title: Extract pgssub subtitles from mkv to txt (srt)
body: <p>How to correctly extract pgssub subtitles to txt file?</p>
  
  <p>I have this target input stream:</p>
  
  <blockquote>
    <p>Stream #0:2(eng): Subtitle: hdmv_pgs_subtitle, 1920x1080 (default)</p>
  </blockquote>
  
  <p>I want save this subtitles like srt into txt file:</p>
  
  <pre><code>$ ffmpeg -i inp.mkv -map 0:2 eng.srt
  </code></pre>
  
  <p>ffmpeg says:</p>
  
  <pre><code>blah blah blah...
  Stream mapping:
    Stream #0:2 -&gt; #0:0 (hdmv_pgs_subtitle (pgssub) -&gt; subrip (srt))
  Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height
  </code></pre>
  
  <p>How to correctly extract this subtitles?</p>