title: How to get the frame numbers that are dropped when changing the frame rate using ffmpeg
body: <p>I have a video that plays at 30fps. I changed the frame rate to 20fps by using the command
  <code>ffmpeg -i lamp02_20210212_060804-compressed.mp4 -filter:v fps=20 xx.mp4</code>. But I want to get the frame numbers of the dropped frames. I saw here <a href="https://stackoverflow.com/questions/1555771/which-frame-does-ffmpeg-get-when-reducing-the-frame-rate">Which frame does ffmpeg get when reducing the frame rate</a> that you can get the frame but I require only the frame number. Is it possible to do this and if yes, how? Any pointers in the right direction will be helpful!</p>