"HD" doesn't actually stand for "high definition," but it's supposed to connote the better image quality that comes with HD TV. Rico Malvar, a Microsoft Research director who helped develop the format, said that compared with JPEG, HD Photo preserves more subtle details, offers richer colors and takes up half the storage space at the same image quality.So why is HD Photo better than JPEG? Here are some quick comparisons:
It is tough to get new image formats to catch on, much less to replace prevailing standards, but Microsoft has two strong forces on its side.
Windows Vista is the first operating system with built-in support for this new image format and Adobe will release a HD Photo plugin for both the Windows and Mac OS X version of PhotoShop CS3.
Per pixel, HD Photo stores at least 16 bits of data for each color. JPEG only stores 8 bits. HD Photo also supports 32 bits per color, which is useful for "high dynamic range" photos. HD Photo promises the same quality as JPEG at half the file size - or twice the quality at the same file size. The algorithm uses simple instructions that can be relatively easily built into cameras' image-processing chips. At the highest standard, HD Photo is lossless, no quality is lost. JPEG is lossy, which means some quality gets lost. HD Photo uses the Microsoft scRGB color space, which has a wider gamut of possible colors than sRGB. The algorithm of HD Photo can decode only a selected portion of a photo, rather than the whole image. This improves performance and reduces memory requirements. The image can also by encoded chunk by chunk, which means the entire image doesn't have to be stored in your system's memory HD Photo images can be gargantuan--262 million pixels on an edge, or 68.6 terapixels total, as long as the compressed image doesn't exceed 32GB in size.
However, it will be hard to replace JPEG.
"JPEG is an industry standard with a variety of quality levels within its architecture," said Sally Smith Clemens, a product manager at Olympus Imaging America. "A replacement format would have to offer very broad support from many developers of both hardware and software to be practical or considered."
Here's a comparison from Microsoft of the JPEG, JPEG-2000 and HD Photo image file formats. The picture in the upper left is the uncompressed raw image file and the other images were compressed to 12.5% of the original file size. The compressed images were then subtracted from the uncompressed image and the absolute value is amplified to compare how many errors there are. In this case, less color means better - totally black would mean there was no difference in quality.