[FFmpeg-devel] [PATCH 1/2] libavformat/matroskadec: set fixed duration for subtitles

[Miguel Borges de Freitas]

From: Miguel Borges de Freitas <enen92 at kodi.tv>

The matroska specification states the start time and duration of
subtitle entries are encoded in the block TimeStamp
and BlockDuration. Furthermore, for all subtitle formats except
S_HDMV/PGS the BlockDuration must always be defined and have an
absolute value even if it is simply 0. ffmpeg assumes that a duration
of 0 means the duration is still unknown and tries to adjust based on
the next packet pts. This is wrong for all formats except S_HDMV/PGS.
Since changing the semantics of duration 0 is not an option (touches
too many parts of the code) this change introduces
AV_PKT_FLAG_FIXED_DURATION flag which decoders might use to flag the
duration of a given packet should not be changed.

Signed-off-by: Miguel Borges de Freitas <enen92 at kodi.tv>
---
 libavcodec/packet.h       | 5 +++++
 libavformat/demux.c       | 3 ++-
 libavformat/matroskadec.c | 4 ++++
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libavcodec/packet.h b/libavcodec/packet.h
index f28e7e7011..699dcc6f79 100644
--- a/libavcodec/packet.h
+++ b/libavcodec/packet.h
@@ -446,6 +446,11 @@ typedef struct AVPacketList {
  * be discarded by the decoder.  I.e. Non-reference frames.
  */
 #define AV_PKT_FLAG_DISPOSABLE 0x0010
+/**
+ * Flag is used to indicate packets in which the duration is absolute
+ * and should not be changed.
+ */
+#define AV_PKT_FLAG_FIXED_DURATION 0x0020
 
 enum AVSideDataParamChangeFlags {
 #if FF_API_OLD_CHANNEL_LAYOUT
diff --git a/libavformat/demux.c b/libavformat/demux.c
index 2dfd82a63c..471be5d3dd 100644
--- a/libavformat/demux.c
+++ b/libavformat/demux.c
@@ -914,7 +914,8 @@ static void update_initial_durations(AVFormatContext *s, AVStream *st,
             pktl->pkt.dts = cur_dts;
             if (!sti->avctx->has_b_frames)
                 pktl->pkt.pts = cur_dts;
-            pktl->pkt.duration = duration;
+            if ((pktl->pkt.flags & AV_PKT_FLAG_FIXED_DURATION) != AV_PKT_FLAG_FIXED_DURATION)
+                pktl->pkt.duration = duration;
         } else
             break;
         cur_dts = pktl->pkt.dts + pktl->pkt.duration;
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index d582f566a2..e887f43e1a 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -3673,6 +3673,10 @@ static int matroska_parse_frame(MatroskaDemuxContext *matroska,
     pkt->pos = pos;
     pkt->duration = lace_duration;
 
+    if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE && 
+        st->codecpar->codec_id != AV_CODEC_ID_HDMV_PGS_SUBTITLE)
+        pkt->flags |= AV_PKT_FLAG_FIXED_DURATION;
+
     res = avpriv_packet_list_put(&matroska->queue, pkt, NULL, 0);
     if (res < 0) {
         av_packet_unref(pkt);
-- 
ffmpeg-codebot