversion with the relative colorimetric intent and activated black point compensation is a default setting across all programs. Black point compensation guarantees that, for example, RGB black is mapped onto the darkest tone of the CMYK target color space. This ensures that no definition is lost in the case of RGB colors that are darker than the darkest CMYK black. Let us assume that you use InDesign to create a PDF/X-3 or X-4 file containing images with embedded profiles. If you do not pay attention when positioning the images in the document, the relative colorimetric intent is individually assigned to the images as the default setting. According to the current ISO or ISO / PDF/X, you cannot include information for activating black point compensation in PDF files. If, in the next step, the RGB images in the PDF file are processed in compliance with the standard, i.e. with the relative colorimetric rendering intent in this case, this leads to a loss of definition in the image shadows and thus to a different result than with RGB-to-CMYK color conversion in InDesign. The answer to this problem consists in deliberately contravening the standard and using a PDF workflow solution that offers black point compensation for ICC-based conversions. This feature is available in ZePrA and activated by default. Black point compensation in ZePrA.

10 10 Another answer would be to carry out conversion with RGBto-CMYK DeviceLink profiles, rather than normal printer profiles. In this case, you need a PDF workflow solution that automatically uses the right DeviceLink profiles for the respective embedded profiles and the required target profile. This is guaranteed by the SmartLink function in ZePrA (see our manual "Introduction to Our Products". Activated SmartLink functions in ZePrA. 2. Problems when handling black and gray Particularly in the printing process, pure 100% black is a color that should be excluded from "normal" color management. Otherwise, it can easily happen that a black text turns into a four-color text with less than 100% black. The situation is similar, but more complex, as regards gradations of gray. When converting PDF files between different printing standards, it is appropriate to also adjust the gradation of pure gray objects, e.g. in order to compensate for the difference in the tone value increase. In many instances, however, this is no trivial matter because, in the event of a classical ICC workflow, it would be necessary to be able to apply Gray profiles in addition to the CMYK profiles, both for the source color space and the target color space. One practicable approach in such cases is to use DeviceLink profiles where pure CMYK gray of the source is mapped onto the CMYK gray of the target. In addition, the PDF workflow system must be in a position to temporarily convert purely gray PDF objects to CMYK objects with CMY=0, in order to then pass them through the DeviceLink profile. In the event of graypreserving conversion, DeviceN black should likewise be preserved and not converted into CMYK. The combination of ZePrA and ColorLogic Standard DeviceLink Profiles offers this function. Diverse setting options for Gray in ZePrA. 3. Unsatisfactory quality of ICC-based CMYK-to-CMYK conversions If you use classical ICC device profiles to convert from CMYK to CMYK, the result is major intervention in the color composition of the files. Moreover, pure CMYK colors do not remain pure, which is undesirable in connection

11 11 with vector graphics and gradients, in particular. The best answer to this problem is to organize the workflow in such a way that, working on the basis of printing standards, you can manage with as few CMYK-to-CMYK color conversions and optimizations as possible. And you should always use DeviceLink profiles if these applications are necessary. The Standard DeviceLink Profiles from ColorLogic comprehensively cover such applications. Particularly in the event of automatic processing of PDF files in printshops, you should think about a data check, such as described in our manual "Enfocus Switch with ColorLogic ZePrA and DocBees-ProfileTagger". If incoming PDF files contain CMYK profiles in individual objects, many printshops usually remove these profiles, in order to avoid effects like iridescent gradients or contamination of pure CMYK colors. In doing so, they deliberately accept the fact that the color impression on the press can differ from the display of the PDF file on the customer s monitor. Solutions for this field of application should, however, take embedded profiles into account, and have a smart CMM (color management module) that is capable of "on-the-fly" calculation of separation-preserving DeviceLink profiles that preserve pure colors. A suitable DeviceLink profile is selected or calculated fully automatically for each color conversion from the profile of an individual CMYK PDF object to the document color space (output intent). The SmartLink option in ColorLogic ZePrA offers such a function. 4. Special features of the overprinting model in PDF files The overprinting model in PDF files sometimes leads to results that differ from what you would visually expect. If a color with 100% cyan overprints on 100% yellow, the When performing color conversion via ICC device profiles, it can occasionally happen, depending on the source and target profiles, that the overprinting properties of objects change and color shifts occur as a result. If, as a result of conversion, a channel that previously contained 0% of a color subsequently contains just 0.2% or more after conversion, this channel is no longer transparent in relation to the background color, but opaque. That cannot happen if you use good DeviceLink profiles. They ensure that channels that previously had 0% still have 0% after conversion - so overprinting still works! result is the anticipated green. However, if yellow containing just small amounts of contaminating cyan overprints a cyan color, the yellow of the overprinting color cuts out the cyan underneath. The result is then a very different color impression than would be expected. As pure colors are easily contaminated with small amounts of other colors, especially during CMYK-to-CMYK conversion using normal ICC device profiles, the consequences are fatal for overprinting PDF objects. The answer to this problem consists either in performing conversion with DeviceLink profiles that preserve pure colors, or in using the SmartLink function, which likewise preserves pure colors. The ColorLogic DeviceLink profiles and the SmartLink feature in ZePrA offer this function.

