[FFmpeg-devel] [PATCH] avformat/movenc: implicitly enable negative CTS offsets for ismv

[Jan Ekström]

ISMV lacks any sort of edit list support, as well as tfxd is
effectively the PTS of the fragment for most intents and purposes.

Thus, if b-frames are requested without negative CTS offsets you
end up with N frames' worth of delay (tfxd PTS plus the CTS offset
of the first sample). Negative CTS offsets enable the first sample
to have CTS=DTS, and thus a/v desync due to b-frame reorder delay
is avoided.
---
 doc/muxers.texi       | 2 ++
 libavformat/movenc.c  | 2 +-
 tests/ref/fate/movenc | 4 ++--
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/doc/muxers.texi b/doc/muxers.texi
index 33f1f7bbaf..268c152023 100644
--- a/doc/muxers.texi
+++ b/doc/muxers.texi
@@ -1344,6 +1344,8 @@ be negative. This enables the initial sample to have DTS/CTS of zero, and
 reduces the need for edit lists for some cases such as video tracks with
 B-frames. Additionally, eases conformance with the DASH-IF interoperability
 guidelines.
+
+This option is implicitly set when writing ismv (Smooth Streaming) files.
 @item -write_prft
 Write producer time reference box (PRFT) with a specified time source for the
 NTP field in the PRFT box. Set value as @samp{wallclock} to specify timesource
diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index d530f40cab..14f85eb2a7 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -6049,7 +6049,7 @@ static int mov_init(AVFormatContext *s)
     /* Set other implicit flags immediately */
     if (mov->mode == MODE_ISM)
         mov->flags |= FF_MOV_FLAG_EMPTY_MOOV | FF_MOV_FLAG_SEPARATE_MOOF |
-                      FF_MOV_FLAG_FRAGMENT;
+                      FF_MOV_FLAG_FRAGMENT | FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS;
     if (mov->flags & FF_MOV_FLAG_DASH)
         mov->flags |= FF_MOV_FLAG_FRAGMENT | FF_MOV_FLAG_EMPTY_MOOV |
                       FF_MOV_FLAG_DEFAULT_BASE_MOOF;
diff --git a/tests/ref/fate/movenc b/tests/ref/fate/movenc
index 872796ebce..5e8f324ea3 100644
--- a/tests/ref/fate/movenc
+++ b/tests/ref/fate/movenc
@@ -17,10 +17,10 @@ write_data len 20, time nopts, type header atom ftyp
 write_data len 1171, time nopts, type header atom -
 write_data len 728, time 0, type sync atom moof
 write_data len 828, time nopts, type unknown atom -
-write_data len 728, time 1013106, type sync atom moof
+write_data len 728, time 1046439, type sync atom moof
 write_data len 812, time nopts, type unknown atom -
 write_data len 148, time nopts, type trailer atom -
-1f37c1a8e01651e8bebcd66f00b6a226 4435 ismv
+49bf122c4c732a344ef68b58acd19be5 4435 ismv
 write_data len 36, time nopts, type header atom ftyp
 write_data len 1123, time nopts, type header atom -
 write_data len 796, time 0, type sync atom moof
-- 
2.17.1