title: ffmpeg motion interpolation for 2x and 4x slowmotion
body: <p>I recently learn that I can use minterpolate option of ffmpeg for frame interpolation in a video. One of the configuration of this option is fps(frame per second) and I am not sure how I should set it to generate 2x and 4x slow-motion videos.
  Here is how I use it right now:
  ffmpeg -i input.avi  -filter "minterpolate='fps=120'" output.avi</p>
  
  <p>seems like the generated output video doesn't have equal number of frames generated between every 2 consecutive frames.
  Can anyone help me here or point me to a helpful document?</p>
  
  <p>Thank you,</p>