Image File Formats; Will WMP Be the Last Word?

Few issues have been so consistently quarrelsome over the years as schemes used to compress images and their associated file formats.

GIF, the "Graphics Interchange Format," is the first that springs to mind. Introduced by CompuServe in the late 1980s, GIF is based on LZW data compression, a technique patented by Unisys--a fact that only came to light after GIF was in widespread use. Royalties were claimed and lawsuit threatened. It was a mess that really only went away when Unisys the patent expired in 2003. (For details on GIF, see "Reading GIF Files" by Wilson MacGyver Liaw; DDJ, February 1995.)

Then in 2004, Forgent Networks threatened to sue a bunch of hardware and software vendors, including the likes of Dell and Apple, for allegedly infringing on its claim to a algorithm in the JPEG, the "Joint Photographic Experts Group" file format. Also developed in the 1980s, JPEG is based on a patented algorithm Forgent laid claim to based on its acquisition of Compression Labs in 1997. That patent is due to expire later this year. (See "JPEG-Like Image Compression," by Craig A. Lindley; DDJ, July and August 1995.)

And then just today (05/26/06), Groklaw reports that the U.S. Patent and Trademark Office has ruled the Forgent patent invalid.

Clarification (06/01/06): Subsequent to this blog entry, I received the following clarification from a Forgent spokesperson:

Please note the USPTO did not reject Forgent’s ‘672 patent. The USPTO rejected 19 of the 46 claims found within the ‘672 patent, not the entire patent. If you would, please correct the wording in this paragraph. It appears Groklaw obtained this piece of information from the announcement the Public Patent Foundation (PUBPAT) put out on May 26.

Then PNG came along, a dollar short and a day late as it turned out. A public domain alternative, PNG ("Portable Network Graphic) avoided most of the acrimonious legal wrangling of GIF and JPEG, but was introduced too near the end of the LZW patent lifecycle to gain much traction. Why shift to PNG, in other words, when the LZW patent will expire in a few months? (See "PNG: The Portable Network Graphic Format," by Lee Daniel Crocker, DDJ, July 1995.)

Over the years, other compressed image file formats came aboard, including the likes of JPEG 2000, Apple's PICT, TIF and PSD from Adobe, SVG, the "Scalable Vector Graphics" from the W3C, and Microsoft's BMP, to mention a few.

You'd think that with all these alternatives, we have all the compressed image files we need. Think again.

Microsoft has jump into the deep end of the file format pool again, this time with its Windows Media Photo Specification. To be built into Windows Vista, WMP is a file format for continuous-tone still images that supposedly delivers 25:1 compression ratios for most uses of digital photography (by comparsion, JPEG delivers a maximum of about 12:1 for consumer JPEG images). According to Microsoft, WMP supports features such as:

• Multiple color formats for display or print
• Fixed or floating-point high dynamic range image encoding
• Lossless or high-quality lossy compression
• Efficient decoding for multiple resolutions and sub-regions
• Minimal overhead for format conversion or transformations during decode

According to EE Times's Rick Merriott:

WMP is based on a symmetrical algorithm that supports both lossless and lossy compression. It requires no complex math or special hardware support, and is based primarily on add and shift operations with few multiplies in its inner loops. Memory requirements are also minimal, in part because the algorithm supports encoding and decoding imagines in stripes that only need small buffers.

To preserve compatibility with existing systems, the WMP format uses the existing TIFF "container" including its approach to metadata. The choice of TIFF however limits file sizes to 4 gigabytes, a limit Microsoft will address for high-end users in the future. Microsoft will also release tools to support WMP on existing Windows XP systems.

Microsoft released to a broad group of licensed development partners a developers kit that includes source code for WMP. The kit will let chip and system makers build support for WMP in their products.

One interesting question data-compression expert Mark Nelson asks over at the Data Compression News Blog is whether WMP will hasten my the demise of the JPEG format.

The one thing for sure is that, while it likely will make a splash, WMP won't be the last word in compressed image file formats.

Posted by Jon Erickson at 09:43 AM  Permalink