thank you for the information. I ran `mplayer -vf framestep=I foo.wmv`, then I got only black screens during the whole playout, no key frames were found. ... it's strange.

Well, maybe the wmv format is so special in a way that the filter doesn't pick the keyframes correctly; maybe wmv has some special kind of keyframes. But I know hardly anything about wmv..However, if there are technical limitations, that should be reflected/commented in the man page. Could a dev comment on that?