avformat/matroskadec: Add support for FlagTextDescriptions

This is the equivalent of the WebM "D_WEBVTT/DESCRIPTIONS" and is
therefore only exported for subtitles.

Reviewed-by: Ridley Combs <rcombs@rcombs.me>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroska.h

@@ -100,6 +100,7 @@
 #define MATROSKA_ID_TRACKFLAGFORCED 0x55AA
 #define MATROSKA_ID_TRACKFLAGHEARINGIMPAIRED  0x55AB
 #define MATROSKA_ID_TRACKFLAGVISUALIMPAIRED   0x55AC
+#define MATROSKA_ID_TRACKFLAGTEXTDESCRIPTIONS 0x55AD
 #define MATROSKA_ID_TRACKFLAGCOMMENTARY       0x55AF
 #define MATROSKA_ID_TRACKFLAGLACING 0x9C
 #define MATROSKA_ID_TRACKMINCACHE 0x6DE7

libavformat/matroskadec.c

@@ -253,6 +253,7 @@ typedef struct MatroskaTrack {
     uint64_t flag_comment;
     uint64_t flag_hearingimpaired;
     uint64_t flag_visualimpaired;
+    uint64_t flag_textdescriptions;
     uint64_t seek_preroll;
     MatroskaTrackVideo video;
     MatroskaTrackAudio audio;
@@ -412,7 +413,7 @@ typedef struct MatroskaDemuxContext {
 // incomplete type (6.7.2 in C90, 6.9.2 in C99).
 // Removing the sizes breaks MSVC.
 static EbmlSyntax ebml_syntax[3], matroska_segment[9], matroska_track_video_color[15], matroska_track_video[19],
-                  matroska_track[30], matroska_track_encoding[6], matroska_track_encodings[2],
+                  matroska_track[31], matroska_track_encoding[6], matroska_track_encodings[2],
                   matroska_track_combine_planes[2], matroska_track_operation[2], matroska_tracks[2],
                   matroska_attachments[2], matroska_chapter_entry[9], matroska_chapter[6], matroska_chapters[2],
                   matroska_index_entry[3], matroska_index[2], matroska_tag[3], matroska_tags[2], matroska_seekhead[2],
@@ -579,6 +580,7 @@ static EbmlSyntax matroska_track[] = {
     { MATROSKA_ID_TRACKFLAGFORCED,       EBML_UINT,  0, 0, offsetof(MatroskaTrack, flag_forced),  { .u = 0 } },
     { MATROSKA_ID_TRACKFLAGHEARINGIMPAIRED, EBML_UINT, 0, 0, offsetof(MatroskaTrack, flag_hearingimpaired), { .u = 0 } },
     { MATROSKA_ID_TRACKFLAGVISUALIMPAIRED, EBML_UINT, 0, 0, offsetof(MatroskaTrack, flag_visualimpaired), { .u = 0 } },
+    { MATROSKA_ID_TRACKFLAGTEXTDESCRIPTIONS, EBML_UINT, 0, 0, offsetof(MatroskaTrack, flag_textdescriptions), { .u = 0 } },
     { MATROSKA_ID_TRACKVIDEO,            EBML_NEST,  0, 0, offsetof(MatroskaTrack, video),        { .n = matroska_track_video } },
     { MATROSKA_ID_TRACKAUDIO,            EBML_NEST,  0, 0, offsetof(MatroskaTrack, audio),        { .n = matroska_track_audio } },
     { MATROSKA_ID_TRACKOPERATION,        EBML_NEST,  0, 0, offsetof(MatroskaTrack, operation),    { .n = matroska_track_operation } },
@@ -2880,6 +2882,9 @@ static int matroska_parse_tracks(AVFormatContext *s)
             }
         } else if (track->type == MATROSKA_TRACK_TYPE_SUBTITLE) {
             st->codecpar->codec_type = AVMEDIA_TYPE_SUBTITLE;
+
+            if (track->flag_textdescriptions)
+                st->disposition |= AV_DISPOSITION_DESCRIPTIONS;
         }
     }