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MPlayer can handle variable-framerate streams where it just has a timestamp for each packet rather than a fixed framerate.

I wish MPlayer worked that way, but unfortunately it doesn't. For MPEG-4 (or H.261, H.263, or motion-JPEG) video streams, MPlayer would complain, and not play video at all, if the user did not use the "-fps" option. My code now overcomes this by looking at (and buffering) incoming packets until it is able to figure out the frame rate; then it sets "sh_video->fps", and from then on the rest of MPlayer is happy.

Well then your demuxer code is probably broken, because variable fps works fine with asf files and quicktime files.

Fact 1: My demuxer code is setting incoming packets' "pts" (presentation timestamp) field correctly. Fact 2: If I leave "sh_video->fps" at zero (and the user has not used "-fps"), then the rest of MPlayer *will complain* about this, and *will not* play video, if the incoming stream is MPEG-4, H.26*, or motion-JPEG video. These facts are *beyond dispute*. If there is something else that I can be doing to stop MPlayer from complaining, other than (i) requiring the user to specify "-fps", or (ii) guessing the frame rate (e.g., from successive packet timestamps) and setting "sh_video->fps" accordingly, then I'm all ears. Ross.