Add postprocessor for converting subtitles (closes #4954)

youtube_dl/postprocessor/ffmpeg.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import io
 import os
 import subprocess
 import sys
@@ -635,3 +636,40 @@ class FFmpegFixupM4aPP(FFmpegPostProcessor):
         os.rename(encodeFilename(temp_filename), encodeFilename(filename))
 
         return True, info
+
+
+class FFmpegSubtitlesConvertorPP(FFmpegPostProcessor):
+    def __init__(self, downloader=None, format=None):
+        super(FFmpegSubtitlesConvertorPP, self).__init__(downloader)
+        self.format = format
+
+    def run(self, info):
+        subs = info.get('requested_subtitles')
+        filename = info['filepath']
+        new_ext = self.format
+        new_format = new_ext
+        if new_format == 'vtt':
+            new_format = 'webvtt'
+        if subs is None:
+            self._downloader.to_screen('[ffmpeg] There aren\'t any subtitles to convert')
+            return True, info
+        self._downloader.to_screen('[ffmpeg] Converting subtitles')
+        for lang, sub in subs.items():
+            ext = sub['ext']
+            if ext == new_ext:
+                self._downloader.to_screen(
+                    '[ffmpeg] Subtitle file for %s is already in the requested'
+                    'format' % new_ext)
+                continue
+            new_file = subtitles_filename(filename, lang, new_ext)
+            self.run_ffmpeg(
+                subtitles_filename(filename, lang, ext),
+                new_file, ['-f', new_format])
+
+            with io.open(new_file, 'rt', encoding='utf-8') as f:
+                subs[lang] = {
+                    'ext': ext,
+                    'data': f.read(),
+                }
+
+        return True, info