[FFmpeg-devel] [PATCH] Matroska demuxer adds WebVTT support

[Matthew Heaney]

WebM files now support inband text tracks, as described in the
following specification:

http://wiki.webmproject.org/webm-metadata/temporal-metadata/webvtt-in-webm

The Matroska demuxer now detects the presence of WebVTT tracks,
synthesizing WebVTT packets (having codec id AV_CODEC_ID_WEBVTT) and
pushing them downstream in the normal way.
---
 libavformat/matroska.c    |   5 ++
 libavformat/matroskadec.c | 130 +++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 134 insertions(+), 1 deletion(-)

diff --git a/libavformat/matroska.c b/libavformat/matroska.c
index ee57c18..953c572 100644
--- a/libavformat/matroska.c
+++ b/libavformat/matroska.c
@@ -57,6 +57,11 @@ const CodecTags ff_mkv_codec_tags[]={
     {"A_VORBIS"         , AV_CODEC_ID_VORBIS},
     {"A_WAVPACK4"       , AV_CODEC_ID_WAVPACK},
 
+    {"D_WEBVTT/SUBTITLES"   , AV_CODEC_ID_WEBVTT},
+    {"D_WEBVTT/CAPTIONS"    , AV_CODEC_ID_WEBVTT},
+    {"D_WEBVTT/DESCRIPTIONS", AV_CODEC_ID_WEBVTT},
+    {"D_WEBVTT/METADATA"    , AV_CODEC_ID_WEBVTT},
+
     {"S_TEXT/UTF8"      , AV_CODEC_ID_SUBRIP},
     {"S_TEXT/UTF8"      , AV_CODEC_ID_TEXT},
     {"S_TEXT/UTF8"      , AV_CODEC_ID_SRT},
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 9a15a2b..3f8eabc 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1566,7 +1566,8 @@ static int matroska_read_header(AVFormatContext *s)
         /* Apply some sanity checks. */
         if (track->type != MATROSKA_TRACK_TYPE_VIDEO &&
             track->type != MATROSKA_TRACK_TYPE_AUDIO &&
-            track->type != MATROSKA_TRACK_TYPE_SUBTITLE) {
+            track->type != MATROSKA_TRACK_TYPE_SUBTITLE &&
+            track->type != MATROSKA_TRACK_TYPE_METADATA) {
             av_log(matroska->ctx, AV_LOG_INFO,
                    "Unknown or unsupported track type %"PRIu64"\n",
                    track->type);
@@ -1860,6 +1861,16 @@ static int matroska_read_header(AVFormatContext *s)
             st->codec->bits_per_coded_sample = track->audio.bitdepth;
             if (st->codec->codec_id != AV_CODEC_ID_AAC)
             st->need_parsing = AVSTREAM_PARSE_HEADERS;
+        } else if (codec_id == AV_CODEC_ID_WEBVTT) {
+            st->codec->codec_type = AVMEDIA_TYPE_SUBTITLE;
+
+            if (!strcmp(track->codec_id, "D_WEBVTT/CAPTIONS")) {
+                st->disposition |= AV_DISPOSITION_CAPTIONS;
+            } else if (!strcmp(track->codec_id, "D_WEBVTT/DESCRIPTIONS")) {
+                st->disposition |= AV_DISPOSITION_DESCRIPTIONS;
+            } else if (!strcmp(track->codec_id, "D_WEBVTT/METADATA")) {
+                st->disposition |= AV_DISPOSITION_METADATA;
+            }
         } else if (track->type == MATROSKA_TRACK_TYPE_SUBTITLE) {
             st->codec->codec_type = AVMEDIA_TYPE_SUBTITLE;
 #if FF_API_ASS_SSA
@@ -2226,6 +2237,115 @@ fail:
     return ret;
 }
 
+static int matroska_parse_webvtt(MatroskaDemuxContext *matroska,
+                                 MatroskaTrack *track,
+                                 AVStream *st,
+                                 uint8_t *data, int data_len,
+                                 uint64_t timecode,
+                                 uint64_t duration,
+                                 int64_t pos)
+{
+    AVPacket *pkt;
+    uint8_t *id, *settings, *text, *buf;
+    int id_len, settings_len, text_len;
+
+    uint8_t *p = data;
+    uint8_t *q = data + data_len;
+
+    if (p >= q)
+        return AVERROR_INVALIDDATA;
+
+    id = p;
+    id_len = strcspn(p, "\r\n");
+
+    p += id_len;
+    if (p >= q)
+        return AVERROR_INVALIDDATA;
+
+    if (*p == '\r')
+        p++;
+    if (p >= q || *p != '\n')
+        return AVERROR_INVALIDDATA;
+    p++;
+
+    if (p >= q)
+        return AVERROR_INVALIDDATA;
+
+    settings = p;
+    settings_len = strcspn(p, "\r\n");
+
+    p += settings_len;
+    if (p >= q)
+        return AVERROR_INVALIDDATA;
+
+    if (*p == '\r')
+        p++;
+    if (p >= q || *p != '\n')
+        return AVERROR_INVALIDDATA;
+    p++;
+
+    if (p >= q)
+        return AVERROR_INVALIDDATA;
+
+    text = p;
+    text_len = q - p;
+
+    while (text_len > 0) {
+        const int len = text_len - 1;
+        const uint8_t c = p[len];
+        if (c != '\r' && c != '\n')
+            break;
+        text_len = len;
+    }
+
+    pkt = av_mallocz(sizeof(AVPacket));
+    if (av_new_packet(pkt, text_len) < 0) {
+        av_free(pkt);
+        return AVERROR(ENOMEM);
+    }
+
+    memcpy(pkt->data, text, text_len);
+
+    if (id_len > 0) {
+        buf = av_packet_new_side_data(pkt,
+                                      AV_PKT_DATA_WEBVTT_IDENTIFIER,
+                                      id_len);
+        if (buf == NULL) {
+            av_free(pkt);
+            return AVERROR(ENOMEM);
+        }
+        memcpy(buf, id, id_len);
+    }
+
+    if (settings_len > 0) {
+        buf = av_packet_new_side_data(pkt,
+                                      AV_PKT_DATA_WEBVTT_SETTINGS,
+                                      settings_len);
+        if (buf == NULL) {
+            av_free(pkt);
+            return AVERROR(ENOMEM);
+        }
+        memcpy(buf, settings, settings_len);
+    }
+
+    // Do we need this for subtitles?
+    // pkt->flags = AV_PKT_FLAG_KEY;
+
+    pkt->stream_index = st->index;
+    pkt->pts = timecode;
+
+    // Do we need this for subtitles?
+    // pkt->dts = timecode;
+
+    pkt->duration = duration;
+    pkt->pos = pos;
+
+    dynarray_add(&matroska->packets, &matroska->num_packets, pkt);
+    matroska->prev_pkt = pkt;
+
+    return 0;
+}
+
 static int matroska_parse_frame(MatroskaDemuxContext *matroska,
                                 MatroskaTrack *track,
                                 AVStream *st,
@@ -2454,6 +2574,14 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, uint8_t *data,
             if (res)
                 goto end;
 
+        } else if (st->codec->codec_id == AV_CODEC_ID_WEBVTT) {
+            res = matroska_parse_webvtt(matroska, track, st,
+                                        data, lace_size[n],
+                                        timecode, lace_duration,
+                                        pos);
+            if (res)
+                goto end;
+
         } else {
             res = matroska_parse_frame(matroska, track, st, data, lace_size[n],
                                       timecode, lace_duration,
-- 
1.8.3