12 12 Overprinting problems can similarly occur when converting spot colors, or process colors created as DeviceN, to CMYK. DeviceN and CMYK have different overprinting properties. Depending on the setting of the PDF-internal overprinting parameters, CMYK will overprint, but not DeviceN, or vice versa, despite identical color values. So, the overprinting properties change when the color space changes. The ZePrA Preserve overprinting effects option ensures that the overprinting parameters are adapted accordingly or, where justifiable, that the resultant colors are expressed in the most suitable color space in order to minimize problems of this kind. However, it is generally not possible to completely avoid these problems without flattening the file. ZePrA "Preserve overprinting effects" option 5. Differences in gamut mapping when combining profiles from different manufacturers This problem mainly affects users who want to establish a high-end RGB workflow and address various printing standards or in-house standards of printshops. In such situations, it is perfectly normal that the profiles used for the printing processes were calculated using different profiling software. According to the ICC standard, there is a good match between profiles from different manufacturers if the colors of the source color space can be converted to the target color space 1:1, and if the absolute colorimetric rendering intent is used. However, this is rarely the case with RGB-to-CMYK conversions, which is why it is better to work with the perceptual rendering intent. How highly saturated colors of the source color space are converted to the target color space is a question of the respective profiling software. Similarly, each manufacturer can calculate the perceptual rendering intent in accordance with his technical skills and preferences. The result of this can be that, depending on the profiling software or depending on the combination of profiles from different manufacturers, highly saturated RGB tones and the gray axis are converted very differently, meaning that the images acquire a different look. If a consistent look is to be achieved across all the target color spaces used, either all the profiles used need to be uniformly recalculated using the same profiling software, or a PDF color server has to be used that recalculates the gamut mapping from source to target "on-the-fly". The former is possible with CoPrA, the latter with ZePrA.

13 13 Comparison of three different gamut-mapping conversions of a highly saturated RGB blue motif with profiles from FOGRA39 characterization data. 6. The soft proofing problem with Multicolor separations Let us assume that you have a Multicolor printer profile with seven color channels, for example. Since the launch of Adobe Photoshop CS4, there has been the option of applying this profile to RGB or CMYK image data. However, this converted file is not displayed in its true colors, since the Multicolor profile is used only for separation, not for display on the monitor. The same problem exists with PDF files separated in DeviceN in Adobe Acrobat. A proposal for resolving this problem can be found in our manual "Multicolor Separations". 7. Transparencies Ever since layouting programs have been capable of blending objects into each other with the help of transparency information, this subject has also played an important role in color management. Unfortunately, however, transparencies result in so many variations as regards blending and the order of semi-transparent objects that there is no simple rule for avoiding unwanted color shifts when converting the colors of transparencies. The reduction (flattening) of transparencies prior to color conversion also often fails to produce the desired effect. Flattening in a DTP program or Adobe Acrobat may well result in unwanted conversions and color shifts. A suggestion for dealing with transparencies and color management is given in the following section.

