PDF and Color Spaces

Editor’s Note: This article is part of the PDF Color Learning Center

The PDF format supports objects specified in a number of device dependent, device independent, and special color spaces. Except for certain controlled environments (i.e. PDF/X), production professionals should assume that different color spaces may be intermingled in any document, perhaps in any page, particularly if you have graphics or pages included from different sources. The trick is to understand what is going on and to plan accordingly.

Device color spaces are those used by the creation device. For images, this would often be the color space of the data from the scanner. However, it could also be the color space applied in Photoshop or Quark, or the result of color conversion by the printer driver, whichever made the last modification. Normally all of these programs go to great lengths to preserve colors. With care, device dependent color reproduces quite well, but that is also its weakness. That care is manifested as art, craftsmanship, or cross-organizational communication – items that are becoming more scarce in our fast-paced, decentralized world.

In an effort to control the variety of color spaces users have resorted to trying to implement color management throughout these processes. The reality is that the scanner data might not display the same colors on a monitor or printer. This has led to the notion that you can attach a profile that will provide the information to convert from the source colors to the colors specific to the destination device. The problem is that if you attach a profile that makes the colors look right on the monitor, then it probably won’t look right on a printed page, and vice versa. To get around this dilemma, some color management systems will have one profile for each destination device or printing condition (print technology, paper, ink) per each source device. If you had N source devices and Y destination devices or print conditions, this could lead to NxY profiles to construct and manage.

Document creators may not know where their files will be printed or on what type of device. Often, the print buyer decides this at the last minute, based on the best price, delivery schedule, and destination. If the creator relies on trying to color manage their PDFs there is no guarantee that they are applying the right profile and that they will get the intended colors.

To get around all this, Adobe, Apple and a few other companies proposed an industry standard for device independent color. Initial implementations included the CIE-based device independent color in PostScript Level 2 and Apple’s ColorSync. The group, and the effort, eventually became the International Color Consortium (ICC).

The premise of ICC color is that you can use a profile to map source colors into a mathematically-rigorous absolute color space and have other profiles to map from this absolute color space to destination colors. This de-couples the creation and rendering of color information. You only need apply the appropriate profile at the destination. Also, it means that N+Y profiles are needed, instead of NxY profiles for traditional color management.

ICC color also recognizes that the same colors can be applied different ways. This has led to the notion of "rendering intents" for different objects. Four standard rendering intents are defined but others are possible. The table below lists the rendering intents and some of their properties.

ICC, PDF & PostScript Rendering Intents
Rendering intent Objective Best for Advantages
Absolute colormetric Color matching Logos, color catalogues Measures the same across media
Relative colormetric Media adjusted color matching Vector graphics Looks the same across media
Saturation Preserve saturation Business graphics Vivid
Perceptual Preserve color relationships Scanned images Pleasing

ICC, PDF & PostScript Rendering Intents
Rendering intent In-gamut mapping Out-of-gamut mapping
Absolute colormetric Exact All to in-gamut nearest color
Relative colormetric Exact, but white point adjusted To nearest in-gamut color, white point adjusted
Saturation Exact sometimes To nearest, most saturated in-gamut color
Perceptual Compressed relatively within in-gamut region Compressed relatively into in-gamut region

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About the Author: Gary Armstrong

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