title: Ffmpeg blur part of video interacively
body: <p>I want to blur part of live stream conditionally in realtime with ffmpeg.<br />
  I read about <code>zeromq</code> filter, but the output of <code>ffmpeg -filters</code> says that <code>boxblur</code> filter doesn't support commands. For example, I want to blur the bottom half of the video then after some time disable blur then blur all video.<br />
  How can I achieve this?</p>