14 14 Transparencies in the PDF/X workflow and color management As the originator of PDF files, you have to decide whether to flatten the transparencies during PDF generation, or whether to retain the transparency settings in the PDF file for printing. With regard to color management, it is also important for a printer to know whether incoming PDF files perhaps contain flattened transparencies, or whether he needs to flatten them himself. If the printer receives a PDF file that has already been flattened, he can process these data directly, without further intervention. If, on the other hand, PDF files containing transparencies are sent to the printer, it may be that the flattening of complex documents by the printer produces a different result than the preview in the layouting program or in Adobe Acrobat Professional. Special features of the color management of PDF files with flattened transparencies If you have a layout where vector graphics or text and pixel images overlap, flattening causes the overlapping areas to be broken up into square fields that are then converted into pure pixel images. After flattening, vector graphics and pixel images with identical color values abut directly. If the color management system handled pixel images and vector graphics differently, the result would be a visually undesirable, visible edge where the two areas abut. Since flattened transparencies are usual in the PDF workflow today, the golden rule applies that vector graphics and pixel images should always be converted using identical color management settings. For CMYK PDF files for printing, this should always be done using optimized DeviceLink profiles in a PDF color server that applies them identically to vector graphics, text and images. This is ensured by the combination of ColorLogic DeviceLink profiles and ZePrA. Unwanted conversions with color shifts and losses of quality are a definite possibility when flattening in a DTP program or Adobe Acrobat. In addition, DTP programs do not use any DeviceLink profiles that would, among other things, support clean conversion of technical colors. Moreover, flattening in DTP programs means that you have to specify the resolution for subsequent platesetting at a very early stage. On top of which, the approach of flattening in the DTP program contradicts the objective (from PDF/X-4) of retaining transparencies up to the last process step. Note: At the time this manual went to press (status May 2011), there were still only very few test tools permitting testing of PDF workflows, including display in Acrobat Professional, to determine whether flattening complies

15 15 with the specifications of PDF/X-4 or ISO However, a tool of this kind is being developed in Altona Technical V2. For high-quality color conversion of PDF files containing transparencies, you first need to normalize the object color spaces in ZePrA and then reduce the transparencies. If you don t do this, you must bear in mind that normal ICC conversion is automatically performed when flattening transparencies between objects in different color spaces or with different embedded profiles. This can then result in the color management problems described in this manual. ZePrA is capable of normalizing the colors of PDF files without changing the transparency settings of the individual objects. Special features of the color management of PDF files including transparencies The PDF transparency model permits transparencies between objects in different color spaces (RGB, CMYK, Gray, spot color) or between objects in the same color space but with different object profiles. ZePrA can be used for the color management of PDF files with transparencies. The ZePrA approach for color conversion is that the typical settings in ZePrA are used to convert the colors of every individual object with or without transparency and the structure of the PDF document is preserved. Depending on the transparency effect and the order of the objects in the PDF, it may be that an unwanted change in a transparency effect can occur following color conversion. Taking all these aspects into consideration, we therefore recommend that you first perform color conversion of transparencies in ZePrA and view the converted file in a transparency-compatible PDF viewer (e.g. Adobe Acrobat 9). If the color-converted files looks correct, you can continue to work with it. If artifacts are evident in the file following color conversion, you should first deactivate the Convert all transparent elements in PDF files checkbox under Configuration/Options, and convert the colors of the file once again. In many cases, deactivating this option helps preserve the impression of the original file (see also the example on the right side), because ZePrA excludes certain transparency modes from color conversion. The option should, however, be activated as standard.

16 16 If even deactivating the option is incapable of avoiding artifacts following color conversion, you need to flatten the file prior to color conversion. If transparencies need to be reduced, you have to do so in suitable tools, separately from ZePrA. If the objects exceed the required total amount of color after any flattening, you should subsequently reduce the total amount of color (TAC) in ZePrA. It is therefore always advisable to check the total amount of color of the file after flattening. Original file with a transparency effect that causes the image to fade to white from the center. Visual artifacts following SaveInk conversion (hard edge and graying of the left-hand side of the image). Perfect SaveInk conversion without flattening of the PDF file; "Convert all transparent elements" option deactivated. Note: When creating the PDF file, note that the transparency color spaces of all objects should always be in the color model (CMYK or RGB) of the document color space (output intent). The default setting for the transparency color space in Adobe Creative Suite is CMYK. Suggestion for applying SaveInk or TAC reduction to PDF files with transparencies If, before flattening, you want to be certain that all color spaces deviating from the document color space are correctly normalized with DeviceLink profiles, instead of being converted with standard ICC device profiles by the flattening tool, we advise you to use the Normalize to document color space or output intent option in the Auto Setup Wizard in ZePrA, and only then flatten transparencies. This is particularly to be recommended if you want to work with SaveInk or TAC reduction profiles, and unwanted color deviations appear during optimization if the transparencies are not flattened. This means that you should apply a three-stage process: 1. Use ZePrA to normalize the color spaces in PDF files containing transparencies. After normalizing, you should check the visual match between the PDF file and the original PDF file using the same color settings, e.g. in the Acrobat output preview. On rare occasions, some transparency effects can already lead to visual problems during normalization. In such cases, you should dispense with normalization and flatten the PDF file directly instead.

