Microsoft has warned open source developers off using its ASF streaming media format.

The author of VirtualDub, an open source video capture and editing package which reverse engineered the ASF format, has dropped ASF support after a call from Microsoft's Windows Media group.

Avery Lee, the author of Virtual Dub, says he received "a polite phone call from a fellow at Microsoft who works in the Windows Media group. He informed me that Microsoft has intellectual property rights on the ASF format and told me that, although I had reverse engineered it, the implementation was still illegal since it infringed on Microsoft patents."

Lee has little option but to comply. "At his request, and much to my own sadness, I have removed support for ASF in VirtualDub 1.3d, since I cannot risk a legal confrontation. This unfortunately means that I can no longer redistribute versions of VirtualDub older than V1.3d." Lee consequently urges other coders not to encode to the ASF format.

ASF is indeed currently the intellectual property of Microsoft, one of a clutch of posthumous - and highly contested - patents the company has been filing since the new year. United States Patent 6,041,345 was granted on March 21 this year.

Carlos Morgado, editor of Gildot, the Portuguese Slashdot-style community, described it as "one of the most bogus patents I have ever seen" in an online forum. ®