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By: Erik Vlietinck
Mon, September 04, 2006 - 9:06:02
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The primary reason for the Windows colour management architecture is to improve the what-you-see-is-what-you-get (WYSIWYG) quality for printing and imaging. Because of limitations in the ICC profile specification, most users are not achieving this today, Microsoft says. The truth is that WYSIWYG is not all about colour management. It’s also about hardware.

WCS is capable of cleanly supporting the wide range of colours that today’s motion pictures and digital cameras are capable of capturing. It eliminates artificial limitation of an ideal print’s dynamic range (recently adopted by the ICC). Instead, a CAM-based profile or a CIEXYZ measurement-based profile can contain a greater dynamic range and improve accurate assessment of white and black point. This should clearly improve quality in workflows that have dynamic ranges greater than print.

One difficult challenge facing colour management systems today is persisting black channel information. Traditionally, highly trained scanner operators would make colour separations targeted at a specific press setup. In such cases, it is very important to preserve the black channel information through the colour management workflow. Although we expect this will become much less important over time, the black channel information can be maintained in WCS.

WCS migrates to a measurement-based profile and a smart colour management module (CMM). In this way, ICC-based workflows become simple to create, manage, edit, learn, and explain. WCS enables vendors to create a single profile for a combination of device states, halftoning settings, media, and viewing conditions. This is done by enabling the use of sophisticated device model plug-ins used in combination with measurement-based profiles.

The Windows colour System includes a new technology called Kyuanos that was developed by Canon. Kyuanos was born from Canon’s leadership in colour digital imaging and printing industries. Today’s digital cameras and colour printers offer advanced colour reproduction capabilities that exceed the potential of today’s colour management systems.

By migrating to a measurement-based profile and a smart CMM, profile-based workflows become simple to create, manage, edit, learn, and explain. This makes it easy for users to get the results they want. Enterprise users are offered a simple model. If they need to recalibrate a device, they just take new measurements. If they want to change the appearance of an image, they select a new gamut mapping algorithm. Photographers and graphic artists can directly examine the values in a measurement-based profile to ensure that the profile is valid.

By providing the capability for simple measurement-based profiles with viewing condition tags that can be changed as necessary, WCS helps to reduce the proliferation of profiles. Because WCS provides measurement-based profiles directly to the CMM, it significantly improves the ease with which profiles can be edited (both manually and automatically) by directly relating primary profile information with adjustable device characteristics. This, in turn, will lead to a lessening of profile explosion. It will, of course, be necessary that CMMs support whatever device (or technology) characterization profiles that the ICC approves.

Finally, Microsoft decided to also change the format of colour profiles or the modules that will change colour behaviour on the fly. Everything will be described using XML, a human-readable file format. While XML is open, it may not be the best choice for all purposes in life.

The ICC Shows Reserve

Microsoft stresses the fact that WCS will use the CIECAM02 colour appearance model, and run-time colour rendering, in which the colour transforms to be applied to source [input] files are determined after the output devices are known.

When using standard ICC profiles in WCS the software developer will have a choice of processing, either run-time colour rendering with CIECAM02 -or- colour rendering using predetermined transforms from the ICC profiles.

According to the ICC, there is a first reason to become confused by Microsoft’s marketing babble: what exactly is meant by colour rendering, gamut mapping, and colour appearance models?

A gamut mapping operation takes the code values from a source image and converts those to the code values of a reproduction [output image] in a way that compensates for differences in the input and output gamut volume shapes. [In printing, gamut volume shape is the result of colorants, media, half toning, etc.]. Gamut mapping does not include adjusting for preferred colours or adapting colours for different lighting [viewing] conditions.

On the other hand, a colour rendering operation begins with an encoded representation of a scene [raw capture] and converts that scene representation to a reproduction in a way that includes gamut mapping and image preference adjustments, and also compensates for differences in viewing conditions, tonal range, etc.

Colour re-rendering is similar to colour rendering, except that it starts with a source image that is already a reproduction, and produces a new different reproduction, typically for a different kind of display.

A colour appearance model uses a parameter set and an algorithm to compute colours encoded in a colour appearance colour space. The value of a colour appearance colour space is that it provides a way to represent colours as a human would see them under a particular defined viewing condition. CIECAM02, the colour appearance model in WCS, is familiar technology.

