Forgent Networks has added Microsoft to the list of companies it has sued alleging infringement of a patent for a data compression technique it claims is used in the JPEG digital image standard.

The lawsuit, filed Thursday in US District Court for the Eastern District of Texas, comes after Forgent was unable to negotiate a licensing agreement with the Redmond, Washington-based software maker, said Michael Noonan, director of investor relations at Forgent.

"We want them to pay a reasonable royalty rate for the technology they are using," Noonan said. "If a company uses JPEG, they are using our patents."

In an apparent preemptive strike, Microsoft last Friday sued Forgent subsidiary Compression Labs in US District Court for the Northern District of California in San Francisco. In the suit, Microsoft asked the court to declare that it is not infringing and invalidate the patent in question, Forgent said in a statement.

Forgent sued 31 companies in April 2004 and several other companies after that. The company has reached licensing agreements with more than 35 companies and received more than $100 million in licensing revenue to this point, it said. Licensees include Sony, Adobe, Macromedia and Onkyo, Noonan said.

Companies that have been sued include Apple, Dell, Kodak, HP, IBM and Xerox.

JPEG (Joint Photographic Experts Group) files are used by a wide variety of hardware and software products to display digital images. The procedure used to compress digital images in order to create a JPEG file infringes on Forgent's patent for a method of digital image compression, the Austin, Texas, company alleges.

In 2002, Forgent announced it held this patent and said it planned to seek licensing agreements from any company that sells products that compress or store JPEG images.

A spokesman for Microsoft had no immediate comment.