How do I get a valid subtitle from a container? #698

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Overview

I need to either entirely demux a subtitle stream from a video container so that I can put it into e.g. an .ass file and it will be valid ass, or transcode a subtitle into a fixed target format like .srt and output that into a valid file.
I thus tried opening a video file and decoding its subtitle stream (ass) and encoding it into a new file with a .srt stream:

import av

cont=av.open("D:/test.mp4")
cont2=av.open('d:/test.srt', 'w')
srtstream = cont2.add_stream('srt')
assframes = cont.decode(subtitles=1)

for f in assframes:

  srtpackets = srtstream.encode(f)

  cont2.mux(srtpackets)
cont2.close()

Expected behavior

I expected calling container.decode() returning frames.

Actual behavior

When decoding a subtitle stream though, its instead returning SubtitleSet objects, which the stream encoder doesn't know to convert them to actual frames.

Traceback:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\scripts\loudspeaker\test.py", line 10, in <module>
    srtpackets = srtstream.encode(f)
  File "av\stream.pyx", line 152, in av.stream.Stream.encode
TypeError: Cannot convert av.subtitles.subtitle.SubtitleSet to av.frame.Frame

Investigation

I tried decoding the subtitle stream itself, like this:

substream = cont.streams.subtitles[1]

frames = substream.decode()

That however crashes the interpreter silently.

I thought about taking the, I suppose, frames, which can be found in SubtitleSet.rects, and simply writing them to a file one by one, but I don't know if that will actually give a correct .ass file in the end.

So is there a way to get either a valid .ass file (demux the ass stream from my container) or, even better, transcode the ass subtitle into a srt one?

Research

I have done the following:

• Checked the PyAV documentation
• Searched on Google
• Searched on Stack Overflow
• Looked through old GitHub issues
• Asked on PyAV Gitter
• ... and waited 72 hours for a response.