[FFmpeg-devel] [PATCH] libavformat/matroskaenc: Add WebM DASH support

Vignesh Venkatasubramanian vigneshv at google.com
Thu May 8 19:11:50 CEST 2014


On Wed, May 7, 2014 at 3:37 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, May 07, 2014 at 02:32:26PM -0700, Vignesh Venkatasubramanian wrote:
>> On Wed, May 7, 2014 at 1:40 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> > On Wed, May 07, 2014 at 09:46:40AM -0700, Vignesh Venkatasubramanian wrote:
>> >> WebM DASH specification [1] requires the Clusters and Cues to be output in a
>> >> specific way. Adding a flag to matroskaenc that will enable support for
>> >> creating WebM/Mkv files conforming to the WebM DASH specification.
>> >>
>> >> [1] http://wiki.webmproject.org/adaptive-streaming/webm-dash-specification
>> >>
>> >> Signed-off-by: Vignesh Venkatasubramanian <vigneshv at google.com>
>> >> ---
>> >>  libavformat/matroskaenc.c | 68 ++++++++++++++++++++++++++++++++++-------------
>> >>  1 file changed, 50 insertions(+), 18 deletions(-)
>> >>
>> >> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
>> >> index e5e6116..3cf6e73 100644
>> >> --- a/libavformat/matroskaenc.c
>> >> +++ b/libavformat/matroskaenc.c
>> >> @@ -70,6 +70,7 @@ typedef struct mkv_seekhead {
>> >>
>> >>  typedef struct {
>> >>      uint64_t        pts;
>> >> +    int             stream_idx;
>> >>      int             tracknum;
>> >>      int64_t         cluster_pos;        ///< file offset of the cluster containing the block
>> >>      int64_t         relative_pos;       ///< relative offset from the position of the cluster containing the block
>> >> @@ -114,6 +115,8 @@ typedef struct MatroskaMuxContext {
>> >>      int cluster_size_limit;
>> >>      int64_t cues_pos;
>> >>      int64_t cluster_time_limit;
>> >> +    int is_dash;
>> >> +    int dash_track_number;
>> >>
>> >>      uint32_t chapter_id_offset;
>> >>      int wrote_chapters;
>> >> @@ -399,8 +402,8 @@ static mkv_cues * mkv_start_cues(int64_t segment_offset)
>> >>      return cues;
>> >>  }
>> >>
>> >> -static int mkv_add_cuepoint(mkv_cues *cues, int stream, int64_t ts, int64_t cluster_pos, int64_t relative_pos,
>> >> -                            int64_t duration)
>> >> +static int mkv_add_cuepoint(mkv_cues *cues, int stream, int tracknum, int64_t ts,
>> >> +                            int64_t cluster_pos, int64_t relative_pos, int64_t duration)
>> >>  {
>> >>      mkv_cuepoint *entries = cues->entries;
>> >>
>> >> @@ -413,7 +416,8 @@ static int mkv_add_cuepoint(mkv_cues *cues, int stream, int64_t ts, int64_t clus
>> >>      cues->entries = entries;
>> >>
>> >>      cues->entries[cues->num_entries].pts           = ts;
>> >> -    cues->entries[cues->num_entries].tracknum      = stream + 1;
>> >> +    cues->entries[cues->num_entries].stream_idx    = stream;
>> >> +    cues->entries[cues->num_entries].tracknum      = tracknum;
>> >>      cues->entries[cues->num_entries].cluster_pos   = cluster_pos - cues->segment_offset;
>> >>      cues->entries[cues->num_entries].relative_pos  = relative_pos;
>> >>      cues->entries[cues->num_entries++].duration    = duration;
>> >> @@ -443,7 +447,7 @@ static int64_t mkv_write_cues(AVIOContext *pb, mkv_cues *cues, mkv_track *tracks
>> >>          for (j = 0; j < num_tracks; j++)
>> >>              tracks[j].has_cue = 0;
>> >>          for (j = 0; j < cues->num_entries - i && entry[j].pts == pts; j++) {
>> >> -            int tracknum = entry[j].tracknum - 1;
>> >> +            int tracknum = entry[j].stream_idx;
>> >>              av_assert0(tracknum>=0 && tracknum<num_tracks);
>> >>              if (tracks[tracknum].has_cue)
>> >>                  continue;
>> >> @@ -646,8 +650,10 @@ static int mkv_write_tracks(AVFormatContext *s)
>> >>              get_aac_sample_rates(s, codec, &sample_rate, &output_sample_rate);
>> >>
>> >>          track = start_ebml_master(pb, MATROSKA_ID_TRACKENTRY, 0);
>> >> -        put_ebml_uint (pb, MATROSKA_ID_TRACKNUMBER     , i + 1);
>> >> -        put_ebml_uint (pb, MATROSKA_ID_TRACKUID        , i + 1);
>> >> +        put_ebml_uint (pb, MATROSKA_ID_TRACKNUMBER,
>> >> +                       mkv->is_dash ? mkv->dash_track_number : i + 1);
>> >> +        put_ebml_uint (pb, MATROSKA_ID_TRACKUID,
>> >> +                       mkv->is_dash ? mkv->dash_track_number : i + 1);
>> >>          put_ebml_uint (pb, MATROSKA_ID_TRACKFLAGLACING , 0);    // no lacing (yet)
>> >>
