Here Comes Yet Another Color Standard

It’s long been appreciated that existing color systems are constrained, not only by current display technologies, but also by the standards that define the color gamuts. While sRGB can be extended to 10 bits per color, and xvYCC can extend the color gamut by non-linear mapping of unused values, these color spaces are still limited with respect to the precision and ultimate color gamut that can be achieved by a "supra-BT.709" definition.

Enter scRGB, whose color encoding extends the sRGB tonal range and bit precision by defining a larger color space. By increasing the precision of color processing, scRGB is said to be "well suited for graphic arts RGB workflows, professional digital photography, computer gamuts and computer graphics." Formerly known as sRGB64, scRGB is based on sRGB, and has the same primaries and white point. However, scRGB offers a larger, scaled color gamut and 48-bit encoding with 16-bits per channel (64 bits including an alpha channel), defining over 65,000 steps for each color instead of the 256 steps available with the 8-bits of sRGB. The extra 8 bits per channel are distributed between the high order bits for the expanded gamut and the low order bits for greater precision within the gamut.

While scRGB is not new, Microsoft recently announced that the Windows Color System (WCS) and display drivers in Windows 7 will support high color, e.g., xvYCC, 10-bit sRGB, and now scRGB. Windows 7 is the next version of Microsoft Windows, and at its Professional Developers Conference in October, Microsoft delivered a pre-beta build of Windows 7 to PDC attendees, and announced plans to release a full Windows 7 beta early next year. A recent Microsoft job posting also indicated that Windows 7 was planning for full release in FY2010.

Microsoft says that their goal for scRGB is to give it the same ease of use and simplicity of sRGB. However, the additional bit-depth will require more GPU/CPU bandwidth, especially as the GPU must convert scRGB images on-the-fly to be viewable on current displays. Nonetheless, Microsoft says that modern GPUs and displays already support these pixel formats, and new brands are shipping with support for 10-bits and beyond.

Cynics feel that these new features are designed to exclude competition, rather than increase interoperability. However, Microsoft must still operate under the terms of the 2002 U.S. District Court anti-trust decree, which requires that for any new middleware feature added to Windows, the company must help rivals integrate their similar software with Windows. We’ll have to see if the reverse is true-that scRGB drivers migrate to other platforms such as Linux or Mac OS as well. Even without help from Microsoft, the equations for scRGB are clear and should pose no problem for Apple or others.

Another issue is that of backward compatibility, since non-compliant decoders may clip colors at the boundaries of their color-space intersection. Transformations between the standards are defined, however, so new devices should be able to properly display images. This backward compatibility may be a minor issue, since graphics designers and video editors upgrade their hardware and software frequently to match improved standards such as this. After all, scRGB, like sRGB or sRGB64, is a standard for professionals, not consumers. At the end of the content creation process, the content creators will map the expanded scRGB colors into xvYCC or even the Rec. 709 format for consumer viewing. Some experts say that using xvYCC to encode DCI colors onto an HDTV signal is a better way to go when the ultimate destination is an HDTV consumer monitor. This would allow a single color grading step to serve for both digital cinema and HDTV, compared to the two color grading steps required now. One of my colleagues at Insight Media suggests that the maker of a TV with such an expanded color gamut could advertise, "See the same colors you see in the movies!"

By Aldo Cugnini, DisplayDaily