[Libav-user] How the ffmpeg command line utility's '-r' flag works?

[Ramil Safin]

I have a usb camera '/dev/videoX' and I've implemented decoding process. Supported camera framerates are 15 and 30.
However, I need to reduce its framerate in order to stream video data (RTP/RTSP) from multiple usb cameras (I have 4 cameras) using H.264 codec and keep quality as good as possible.
Also it is essential to be in real-time (small delays). I've been searching on how to reduce the framerate of the output stream, for example to 5 fps.
And I know that in order to reduce the framerate in CLI (ffmpeg)  '-r' flag is used.

And my question is how '-r' functionality is implemented?
Is it filter (AVFilter) or some kind of algorithm?

Thanks.