[downloader/ism] Support muxing TTML subtitles

yt_dlp/downloader/ism.py

@@ -118,8 +118,7 @@ def write_piff_header(stream, params):
         vmhd_payload += u16.pack(0) * 3  # opcolor
         media_header_box = full_box(b'vmhd', 0, 1, vmhd_payload)  # Video Media Header
     elif stream_type == 'text':
-        sthd_payload = u16.pack(0) * 2
-        media_header_box = full_box(b'sthd', 0, 1, sthd_payload)  # Subtitle Media Header
+        media_header_box = full_box(b'sthd', 0, 0, b'')  # Subtitle Media Header
     else:
         assert False
     minf_payload = media_header_box
@@ -173,6 +172,14 @@ def write_piff_header(stream, params):
             sample_entry_box = box(b'avc1', sample_entry_payload)  # AVC Simple Entry
         else:
             assert False
+    elif stream_type == 'text':
+        if fourcc == 'TTML':
+            sample_entry_payload += b'http://www.w3.org/ns/ttml\0'  # namespace
+            sample_entry_payload += b'\0'  # schema location
+            sample_entry_payload += b'\0'  # auxilary mime types(??)
+            sample_entry_box = box(b'stpp', sample_entry_payload)
+        else:
+            assert False
     else:
         assert False
     stsd_payload += sample_entry_box

yt_dlp/extractor/common.py

@@ -2816,7 +2816,7 @@ class InfoExtractor(object):
             for track in stream.findall('QualityLevel'):
                 fourcc = track.get('FourCC', 'AACL' if track.get('AudioTag') == '255' else None)
                 # TODO: add support for WVC1 and WMAP
-                if fourcc not in ('H264', 'AVC1', 'AACL'):
+                if fourcc not in ('H264', 'AVC1', 'AACL', 'TTML'):
                     self.report_warning('%s is not a supported codec' % fourcc)
                     continue
                 tbr = int(track.attrib['Bitrate']) // 1000