Managing colours in Windows Vista

An email from reader Peter Webster prompted the subject of this article: how colour management works (or, in his case, how it doesn’t work) in Windows Vista.

Originally only of interest to digital imaging professionals, whose daily work depends on consistent colour, colour management is slowly moving into the mainstream.

It’s a shame that this subject isn’t getting easier ­ for the uninitiated and those who have dipped a toe in the colour management water and been scalded, colour management remains a bit of a dark art and something you either ignore or attempt with limited expectations of success.

I’m not going to attempt a comprehensive review of colour management here, but I have listed resources at the end of the article that will give you a better understanding. For now, I’ll explain colour management in Vista and how it differs from XP.

The colour management system (CMS) used in Windows 2000 and XP was based on colour profiles, the standard for which was defined by the International Colour consortium (ICC). ICC profiles describe the colour characteristics of specific devices and contain data on how to translate colour consistently between devices with differing colour characteristics.

Translation takes place by converting the colour data to an intermediate colour space (the Lab colour space), hence ICC profiles need only know how to translate to and from their own colour space to Lab for the system to translate between two profiles.

Windows Colour System
Windows Vista introduced a new CMS called Windows Colour System (WCS), which doesn’t use ICC colour profiles. In an ICC-based CMS, all the information required for translation is contained within the profile. This provides amazing flexibility but it’s a translation system that’s half blind, with each half of the translation process working in ignorance of the other; the source isn’t aware of the destination and vice-versa, which can result in conversion inaccuracies.

WCS overcomes these drawbacks, and uses three kinds of profiles ­ - device model profiles contain data describing device colour characteristics, the gamut mapping method profiles define how colour data is transformed between profiled devices, and appearance model profiles account for viewing conditions such as background colour around the image.

Appearance model profiles are an interesting development because they allow colour management systems to take viewing conditions into consideration. Though practical implementations of appearance models are not in widespread use, you can define the appearance model profile in the Advanced tab of the Vista display properties (colour management) control panel.

The default setting ‘WCS profile for sRGB viewing conditions’ and the alternative ‘WCS profile for ICC viewing conditions’ allow you to alter the monitor appearance to simulate viewing under the CIE (International Commission for Illumination) standard D65 and D50 illumination conditions respectively.

These standards are used in professional proofing applications, such as viewing rooms for printing proofs. Though unlikely to be of much value to the hobbyist photographer, appearance model profiles can help narrow the gap between monitor displays and printed output, allowing for more accurate soft proofing.

So what are the practical consequences of these different systems for people upgrading to Vista who still want to use their ICC device profiles for their devices and who have large libraries of images tagged with ICC colour space profiles?

Most professional imaging and publishing applications don’t use OS colour management engines. If you’re using an Adobe product, for example, then Adobe’s ACE Colour Management Module (CMM) will be performing the conversions. This ensures information, such as image colour data, is transformed from the document or working colour space profile to your display profile. It also provides soft proofing options to simulate printed output on your monitor.

In Vista, WCS will carry out colour management functions when you preview images in Windows Explorer, display them in Windows Photo Gallery or work with them using a Microsoft Office application, but not Internet Explorer.

If you’re having problems maintaining colour consistency across applications you can download the Adobe CMM from the the company’s website. As long as your other applications support use of a third-party CMS, you can use the same Adobe CMM across all your applications.

WCS is backwards compatible with ICC colour profiles and if all of the profiles in a CMS chain are ICC profiles, Vista will use them. If one or more of the profiles is in WCS format then WCS takes over, converting the ICC profiles to WCS device model profiles.