>> >>          if ((tag = av_dict_get(st->metadata, "title", NULL, 0)))
>> >> @@ -1425,7 +1431,8 @@ static void mkv_write_block(AVFormatContext *s, AVIOContext *pb,
>> >>
>> >>      put_ebml_id(pb, blockid);
>> >>      put_ebml_num(pb, size+4, 0);
>> >> -    avio_w8(pb, 0x80 | (pkt->stream_index + 1));     // this assumes stream_index is less than 126
>> >> +    // this assumes stream_index is less than 126
>> >> +    avio_w8(pb, 0x80 | (mkv->is_dash ? mkv->dash_track_number : (pkt->stream_index + 1)));
>> >>      avio_wb16(pb, ts - mkv->cluster_pts);
>> >>      avio_w8(pb, flags);
>> >>      avio_write(pb, data + offset, size);
>> >> @@ -1539,7 +1546,7 @@ static void mkv_flush_dynbuf(AVFormatContext *s)
>> >>      mkv->dyn_bc = NULL;
>> >>  }
>> >>
>> >> -static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
>> >> +static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt, int add_cue)
>> >>  {
>> >>      MatroskaMuxContext *mkv = s->priv_data;
>> >>      AVIOContext *pb = s->pb;
>> >> @@ -1596,8 +1603,13 @@ static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
>> >>          end_ebml_master(pb, blockgroup);
>> >>      }
>> >>
>> >> -    if ((codec->codec_type == AVMEDIA_TYPE_VIDEO && keyframe) || codec->codec_type == AVMEDIA_TYPE_SUBTITLE) {
>> >> -        ret = mkv_add_cuepoint(mkv->cues, pkt->stream_index, ts, mkv->cluster_pos, relative_packet_pos,
>> >> +    if ((codec->codec_type == AVMEDIA_TYPE_VIDEO && keyframe) ||
>> >> +        codec->codec_type == AVMEDIA_TYPE_SUBTITLE ||
>> >> +        add_cue) {
>> >> +        ret = mkv_add_cuepoint(mkv->cues,
>> >> +                               pkt->stream_index,
>> >> +                               mkv->is_dash ? mkv->dash_track_number : pkt->stream_index + 1,
>> >> +                               ts, mkv->cluster_pos, relative_packet_pos,
>> >>                                 codec->codec_type == AVMEDIA_TYPE_SUBTITLE ? duration : -1);
>> >>          if (ret < 0) return ret;
>> >>      }
>> >> @@ -1615,6 +1627,7 @@ static int mkv_write_packet(AVFormatContext *s, AVPacket *pkt)
>> >>      int64_t cluster_time;
>> >>      AVIOContext *pb;
>> >>      int ret;
>> >> +    int start_new_cluster;
>> >>
>> >>      if (mkv->tracks[pkt->stream_index].write_dts)
>> >>          cluster_time = pkt->dts - mkv->cluster_pts;
>> >> @@ -1632,11 +1645,26 @@ static int mkv_write_packet(AVFormatContext *s, AVPacket *pkt)
>> >>          cluster_size = avio_tell(pb);
>> >>      }
>> >>
>> >> -    if (mkv->cluster_pos != -1 &&
>> >> -        (cluster_size > mkv->cluster_size_limit ||
>> >> -         cluster_time > mkv->cluster_time_limit ||
>> >> -         (codec_type == AVMEDIA_TYPE_VIDEO && keyframe &&
>> >> -          cluster_size > 4 * 1024))) {
>> >> +    if (mkv->is_dash && codec_type == AVMEDIA_TYPE_VIDEO) {
>> >> +        // WebM DASH specification states that the first block of every cluster
>> >> +        // has to be a key frame. So for DASH video, we only create a cluster
>> >> +        // on seeing key frames.
>> >
>> >> +        start_new_cluster = keyframe;
>> >
>> > is it intended to have 1 frame per cluster for intra only videos ?
>>
>> yes. but this only done when -dash parameter is explicitly set. so
>> it's very unlikely that someone would want intra-only video for dash.
>
> Someone might already have a intra only video and want to transmit
> it without reencoding or
> a encoder might choose to encode multiple frames in a row to be
> encoded as intra frames (for example in a very high motion scene)
>
>

that looks like a valid use case. i will rather limit it by
cluster_time_limit and force a keyframe with the encoder whenever a
cluster is to be created. will update the patch soon.

>>
>> >
>> > also, is there an easy way by which this patch can be tested ?
>>
>> i'm not really sure. i have tested it manually. should i see if i can
>> come up with a fate test?
>
> a fate test would certainly be a good idea

will add a fate test too in the follow up patch.

>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> Awnsering whenever a program halts or runs forever is
> On a turing machine, in general impossible (turings halting problem).
> On any real computer, always possible as a real computer has a finite number
> of states N, and will either halt in less than N cycles or never halt.
>
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-- 
Vignesh


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