17 17 2. If the file is visually OK after normalization, you can flatten it using a separate program. 3. Finally, apply the required SaveInk or TAC reduction profile in a separate queue in ZePrA. Checking for visual match, e.g. with Acrobat output preview Fig.: SaveInk workflow for PDFs containing transparencies Normalization with SmartLink Flattening into a PDF/X-1a SaveInk or TAC reduction PDF/X-4 with mixed color spaces and transparencies PDF/X-4 with transparencies, normalized to the document color space Flattened PDF/X-1a without transparencies PDF/X-1a file, ready for the press

18 18 Preparation of media-neutral PDF files Brief description Strategies for generating and processing media-neutral PDF files. Two workflow versions are discussed in detail, and the settings in ZePrA and CoPrA presented. Application programs CoPrA ZePrA Customary DTP programs (Adobe InDesign, Acrobat, Photoshop, QuarkXPress) Target groups Agencies Prepress Printers with their own prepress operations Preliminary thoughts on media-neutral PDF generation When creating print products, there is often uncertainty regarding the printing process ultimately to be used to print the advertising campaign, the flyer, the brochure, etc. So, how should you best create the layout? It would be very much a case of overkill if you were in each instance to individually prepare the image and graphic data for every conceivable printing process (web or sheetfed offset, coated or uncoated paper, gravure or newspaper printing, digital printing or poster printing by inkjet, or for the Internet). The best solution in this case is media-neutral preparation of PDF files, and color conversion to the target color space with the help of DeviceLink profiles. This chapter presents two different approaches for media-neutral processing: Media-neutral production in the CMYK workflow Media-neutral production in the RGB workflow Diagram of a media-neutral workflow. Starting from a master PDF/X file, ZePrA is used for correct color conversion to the various printing standards.

19 19 Taken together, the color correction of RGB images, the composition of complete documents, the generation of PDF files, and their handling in PDF color servers for media-neutral production, add up to the toughest discipline in color management. Depending on the task in hand, the workflows to be set up for this purpose can be very different. Note: This manual, and specifically this chapter, essentially deal with the use of ColorLogic products in the PDF color management workflow. The Color- Logic products are based on open standards, such as ICC, PDF and PDF/X, not on proprietary solutions. The finer points of RGB-based color correction of images or the composition of DTP documents, and detailed settings for PDF generation, can be found in other publications. Specimen layout with images, technical colors and Corporate Identity colors that impose different demands on color management. Generally speaking, you need to work out clear strategies for media-neutral production. Based on our experience, you should be guided by the following considerations: 1. Generation of media-neutral PDF files and color conversion to CMYK The generation of media-neutral PDF files, and color conversion to the right printing standard for output, should be dealt with at the prepress stage, before handover to the printer unless the printer is also a prepress provider and has complete, in-house control of the composition of the documents, the profiles used, and the settings for PDF generation. 2. Realization of simple, reliable and efficient production Highly complex workflow scenarios can be constructed in media-neutral production, requiring a major quality assurance effort. Particularly critical points in this context are the contract proofing of a master file in the workflow, the color management of technical colors, the flattening of transparencies, and the quality control of the profiles used in the workflow. Important and less important colors in the layout In media-neutral color management, different objects in the PDF document are of different importance. In the example shown on the left, the objective is true-to-original reproduction of the images on all papers and in all media. Similarly, important technical colors (vector graphics) are to be reproduced faithfully on all media. This includes Corporate Identity colors, as well as defined spot colors (in our example, the ColorLogic, ZePrA and CoPrA logos). Images and key vector graphics impose very high demands on color management. In contrast, technical colors with

