Filter ssim
  Calculate the SSIM between two video streams.
    Inputs:
       #0: main (video)
       #1: reference (video)
    Outputs:
       #0: default (video)
ssim AVOptions:
  stats_file        <string>     ..FV...... Set file where to store per-frame difference information
  f                 <string>     ..FV...... Set file where to store per-frame difference information

framesync AVOptions:
  eof_action        <int>        ..FV...... Action to take when encountering EOF from secondary input  (from 0 to 2) (default repeat)
     repeat          0            ..FV...... Repeat the previous frame.
     endall          1            ..FV...... End both streams.
     pass            2            ..FV...... Pass through the main input.
  shortest          <boolean>    ..FV...... force termination when the shortest input terminates (default false)
  repeatlast        <boolean>    ..FV...... extend last frame of secondary streams beyond EOF (default true)


