title: Recording video with unknown framerate with FFmpeg
body: <p>I am recording video with FFmpeg and I would like the frame rate it is written with to at least be in the right ballpark. Right now I take the frame rate that my input claims to have and use that to set the frame rate (time_base) for my output video stream. However this is sometimes wildly different from the actual frame rate that I am getting (I have seen a stream claiming 50 fps but publishing at 9 fps). </p>
  
  <p>What I would like to do is use a elapsed timer and count the frames I record to calculate the actual frame rate I recorded at when I am finished recording. I would seem though that the frame rate is set in my AVStream is used in avcodec_open2 before I write any frames. If I set it later (such as while I am writing frames) while ffplay can play it (complaining that the time increment bits is 6 not 4) other video players cannot. Is there a way to set the frame rate for the whole file <strong>after</strong> writing the frames? If not is there a way to tell the frames themselves some sort of timestamp or frame rate while I am recording that will result in a valid recorded file? </p>