title: FFMPEG Dash with tiles of thumbnail images
body: <p>As of DASH-IF IOP version 4.2, section 6.2.6 defines the notion of image-based tracks in DASH:
  <a href="https://dashif.org/docs/DASH-IF-IOP-v4.3.pdf" rel="nofollow noreferrer">https://dashif.org/docs/DASH-IF-IOP-v4.3.pdf</a>.</p>
  
  <p>This is the ability to have an adaption set made up of mime type images that themselves are a strip of low resolution thumbnails. 
  A player will use these thumbnails when the user hovers their mouse over the video timeline and get a 
  preview of the the frame at that approximate timecode. </p>
  
  <p>Theo player website has a page dedicated to this function for playback:
  <a href="https://www.theoplayer.com/blog/in-stream-thumbnail-support-dvr-dash-streams" rel="nofollow noreferrer">https://www.theoplayer.com/blog/in-stream-thumbnail-support-dvr-dash-streams</a></p>
  
  <p>I need to generate a dash stream (not live) using ffmpeg that also contains these thumbnails. 
  I already have an ffmpeg command that will generate the film strip of jpgs which outputs a thumbnail every 5 seconds of input video and joins 5 of these together in a single jpg:</p>
  
  <p><code>ffmpeg -i INPUT -q:v 20 -vf "select=not(mod(n\,125)),scale=480:270,tile=5x1" -vsync vfr output%d.jpg</code></p>
  
  <p>and the mpeg dash itself:</p>
  
  <p><code>ffmpeg -i INPUT -y -map 0 -acodec aac -ac 2 -ar 48000 -s 960x540 -vcodec libx264 -f dash -preset veryfast -b:v:2 1500k -seg_duration 2 output.mpd</code></p>
  
  <p>But I cannot find a way in ffmpeg to include the thumbnails in the dash mpd file.</p>