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Introduction to Color Management with PDF Files Preparation of Media ColorLogic GmbH Introduction to Color Management with PDF files Preparation of Media-Neutral PDF Files 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 stan- dards 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 regis- tered 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 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 ..........................................8 1. Lack of black point compensation .................................................................9 2. Problems when handling black and gray .....................................................10 3. Unsatisfactory quality of ICC-based CMYK-to-CMYK conversions ......................................................................................................10 4. Special features of the overprinting model in PDF files ..........................11 5. Differences in gamut mapping when combining profiles from different manufacturers.........................................................................12 6. The soft proofing problem with Multicolor separations .........................13 7. 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 .................................18 1. Generation of media-neutral PDF files and color conversion to CMYK ............................................................................................................19 2. 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 ....................24 1. Selection of the perceptual rendering strategy .........................................24 ColorLogic Black Point Compensation ...........................................................24 ColorLogic Absolute Compression .................................................................24 DeviceLink-based conversion from RGB to CMYK..........................................25 2. Soft proof and proof of an RGB master image .........................................26 3. Composition of master documents in InDesign .......................................27 4. Color conversion of the master documents in ZePrA ...........................28 5. Switching between true spot colors and their CMYK rendering .........30 Printing spot colors as process colors .................................................................31 4 Introduction to color management with PDF files PDF is the most versatile, but also the most complex, data format in the gra- phic arts industry. It is a standard format for submitting printing data to print- shops, 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 15930 - PDF/X: PDF files for exchanging printing data • ISO 19005 - 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 ma- kes 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 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 15930 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 theICC standard. The addi- tional 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 com- position of layout documents and subsequent PDF generation take place un- der 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 colors, DeviceN and alternate color spaces Spot color patches with It is possible to print not only with CMYK on the press, but also with spot alternate CMYK colors. 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 ge- nerally either a CMYK or a Lab color space. 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
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