20 20 information colors, i.e. colored headings and vector design elements (e.g. the red area in the title or the gray background around the body text), involve the reproduction of pure, often highly saturated colors. The demands on color management in this context are that pure colors are not contaminated and have high color saturation, that gray and black are not composed of four colors, and that gradients are reproduced cleanly and without breaks. While a color match that is as true to the original as possible on different media is certainly desirable for information colors, it is not an absolute prerequisite as in the case of images and key vector graphics. Media-neutral production in the CMYK workflow In this workflow version, technical colors (vector graphics) and images are consistently composed in one CMYK master color space in the layouting program. The most suitable choice as the CMYK master color space is a large color space displaying a large dynamic range and thus covering numerous printing color spaces. The CMYK master color space should additionally have a very bright paper white, preferably containing only little or no optical brightener, and a neutral gray balance. Although a CMYK workflow is not easy to classify under the heading "media-neutral" and is more likely to be of importance for legacy data, it can prove to be extremely useful. Moreover, a CMYK workflow offers a number of advantages as regards layout creation. ISO Coated v2 is a CMYK master color space frequently encountered in European production operations. This color space covers all offset printing processes, as well as the newspaper printing color space. For color conversion from ISO Coated v2 to the globally customary printing standards, ColorLogic supplies preconfigured Device- Link profiles that are available as demo profiles in ZePrA (see also our manual "Introduction to Our Products"). If you want to use the print layout for the Internet, new media or Office applications (presentations), we recommend that you convert the entire document from the CMYK master color space (e.g. ISO Coated v2) to srgb. However, when converting from CMYK to RGB, you

21 21 must expect color losses that would not occur if the image data were already in RGB. Advantages of the CMYK method Simple, efficient production on the basis of CMYK images in the CMYK master color space. Key vector graphics, such as house colors, can easily be created in the CMYK master color space. Automatic conversion of grayscale images and vector graphics with the DeviceLink profile from the CMYK settings in ZePrA. Pure colors in technical colors remain pure, even after color conversion. Flattened transparencies with directly abutting vector graphics and pixel images of the same color are no problem thanks to identical processing (see also the screenshot of a layout with a CMYK image and a CMYK vector area in the same color as the image). Creation of queues with just a few mouse clicks in ZePrA. Simple creation of proofs. CMYK-to-CMYK DeviceLink creation in CoPrA, with optimum settings for preserving separations, as well as for achieving pure primary, secondary and triplex colors. The ColorLogic DeviceLink profiles permit conversion from a CMYK master color space, such as ISO Coated v2, to target color spaces with (partially) larger gamuts, such as PSR LWCplus V2 (gravure). The entire color space is used in this context. The saturated colors of a CMYK color space are located in the area of the pure colors. The special CoPrA options for preserving pure colors while retaining the correct hue enable harmonious expansion from the CMYK master color space to slightly or partially larger target color spaces (e.g. gravure). Starting from a large CMYK master color space thus means that no gamut is wasted in the target color space. All the above attributes of DeviceLink conversion using ColorLogic profiles make a media-neutral CMYK workflow possible. However, limitations must be expected in the event of substantially larger target color spaces, such as inkjet color spaces for poster production, RGB imagesetting, or RGB output for the Internet/ new media. Note: As you will see in the following section, production workflows with RGB images have additional advantages in media-neutral production. However, they also call for more color management and PDF expertise if you want to make targeted use of this potential and safely avoid possible

22 22 problems. We therefore recommend that you first acquire a good mastery of a CMYK-based workflow, so that you can then start to integrate RGB images. Media-neutral production in the RGB workflow In this future-oriented version, image data are created in RGB, but vector graphics and text in CMYK. However, vectors can also be created in RGB in exceptional cases, i.e. if you have vector graphics whose colors are important. Image data should consistently be created in an RGB master color space, such as AdobeRGB. If you want to integrate image data that are in other RGB color spaces, make sure to embed the profiles. Less important vector graphics with information colors should be created in a CMYK master color space. The RGB color space for image data and key vector graphics has a number of positive features particularly if you use AdobeRGB or ecirgbv2 as the RGB master color space. The size and smoothness of this color space are superior to the CMYK color space, making it a safe choice for the future. Owing to its widespread international use, AdobeRGB is the obvious candidate as the RGB master color space for international production. Modern layouting programs, such as Adobe InDesign CS4/5 or QuarkXPress 8, are necessary for creating media-neutral documents with RGB images and RGB vector elements. Other important functions of modern layouting applications are the option of creating the transparency color space for flattening either in RGB or in CMYK, and of exporting as a PDF/X file including all embedded RGB profiles and transparencies. In Ze- PrA 2.0 and higher, PDF/X files can be processed with a CMYK, RGB, Gray or Multicolor output intent.

