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Prof. Dr. Stefan Brües*, University of Wuppertal, Germany
The transition from the reproduction technology of the 80s to the modular prepress/DTP world of the 90s made colour calibration a new task for prepress companies. Colour management systems produce profiles of scanners and digital cameras, colour monitors and printing results to ensure that colour originals are accurately reproduced in the print output. In the space of just ten years, colour management has evolved from the experimental stage into an essential technology in the modern-day media industry. Today in 2003, colour management technology has reached a high level of maturity. Only ten years after it first appeared on the market, ICC colour profiles have become indispensable in modern premedia. The operation of digital proofing systems would be simply impossible without modern colour management. It can be said without exaggeration that colour management has revolutionised digital proof printing. It is only thanks to targeted colour transformations that standard printers from Canon, Epson, HP and other manufacturers from the office area can be used as proofers. A whole series of newcomers in the print industry supplier community offer an intelligent combination of a PostScript RIP with integral colour management functions. The quality of these low-priced proofing systems has now reached such a high level that even discerning customers accept such digital proofs as contract proofs. Modular proofing solutions are regularly to be found among the leaders in the digital proofer tests conducted by the European Color Initiative (ECI) together with the Bundesverband Druck und Medien e.V. (German Printing and Media Industries Federation – bvdm).
Only the profiles are standardised
Meanwhile, the fourth version of the ICC profile specification is available (see www.color.org). It is a little-known fact that to date only the colour profiles have been specified and thus de facto standardised by the ICC. The functions of the second main component of a colour management system, the so-called colour management module (CMM), have not yet been precisely specified by the ICC. To a certain extent, the functions of a CMM can be derived from the profile specifications, but this does not apply to all operators. Especially in the past, different interpretations of a job could lead to visibly different transformation results. For example, different interpolation algorithms can produce slight tone value differences. In most practical cases, this will not be noticed. If, however, a 2% grey is calculated instead of a tone value of zero (paper white), in most cases this presents a massive production problem. The grey then has to be removed manually, often involving major editing effort. In general, the CMMs available today from established manufacturers of colour management components have a very high degree of compatibility. Practical everyday problems are only rarely attributable to an error in the CMM. So far, there has not been an official ISO standard for colour management. However, a draft standard has been prepared and, in 2003, a joint task force of the ISO and ICC was launched to convert the results of the ICC work into an official ISO standard.
Deliberate and coincidental ‘incompatibilities’
Despite the intensive work carried out on the ICC profile standard, potential sources of errors remain in the application. For instance, the use of perceptual rendering intent (PRI) for colour separation can sometimes produce very different results when using profiling programs from different manufacturers. Fig. 1, for example, shows the implementation of a media-neutral image file coded in an ECI-RGB colour space for offset printing with the PRI of two different profiling programs. Production-critical differences in reproduction output are obtained from one and the same starting file depending on the PRI of the colour profile.
To avoid such problems, for instance, in publishing production to obtain predictable reproduction results for an advertisement file from different print companies, users experienced in ICC applications can switch from perceptual to relative colorimetric rendering intent and activate the black point compensation option in Adobe Photoshop. In this case, both profiling programs produce attractive and very similar reproduction results (Fig. 2).
The ICC, incidentally, is responsive to users’ ideas. Following the positive experience gained with this specific Photoshop function, a similar solution is to be adopted shortly in the general ICC standard. In the longer term, potential problems could be solved by an amendment to the ICC architecture.
Problematic: skills in working with colour profiles
While the technology of colour management has reached a high degree of maturity, the same does not yet apply to all users. Working with colour profiles and profiled workflows is still a major problem area. It is no mere chance that notably digital proofing is the field in which users work with colour profiles in virtually all systems, since applications here are locally limited. In fact, operation of such systems is actually particularly straightforward. In most cases, proofing simply requires two local colour profiles: one to characterise the print scale to be simulated and another to characterise the rendering properties of the proof printer. If both colour profiles describe the actual rendering conditions, excellent results should be achieved. However, users often somewhat hastily assume that their production process ideally corresponds to a standardised print scale (e.g. ISO 12647-2). If the digital proof is not accurate, the first place to look should be in actual production. All too often, standard print conditions are applied without criticism or colour profiles are taken from the Internet and used without question.
Photoshop is exemplary
An important step for the widespread use of colour management technology is the now very extensive implementation of ICC mechanisms in the application programs of the print and media industry. Special importance is attached to Adobe Photoshop, the dominant tool for image processing. After several attempts, Photoshop in its seventh version is regarded as a model of well thought-out ICC-based colour management implementation. Users skilled in working with colour profiles will find in Photoshop all the tools they need to initiate colour transformations or address profiles. One interesting aspect is that the Photoshop architects have even added a number of functions which are not yet covered by the ICC standard but which are very useful in modern workflows. For example, the black point compensation function mentioned earlier carries out scaling of the black point, which is often desired to make image reproduction with colour profiles more predicable and altogether simpler.
Microsoft neglecting the ICC standard
As Microsoft has neglected the ICC standard in recent years, Adobe has created its own colour management interface, called ACE. Based on the ICC standard, it is regarded by many experts as a reference implementation. Adobe products, which are so important for the graphic arts industry, now contain a standard colour management platform across all supported operating systems. Incompatibilities at system level (often used as an excuse for application problems) can now be virtually ruled out, at least across the Adobe product line, including the important interfaces with PostScript and PDF.
Information on the subject of colour management is widely available in the world’s largest library, the World Wide Web. However, this information should also be treated with caution. Only few sources can be truly regarded as comprehensive and accurate. Interested users are advised to subscribe to the mailing lists of the ECI (www.eci.org, in German and English) and Apple (www.apple.com/colorsync, in English only), which provide advice on problems from a now extremely competent international user community.
Colour management becoming increasingly important
Modern prepress technology is set to move ever further away from exclusive print production for standardised offset print. Multiple use of production data is still at an early stage. Numerous large and small production jobs are being run at customer request from media- or process-neutral data files (e.g. IKEA and Neckermann catalogues). Colour management is a key technology, and its importance is set to increase further.
There is still a lot to do for the ICC as far as colour management functions are concerned. In a worldwide user survey, respondents called above all for improvements in handling CMYK data, greater predictability of transformation results and increased user-friendliness. Lars Borg, colour management expert at Adobe Systems and current Chairman of the International Color Consortium, is working flat out to develop the ICC standard further. The criticisms and ideas put forward by users have been distributed among the ICC taskforces and will soon result in further improvements.
*Prof. Dr. Stefan Brües lectures at the University of Wuppertal and played a major role in establishing the ICC Committee and developing the ICC standard in his earlier work with FOGRA. brues@uni-wuppertal.de
