Microsoft hopes scRGB will improve photo colours

For a computer, dealing with colour is just another maths problem. And Microsoft wants to change the way your PC counts.

The company has developed a colour space -- a way to encode colours as numbers a computer can process -- called scRGB. If the company succeeds in getting it to catch on, the technology could help add depth and richness to photos taken with digital cameras and viewed on a computer or TV screen.

This image shows overexposed highlights, a common problem with digital photography. Look below to see some image-editing results. (Credit: Microsoft/Bill Crow)

Today's cameras and computers usually employ a colour space called sRGB, developed in the 1990s by Microsoft and Hewlett-Packard, that describes colours as a particular combination of red, green, and blue. But sRGB is limited both in the breadth of colours it can display and in the subtlety of the tonal shades that separate bright from dark, and scRGB is designed to lift those limits.

The sRGB goal was to make Web page colours more consistent from one computer to the next, and it succeeded at that, said Bill Crow, who has led Microsoft's HD Photo effort to improve digital imagery and who has just been named group manager of the company's Microsoft Live Labs Seadragon imaging effort. But it's designed to match the performance only of decade-old CRT monitors, which fall short of the spectrum of hues that human eyes can distinguish and that newer monitors can display, he said.

"The challenge of sRGB is that it's a subset of the total colour space. We are discarding colours when we encode ... into sRGB," Crow said. "ScRGB would allow a richer saturated red value ... than the sRGB limit for red."

The human eye is good at discerning the subtleties of shadows even on a sunny day, but not so digital cameras, monitors, and file formats such as JPEG. And although most people aren't discontent with the state of image display on their computers, digital photography is exposing limits of sRGB for experts, enthusiasts, and even ordinary photographers trying to edit their photos.

Boosting HDR
The shortcomings of today's technology are particularly glaring for one burgeoning photography technique taking root among enthusiasts, high-dynamic range, or HDR, photography. This technique merges multiple photos taken at different exposures into a single shot whose tonal range can accommodate the brightest brights, the darkest darks, and everything in between.

The left image shows how JPEG, with its narrow dynamic range, sometimes loses data when edited to correct for overexposure. The right HD Photo image, encoded with scRGB, has more range, so highlights can be recovered better. (Credit: Microsoft/Bill Crow)

The first problem is the initial photos can't manage the whole dynamic range. The second is that once the HDR image is created, computer monitors can't display it the way a human eye would see it. Instead, HDR today involves a process called "tone mapping" to pick which shades to display or not. The result can be halos around dark areas and other peculiarities.

"Dealing with HDR information in general right now is challenging, because you can't see it on your monitor," said Kevin Connor, senior director of product management for Adobe's digital imaging group. "Until the display technologies pick up, it's hard for the average person to actually make broad use of it."

For the more mainstream case of photo editing, Crow said, scRGB can help bring about a future in which a washed-out patch of white in a photo can be corrected to recover different gradations of brightness.

How scRGB works
The scRGB colour space is an extension of sRGB, Crow said, and colours such as black or 100 percent green looks the same with either. What's different with scRGB is that it can extend farther -- well beyond the colours that can be seen by the human eye -- and subdivide more finely than sRGB.

ScRGB can lavish double or even quadruple the number of bits of data used to describe each pixel. And it can use not only integers but also floating-point numbers that enable much narrower steps on the path from light to dark.

ScRGB isn't just an academic idea. It's been a royalty-free standard (click for PDF) since 2003 and therefore is "free for anyone to use," Crow said. But the more likely impetus for adoption is that scRGB support is built into Windows Vista and into the HD Photo file format Microsoft is working to standardise as JPEG XR.

If Microsoft succeeds in getting HD Photo to catch on as JPEG XR, it could help carry scRGB beyond the PC.

"I absolutely expect scRGB support in cameras to accompany JPEG XR," Crow said. One sRGB weakness is that it's designed to be an "output-referred" colour space -- one geared specifically for showing information on displays. In contrast, scRGB is also designed for input devices such as cameras and processing such as Windows Vista's image-handling mechanism.

Chipmakers have already implemented the technology in camera image processors, but Crow wouldn't estimate when that technology might make it to market.

Recording more data per pixel makes for larger file sizes, but Microsoft argues that the space is regained when using its scRGB-compatible image file format, HD Photo, which has roughly twice the compression efficiency as today's JPEG standard.

With fewer colours and smaller numbers, sRGB may sound simpler to deal with, but those limits bring complications.

"The smaller the colour space, the bigger the colour-management problem," Connor said. "Each device has different set of trade-offs about which colours it's trying to capture and which it says it's not going to bother with."

For example, to best use sRGB's maximum of 256 levels of brightness, computers were adapted to put the gradations where people are most sensitive to their presence: in the shadows. A mathematical parameter called "gamma" determines the location of the midpoint on the curve from dark to bright and therefore where the shades of grey show up. But not every software package and computer monitor is set up with the same gamma setting.

In contrast, scRGB ignores gamma altogether, evenly dividing the tonal gradations. That simplifies the mathematical operations of image-editing software, Crow said.

Slow transitions
Another colour space, AdobeRGB, gradually has caught on as an alternative that can show a wider gamut of colours than sRGB. But it illustrates a challenge of getting a new colour space to catch on: it's supported generally only in higher-end cameras, and most software, such as the current versions of Internet Explorer and Firefox, can't handle it. An image on the Web that's encoded with AdobeRGB looks drained of colour unless you're viewing it with Apple's Safari browser.

AdobeRGB caught on in part through the success of Photoshop, the king of the image-editor mountain. Photoshop 5 introduced the color space as a way to accommodate blue colours used in printing that weren't described by sRGB, Connor said.

Microsoft lacks Adobe's imaging clout, but possesses a different power: Windows. "Any time you have Microsoft endorsing a standard," said Erik Willey, director of product marketing for desktop displays at monitor maker ViewSonic, "then it's got momentum behind it."