23 23 Color presets in Adobe InDesign. The choice of the RGB and CMYK working color spaces determines how untagged images are to be processed. Change the rendering intent to Perceptual, so that the default setting (Relative colorimetric) is not used in the PDF. Advantages of the RGB method Positioning of RGB image data directly in the layout document, without conversion to the specific CMYK color space of the printing process. RGB image data permit better exploitation of the gamut of the target color space. However, this presupposes very good printer profiles or, better still, DeviceLink profiles. Better control of the gray balance in master images, since RGB values with equal components always produce a neutral gray, whereas the gray balance in a CMYK master color space is dependent on the paper color and/or the profile used. Fewer conversion steps are required, since only a single color conversion is needed to get from the RGB image to the CMYK target color space. Better quality when preparing the layout for the Internet and new media, as well as for large color spaces, e.g. for large-format printing with inkjet printers or Multicolor/HiFi printing. Creation of queues with just a few mouse clicks in ZePrA, since the Auto Setup function in ZePrA 2.0 provides a preset for media-neutral data processing. Key vector graphics, such as house colors, can be created in the same RGB master color space as the images. Flattened transparencies with directly abutting vector graphics and pixel images of the same color are no problem thanks to identical RGB values (see also the screenshot of a layout with an RGB image and an RGB vector area on the previous page). A media-neutral workflow with RGB images always makes sense if: You have control of the color correction and approval of RGB master images You have, or can create, you own, high-quality ICC device profiles and DeviceLink profiles for the target color spaces needed You have a color server, such as ZePrA, at your disposal You have flattening outside the layouting program under control.

24 24 The RGB workflow in Adobe Photoshop, InDesign and ZePrA 1. Selection of the perceptual rendering strategy Our manual "Principles of DeviceLink Technology" describes the various options for perceptual rendering in ColorLogic products. When constructing an automated workflow with media-neutral data, it is advisable to calculate all the target profiles used with a consistent rendering strategy. Two different possibilities will again be briefly outlined at this point with a view to medianeutral production. ColorLogic Black Point Compensation In this case, the gray axis remains relative to the paper color, and perceptual rendering tends towards the relative colorimetric with black point compensation version in Adobe applications. In the post-production of RGB images, a customary workflow involves the delivery of RGB data plus ISO Coated v2 proof. Relative colorimetric rendering with black point compensation is often used for color conversion from RGB to CMYK. In this case, existing RGB data can easily be transferred to a workflow where perceptual color conversion is performed using ColorLogic Black Point Compensation gamut mapping. When perceptual conversion of highly saturated blue tones using ColorLogic Black Point Compensation is compared with the conversion of the ECI profile ISOcoated_v2_eci.icc with relative colorimetric + black point compensation, many users prefer the less violet ColorLogic conversion with the CL_ISOcoated_V2_TAC330.icc profile (see also the gamut mapping example in the chapter Introduction to color management with PDF files on page 4). ColorLogic Absolute Compression This is a perceptual conversion, the characteristics of which resemble those of absolute colorimetric conversion with preservation of detail definition in out-of-gamut areas. The paper color is compensated for in the gray balance. Perceptual conversions to very yellowish papers, for example, are thus much cooler than with standard profiles commonly used on the market, but match the original image far better in the hardcopy proof, the soft proof, and the ultimate printing process. Paper tone simulation always has to be activated for soft proofing. Because the paper color is taken into account during color conversion, Absolute Compression is recommendable for media-neutral production with very different papers and paper colors.

25 25 Left: Normal ICC conversion from AdobeRGB to ISO Coated V2 (relative colorimetric + black point compensation) with irregular gradients, losses of definition in highly saturated colors, and blue tones with a violet cast. Right: DeviceLink conversion from AdobeRGB to ISO Coated V2 with pure, saturated primary and secondary colors, harmonious separation without losses of definition, and without color casts in the blue test image. RGB-to-CMYK Device- Link profile creation in CoPrA with optimum settings for achieving pure primary and secondary colors. DeviceLink-based conversion from RGB to CMYK A DeviceLink profile for RGB to CMYK can be seen as a special form of gamut mapping from a defined RGB source color space to a CMYK target color space. As a rule, the red, green and blue regions of the AdobeRGB color space, for example, are significantly larger than even large CMYK color spaces like ISO Coated V2. However, specifically in the region of dark, saturated colors, large CMYK color spaces (e.g. on inkjet printers) have a larger gamut than customary RGB color spaces. The use of DeviceLink profiles enables you to specifically exploit these color space potentials in order to obtain maximum quality during separation. The same applies when converting pure RGB to pure CMYK colors. If technical colors are created with pure colors in the RGB color space, you have to use DeviceLink profiles for conversion to CMYK in order to get truly pure colors in CMYK.

