Successful color management in PDF workflows

Editor's Note: Adobe introduced the concept of color management for the first time in Acrobat 4 and redefines it in Acrobat 5, consolidating the dominance of the PDF file format in multiple workflows. If you want to achieve a true "device-independent" concept, it is necessary to install the correct Distiller software and use third-party plugins wisely.

Over the years, Adobe's Portable Document Format (PDF file) has undergone training as a promising digital file format. It solves a number of problems that have plagued the industry, such as font fidelity, layout integrity, device and model independence, and single manufacturer support. This article describes how the color characteristics in PDF 1.3 and PDF 1.4 make PDF the preferred file format in device-independent color workflow applications based on ICC profiles.

Looking for a digital file format

For some workflows, its job is to archive the original application files for multiple printing, reprinting, and the use of content for cross-media publishing purposes. Many companies choose to save the file in the final Postscript file format. The advantage is that the output of the Postscript file will be exactly the same as the original, and if the layout is done properly, the output will contain all the page content created by the different applications, as well as some necessary content resources such as fonts and images. This digital management approach has solved some kind of production problems, such as mapping spot colors on the correct plate and trapping. But in most cases, Postscript has the same limitations in color as the original application file used: the color has been converted according to a specific output condition, and recorded in a Postscript file in a device-dependent color space manner.

With the advent of Postscript Level II in 1990, it became possible to generate device-independent Postscript files. But even in Level II, the interpretation and conversion of Postscript's device-independent color space can vary depending on the Postscript device used, and users cannot control or anticipate these changes. This is why most users still create device-related Postscript files during production and archiving.

Why is the PDF file better?
PDF files have many advantages in recording device-independent color spaces. Since a separate application (Acrobat Distiller) can be used to convert Postscript files to PDF files, the user can define several sets of parameters during the conversion and encoding process to ensure the conversion accuracy, as opposed to Postscript files recorded on disk via a print driver. In contrast, generating a color PDF file may be subject to more control. Converting a device-independent color map in a PDF file to the currently selected output device's color space is more convenient and reliable than outputting to a Postscript file. Finally, because ICC color profiles may be used directly in the PDF to define a device-independent color space, the use of applications that support ICC profiles ensures color consistency.

In this article, we will discuss the application of PDF files in the field of color publishing. Then we will discuss Postscript color management, techniques for creating PDF files from multiple applications, and how to convert Postscript files into PDF files via Distiller. Finally, we will also introduce some third-party tools for viewing and editing color PDF files.

Features of Acrobat and PDF The version of the PDF language is updated with the version of Acrobat, from PDF 1.0 in Acrobat 1.0, PDF 1.1 in Acrobat 2, and so on, to PDF 1.4 in Acrobat 5. The concept of a color space emerged in PDF1.3 and later versions for encoding device-dependent and device-independent colors.

Version 1.0 of the PDF was first introduced in 1992. The main feature that corresponds to the corresponding Acrobat 1.0 product is the ability to embed fonts for printouts in PDFs and to support click-through links in PDF documents. However, only RGB colors were supported at the time. Acrobat 2.0 appeared in 1994 with a new set of architectures that allow third-party plug-ins to extend Acrobat's functionality. Acrobat 2.0 adds support for external links and additional annotations. It also encodes device-independent colors using CIELAB value encoding.
At about the same time, Adobe began offering PDF support in the company's published applications, particularly PageMaker and FrameMaker. While this has many benefits, it creates new hurdles for reliable color management workflows because different PDF export execution tools handle different ways of encoding information for page objects.

In 1996, Acrobat 3.0 (corresponding to the PDF version 1.2) added many support for Postscript prepress special features, including halftone and overprint specifications; support for spot colors, tables, OPI 1.3; and CMYK color space coding. In the past, people thought that the workflow of using a single digital file other than Postscript was only a theoretical idea, but these features made this idea a reality. Some plug-in programs from third parties, such as Lantana's products, are also taking advantage of these features, thus further consolidating the importance of PDF as a publishing workflow digital format.

Acrobat 4.0 and PDF 1.3 were released in 1999. This version of the new features support OPI 2.0, double-byte CID fonts, smooth gradient technology, and further support for spot colors, supporting the new "Device N" color space. Perhaps from a color standpoint, the most important is that Distiller 4.0 gives users the ability to use the new "ICC-based" color space standards to redefine the device-related colors in Postscript files, assigning them the correct ICC characteristics. Documentation to ensure that the final PDF can be easily converted between different output systems. Unfortunately, Acrobat applications do not have these features. For a long period of time, ICC-based colors can only be accurately converted via Postscript color management in Postscript 3 devices.

Acrobat 5.0 was introduced in 2000 and can read and write 1.4 versions of PDF files (Acrobat is not the first application with this capability). The most critical publishing feature of PDF 1.4 is support for transparency. From a color publishing point of view, the key feature is the Acrobat 5.0 application itself, which allows the output of the ICC profile to be specified, ensuring that ICC-based colors in the PDF file can be converted without Postscript color management. Since Acrobat 5 can use the Adobe Color Engine introduced in Photoshop 6.0 (now supported by many Adobe applications), the accuracy of color conversion in Acrobat is comparable to these important publishing applications.

From a color management point of view, only the 1.3 and 1.4 versions of PDF have attracted a lot of attention, because only these versions contain ICC-based color definitions, allowing for the easy reprocessing of color PDF files after they are initially generated. These special color spaces that exist in the PDF will be discussed later in this article.