CIECAM02, and other colour appearance models, are commonly used in the construction of ICC profiles. I’ll repeat this: CIECAM02, and other colour appearance models, are commonly used in the construction of ICC profiles.

The idea of run-time colour rendering is that the complete colour transformation is constructed at run-time from various available components, and that the complete transform is specific to the imaging conditions required at that time.

According to the ICC, its member companies have provided various kinds of run-time colour rendering solutions to market as far back as the mid- to late-1990s. Scitex’s Profile Wizard is one example. Enhanced support for run-time colour rendering was one of the design objectives in the ICC version 4 revision work, completed in 2001.

Although ICC profiles can be used to construct run-time colour rendering transforms, and some ICC-based applications are available, the dominant modes of ICC operation have used predetermined transforms. This is because quality, predictability, and repeatability have generally been more important to ICC users than run-time output flexibility. That is, to professional users who have to comply to quality standards, this is more important for obvious reasons. Consumers don’t have to deal with these issues, but the ICC system has been made so that they can benefit from it, given some effort in terms of learning.

Across the markets that the ICC serves, there are business-critical use cases that require the specification of predetermined colour behaviour, e.g. conversion rules to be carried from the design approval point in a workflow to the later implementation stages, and to be archived with digital colour files for later matching reproduction. Predetermined transforms encoded in ICC profiles provide this capability. What the ICC says here is that Microsoft is focussing on several markets outside the professional colour management markets.

Also, the ICC draws the attention to another important issue. It says: “It is important to keep in mind that a typical colour conversion transform, whether it is constructed at run-time, or predetermined, will incorporate a number of features. Colour appearance models deal with viewing adaptation adjustments between source and destination, but do not address optimization for a variety of output condition particulars, e.g. gamut reshaping from monitor to print, printing ink limit, etc.”

In many cases the quality of output is determined by such particulars, the ICC points out. ICC profiles include pre-optimized transform elements that deal with all aspects of cross-media reproduction. E.g. the predetermined perceptual rendering intent transforms in ICC Version 4 profiles are pre-optimized for print production gamut mapping. 

Version 4 Profiles Meet Microsoft's "Concerns"

Version 4 profiles ensure correct interconnection between the predetermined transforms in source and destination profiles through the use of a common well-defined reference print colour gamut.

The ICC has chosen not to lock-in colour management systems to a particular version of a colour appearance model due to the rapid pace of advancement in colour appearance and colour rendering research. While Microsoft seems to believe CIECAM02 is the ultimate scientific colour appearance model, around the same time as the establishment of CIECAM02, M. Sarifuddin and Rokia Missaoui from the University of Quebeq proposed a different uniform colour space which claimed better performance.

ICC colour management and virtually all colour appearance models, are based on CIE colorimetry, which has remained stable since 1931. Basing colour source interpretation on CIE colorimetry does not limit quality, and maintains a consistent colour conversion basis for any colour rendering algorithm that may be used. For ICC profile users, flexibility in choosing gamut mapping to convert between similar colour encodings, colour rendering to create an image from a scene, or colour re-rendering, e.g. to create an optimized print from a monitor display image, is provided through the predetermined [and pre-optimized] rendering intent transforms in ICC profiles, and the colorimetric encoding of the PCS.

When a run-time colour appearance adaptation is required, support is provided by the chromatic adaptation and viewing conditions tags in ICC profiles.

The Microsoft Windows Colour System will allegedly provide the following benefits to customers:

• A completely revamped colour infrastructure and translation engine (CITE). CITE is the core of a novel colour management paradigm that is modular, transparent, unambiguous, and understandable.
• An unparalleled extensibility framework through which imaging IHV and ISV partners can enhance and improve the components of the CITE for their business-critical scenarios.
• An enhanced colour processing pipeline with support for greater bit depths, multiple colour channels, and alternative colour spaces. This includes support for instances of extended dynamic range and gamut colour space families such as IEC 61966-2-2 (scRGB).
• Seamless interoperability with ICC-based workflows, including continued support for current ICM2 application programming interfaces (APIs) and sRGB (IEC 61966-2-1, a standard RGB colour space) with extended APIs to support new functionality.
• A notably improved user experience through a centralized colour control panel and an intuitive and easy-to-use monitor calibration wizard.
• Important enhancements to Microsoft imaging codecs as well as key improvements to the core colour management and print infrastructure to support very specific target scenarios such as enterprise colour printing and out-of-box digital photography.