26 26 Pure ICC device profile conversion can give rise to contaminating values of between 1 and 9% for individual color channels. See also our manual "Highquality RGB-to-CMYK Conversions in Photoshop". 2. Soft proof and proof of an RGB master image To properly correct the colors of RGB image data for an automated workflow, you need to define what a digital proof and a matching soft proof are to look like. One common approach is to define the FOGRA39 / ISO Coated v2 color space as the proof reference for RGB master images. For checking in Photoshop, create a soft proof setting that shows conversion from RGB to ISO Coated v2 with either a printer profile or a DeviceLink profile with the selected perceptual rendering strategy. When using a printer profile in Photoshop, the ISOcoated_v2_eci.icc profile is replaced by the ISO Coated V2 profile created in CoPrA with the required gamut mapping version. If, when retouching images in Photoshop, you want to check the influence of the DeviceLink profile on color conversion by soft proof, you need specially calculated soft proof profiles, since Photoshop is not capable of configuring an RGB-to- CMYK DeviceLink profile in the soft proof. However, CoPrA 2.0 or higher offers you the possibility of using a DeviceLink profile to create a special soft proof profile that can simulate both the gamut mapping and pure colors and thus the DeviceLink conversion. If you use a soft proof profile of this kind and activate the soft proof preview in Photoshop, you can optimize and visually check your RGB image data for final image retouching, without having to convert to the CMYK color space. For digital proofing, also remember to likewise convert the RGB data to CMYK with the help of the respective CoPrA profile. Creation of a preview or soft proof profile from a CoPrA DeviceLink profile Note: When choosing the reference for your RGB master image data, you can, of course, also define an entirely different CMYK color space than the ISO Coated V2 mentioned in this document. In this case, however, please note that your technical colors should also be created in this CMYK reference color space for reasons of consistency. Similarly, you need CMYK-to-CMYK DeviceLink profiles for conversion from your CMYK reference color space to the respective target color spaces. You may need to create and save various DeviceLink profile combinations in line with your requirements.

27 27 3. Composition of master documents in InDesign When composing the master document in InDesign, the rule applies that all images must be in the RGB master color space, technical colors with information colors must be created as CMYK tones in the CMYK master color space (e.g. as ISO Coated V2), and technical colors with key image colors must be created in the RGB master color space. In the color settings, make sure that perceptual is activated, instead of the default setting (relative colorimetric with black point compensation), since this general specification in the color settings is later permanently embedded in every single PDF object during PDF generation (see also the screenshot on page 23). If, contrary to our recommendation, key technical colors are always to be created in the CMYK master color space, rather than using RGB vectors, the only answer is to fall back on a trick. In areas where images and vectors having the same image color overlap, you have to make the image areas transparent in Photoshop, and position the technical color underneath the transparent image in the layouting program (see screenshot). It is then no problem to convert from the RGB image directly to the target color space, in order to then reduce the transparency. For PDF generation, we recommend PDF/X-4 without flattening (InDesign CS4 and higher). Should you intend to have flattening done by the printer, we advise you to talk to everyone involved in the printing process at a very early stage and to run production tests. If the layout does not need transparencies, select PDF/X-3 as the standard output format in InDesign. Note: If RGB images and technical colors in CMYK with transparencies are used in the layout, no flattening may take place during PDF generation from the layout (also meaning that no PDF/X-3 file may be created), since this can entail unwanted CMYK conversion of RGB images.

