Fixed comments and added more supported formats

yt_dlp/postprocessor/ffmpeg.py

@@ -1047,21 +1047,32 @@ class FFmpegSplitChaptersPP(FFmpegPostProcessor):
             ['-ss', str(chapter['start_time']),
              '-t', str(chapter['end_time'] - chapter['start_time'])])
 
+    # Extends opts with chapter specific metadata for the supported formats.
+    #
+    # Tested and supported on opus, m4a, webm and mp4.
+    def _set_metadata_arg(self, opts, ext, key, value):
+        if ext == 'opus':
+            # opus file requires a stream to keep title, artist etc metadata.
+            # FFmpegMetadataPP already set metadata and created that stream.
+            # Futher metadata updates should be set on the stream(:s)
+            # -metadata will do nothing and needs to be -metadata:s
+            opts.extend(['-metadata:s', f'{key}={value}'])
+        elif ext in ['m4a', 'webm', 'mp4']:
+            opts.extend(['-metadata', f'{key}={value}'])
+
     # FFmpeg adds metadata about all chapters from parent file to all split m4a files.
     # This is incorrect since there must be only single chapter in each file after split.
     # Such behavior confuses players who think multiple chapters present
-    def _set_out_opts(self, ext, chapter_title):
+    def _set_out_opts(self, ext, chapter_title, track_number):
         out_opts = [*self.stream_copy_opts()]
         out_opts.extend(['-map_metadata', '0'])
         # exclude chapters metadata but keep everything else
         out_opts.extend(['-map_chapters', '-1'])
-        if not chapter_title:
-            return out_opts
+
         # replace global title with chapter specific title in split files
-        if ext == 'm4a':
-            out_opts.extend(['-metadata', 'title={}'.format(chapter_title)])
-        if ext == 'opus':
-            out_opts.extend(['-metadata:s', 'title={}'.format(chapter_title)])
+        if chapter_title:
+            self._set_metadata_arg(out_opts, ext, "title", chapter_title)
+        self._set_metadata_arg(out_opts, ext, "track", track_number)
         return out_opts
 
     @PostProcessor._restrict_to(images=False)
@@ -1078,7 +1089,7 @@ class FFmpegSplitChaptersPP(FFmpegPostProcessor):
         self.to_screen('Splitting video by chapters; %d chapters found' % len(chapters))
         for idx, chapter in enumerate(chapters):
             destination, opts = self._ffmpeg_args_for_chapter(idx + 1, chapter, info)
-            out_file_opts = self._set_out_opts(info['ext'], chapter.get('title', ''))
+            out_file_opts = self._set_out_opts(info['ext'], chapter.get('title', ''), str(idx + 1))
             self.real_run_ffmpeg([(in_file, opts)], [(destination, out_file_opts)])
         if in_file != info['filepath']:
             self._delete_downloaded_files(in_file, msg=None)