sd_lavc: display DVD subs with unknown duration

DVD subs (rarely) have subtitle events without end timestamp. The
duration is unknown, and they should be displayed until they're
replaced by the next event.

FFmpeg fails hard to make us aware whether duration is unknown or
actually 0, so we can't distinguish between these two cases. It fails
at this twice: AVPacket.duration is set to 0 if duration is unknown,
and AVSubtitle.end_display_time has the same issue.

Add a hack that considers all bitmap subtitles with duration==0 as
events with uknown length. I'd rather accidentally display a hidden
subtitle (if they exist at all), instead of not displaying random
subtitles at all.

See github issue #325.

1 files changed, 6 insertions, 0 deletions

diff --git a/sub/sd_lavc.c b/sub/sd_lavc.c
index a906e72f10..b03861025d 100644
--- a/sub/sd_lavc.c
+++ b/sub/sd_lavc.c
@@ -136,6 +136,12 @@ static void decode(struct sd *sd, struct demux_packet *packet)
     AVSubtitle sub;
     AVPacket pkt;
 
+    // libavformat sets duration==0, even if the duration is unknown.
+    // Assume there are no bitmap subs that actually use duration==0 for
+    // hidden subtitle events.
+    if (duration == 0)
+        duration = -1;
+
     clear(priv);
     av_init_packet(&pkt);
     pkt.data = packet->buffer;