28 28 4. Color conversion of the master documents in ZePrA For color conversion directly to the target color space, the RGB images are converted to CMYK using a matching RGB-to-CMYK DeviceLink profile with ColorLogic gamut mapping, while the technical colors are converted from the CMYK master color space to the target color space by means of a DeviceLink profile. So, for media-neutral production, you deviate from the typical path of normalization (see the section Color management of PDF files: Normalizing on page 7) in order to maximally exploit the potential of RGB color spaces. Should the document contain several different color spaces with embedded profiles, ZePrA s SmartLink technology enables you either to dynamically apply existing DeviceLink profiles for known combinations of source and target profile to the corresponding images and vectors, or to calculate new ones "on-the-fly" if no suitable DeviceLink profiles are available. ZePrA 2 color settings for media-neutral color conversion with active SmartLink function.

29 29 If you use the customary European and American printing standards as the target color space, for which ColorLogic provides preconfigured DeviceLink profiles, it takes only little time to create a queue, including hot folders, with the help of the Auto Setup Wizard. For media-neutral processing, select the Direct color conversion to new output condition option in Auto Setup. You merely have to select the matching DeviceLink profile that converts from the document color space to the target color space. Moreover, the most important settings for the media-neutral workflow are selected automatically in ZePrA. All you need to do in the RGB settings, if necessary, is select the required gamut mapping through the rendering intent in the RGB settings, and also the required RGB default profile to serve as the source profile if the PDF file to be processed does not contain any embedded profiles for RGB objects. If you already use high-quality DeviceLink profiles and want to apply them using SmartLink technology, create the profiles in ZePrA s SmartLink Setup. Note: If, in medianeutral production, you use target color spaces for which no ColorLogic Standard DeviceLink Profiles are available, you can calculate corresponding DeviceLink profiles using CoPrA.

30 30 5. Switching between true spot colors and their CMYK rendering If, depending on project, you sometimes want to print spot colors with true spot colors and sometimes with the CMYK rendering, you must ensure that they are created correctly in the master layout. Converting spot colors to CMYK For CMYK rendering of a spot color, you have to specify in Adobe InDesign whether a CMYK color or a Lab color is to be created as the replacement color. This InDesign setting controls the alternate color space in the exported PDF file, and thus also influences conversion in color servers like ZePrA, if use is made of the Convert spot colors to target color space option. Definition of a Lab-based replacement color permits the best possible rendering of a spot color in the soft proof. Spot colors can effectively be converted to target color spaces with a large gamut by means of classical ICC profile conversion and the absolute colorimetric rendering intent. In the case of target color spaces with a small gamut, such as for newspaper printing, this absolute colorimetric color conversion of a Lab-based spot color leads to unsatisfactory results if the spot color cannot be mapped in the target color space. A more appropriate method in such cases is to first convert the spot color to a CMYK master color space, such as ISO Coated v2, and then use a DeviceLink profile for conversion to the target color space, such as ISO Newspaper 26. The following recommendation can generally be given for the CMYK conversion of spot colors: If you predominantly work to different printing standards for which preconfigured DeviceLink profiles are available, including newspaper printing, it is advisable to use the alternate color space CMYK in the CMYK master color space (e.g. ISO Coated v2) for spot colors in the master layout.

31 31 If you use ZePrA s Convert spot colors to target color space option, conversion to the target color space is performed using a DeviceLink profile. In contrast, if you primarily work to printing standards with a large gamut, it is recommendable to use a Lab-based replacement color. If you use ZePrA s Convert spot colors to target color space option, this permits direct color conversion from Lab to the target color space. Note: For conversion of neutral technical colors, it is advisable in this case to calculate the ICC target profile with strong to maximum GCR, at least in the neutral colors. This is necessary to enable the printer to obtain stable printing results over the entire run, particularly as regards neutral technical colors. The ZePrA Gradations module with set tone value adjustment for spot color channels Printing spot colors as process colors If you do not convert the spot color to CMYK, it is ultimately printed as the 5th, 6th or 7th, etc. process color in addition to CMYK. This gives rise to the question of how to handle true spot colors in printing processes with substantially different tone value increases. Depending on the spot color, the usual procedure here is to adapt the gradation, since a dark spot color tint is more likely to have a larger tone value increase than a light spot color tint. The Gradations module in ZePrA can be used to make individual gradation adjustments for each spot color. ZePrA comes with predefined adjustments that compensate for the tone value increase in the mid-tone range. If you know the exact tone value increase, you can use ZePrA 2 to very easily create your own adjustment and assign it to the spot color in question. If you do not know the tone value increase, the general rule is that you should assume the same tone value increase as for the black channel. This option, too, can be selected in the Gradations module.