movenc: Add an option for delaying writing the moov with empty_moov

This delays writing the moov until the first fragment is written,
or can be flushed by the caller explicitly when wanted. If the first
sample in all streams is available at this point, we can write
a proper editlist at this point, allowing streams to start at
something else than dts=0. For AC3 and DNXHD, a packet is
needed in order to write the moov header properly.

This isn't added to the normal behaviour for empty_moov, since
the behaviour that ftyp+moov is written during avformat_write_header
would be changed. Callers that split the output stream into header+segments
(either by flushing manually, with the custom_frag flag set, or by
just differentiating between data written during avformat_write_header
and the rest) will need to be adjusted to take this option into use.

For handling streams that start at something else than dts=0, an
alternative would be to use different kinds of heuristics for
guessing the start dts (using AVCodecContext delay or has_b_frames
together with the frame rate), but this is not reliable and doesn't
necessarily work well with stream copy, and wouldn't work for getting
the right initialization data for AC3 or DNXHD either.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -61,6 +61,7 @@ static const AVOption options[] = {
     { "default_base_moof", "Set the default-base-is-moof flag in tfhd atoms", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_DEFAULT_BASE_MOOF}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
     { "dash", "Write DASH compatible fragmented MP4", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_DASH}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
     { "frag_discont", "Signal that the next fragment is discontinuous from earlier ones", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_FRAG_DISCONT}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
+    { "delay_moov", "Delay writing the initial moov until the first fragment is cut, or until the first fragment flush", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_DELAY_MOOV}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
     FF_RTP_FLAG_OPTS(MOVMuxContext, rtp_flags),
     { "skip_iods", "Skip writing iods atom.", offsetof(MOVMuxContext, iods_skip), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, AV_OPT_FLAG_ENCODING_PARAM},
     { "iods_audio_profile", "iods audio profile atom.", offsetof(MOVMuxContext, iods_audio_profile), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 255, AV_OPT_FLAG_ENCODING_PARAM},
@@ -1260,7 +1261,7 @@ static int mov_write_stbl_tag(AVIOContext *pb, MOVTrack *track)
     if (track->mode == MODE_MOV && track->flags & MOV_TRACK_STPS)
         mov_write_stss_tag(pb, track, MOV_PARTIAL_SYNC_SAMPLE);
     if (track->enc->codec_type == AVMEDIA_TYPE_VIDEO &&
-        track->flags & MOV_TRACK_CTTS)
+        track->flags & MOV_TRACK_CTTS && track->entry)
         mov_write_ctts_tag(pb, track);
     mov_write_stsc_tag(pb, track);
     mov_write_stsz_tag(pb, track);
@@ -1771,6 +1772,12 @@ static int mov_write_trak_tag(AVIOContext *pb, MOVMuxContext *mov,
                               MOVTrack *track, AVStream *st)
 {
     int64_t pos = avio_tell(pb);
+    int entry_backup = track->entry;
+    /* If we want to have an empty moov, but some samples already have been
+     * buffered (delay_moov), pretend that no samples have been written yet. */
+    if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV)
+        track->entry = 0;
+
     avio_wb32(pb, 0); /* size */
     ffio_wfourcc(pb, "trak");
     mov_write_tkhd_tag(pb, mov, track, st);
@@ -1802,6 +1809,7 @@ static int mov_write_trak_tag(AVIOContext *pb, MOVMuxContext *mov,
         }
     }
     mov_write_track_udta_tag(pb, mov, st);
+    track->entry = entry_backup;
     return update_size(pb, pos);
 }
 
@@ -1876,6 +1884,12 @@ static int mov_write_mvhd_tag(AVIOContext *pb, MOVMuxContext *mov)
                 max_track_id = mov->tracks[i].track_id;
         }
     }
+    /* If using delay_moov, make sure the output is the same as if no
+     * samples had been written yet. */
+    if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV) {
+        max_track_len = 0;
+        max_track_id  = 1;
+    }
 
     version = max_track_len < UINT32_MAX ? 0 : 1;
     (version == 1) ? avio_wb32(pb, 120) : avio_wb32(pb, 108); /* size */
@@ -3110,8 +3124,18 @@ static int mov_flush_fragment(AVFormatContext *s)
         for (i = 0; i < mov->nb_streams; i++)
             mov->tracks[i].data_offset = pos + buf_size + 8;
 
+        if (mov->flags & FF_MOV_FLAG_DELAY_MOOV)
+            mov_write_identification(s->pb, s);
         mov_write_moov_tag(s->pb, mov, s);
 
+        if (mov->flags & FF_MOV_FLAG_DELAY_MOOV) {
+            if (mov->flags & FF_MOV_FLAG_FASTSTART)
+                mov->reserved_moov_pos = avio_tell(s->pb);
+            avio_flush(s->pb);
+            mov->fragments++;
+            return 0;
+        }
+
         buf_size = avio_close_dyn_buf(mov->mdat_buf, &buf);
         mov->mdat_buf = NULL;
         avio_wb32(s->pb, buf_size + 8);
@@ -3194,6 +3218,19 @@ static int mov_flush_fragment(AVFormatContext *s)
     return 0;
 }
 
+static int mov_auto_flush_fragment(AVFormatContext *s)
+{
+    MOVMuxContext *mov = s->priv_data;
+    int ret = mov_flush_fragment(s);
+    if (ret < 0)
+        return ret;
+    // If using delay_moov, the first flush only wrote the moov,
+    // not the actual moof+mdat pair, thus flush once again.
+    if (mov->fragments == 1 && mov->flags & FF_MOV_FLAG_DELAY_MOOV)
+        ret = mov_flush_fragment(s);
+    return ret;
+}
+
 int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
 {
     MOVMuxContext *mov = s->priv_data;
@@ -3206,7 +3243,7 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
 
     if (mov->flags & FF_MOV_FLAG_FRAGMENT) {
         int ret;
-        if (mov->fragments > 0) {
+        if (mov->fragments > 0 || mov->flags & FF_MOV_FLAG_EMPTY_MOOV) {
             if (!trk->mdat_buf) {
                 if ((ret = avio_open_dyn_buf(&trk->mdat_buf)) < 0)
                     return ret;
@@ -3334,11 +3371,12 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
             trk->frag_start   = pkt->dts;
             trk->start_dts    = 0;
             trk->frag_discont = 0;
-        } else if (pkt->dts && mov->flags & FF_MOV_FLAG_EMPTY_MOOV)
+        } else if (mov->fragments >= 1)
             av_log(s, AV_LOG_WARNING,
-                   "Track %d starts with a nonzero dts %"PRId64". This "
+                   "Track %d starts with a nonzero dts %"PRId64", while the moov "
-                   "currently isn't handled correctly in combination with "
+                   "already has been written. Set the delay_moov flag to handle "
-                   "empty_moov.\n", pkt->stream_index, pkt->dts);
+                   "this case.\n",
+                   pkt->stream_index, pkt->dts);
     }
     trk->track_duration = pkt->dts - trk->start_dts + pkt->duration;
 
@@ -3413,7 +3451,7 @@ static int mov_write_packet(AVFormatContext *s, AVPacket *pkt)
               enc->codec_type == AVMEDIA_TYPE_VIDEO &&
               trk->entry && pkt->flags & AV_PKT_FLAG_KEY)) {
             if (frag_duration >= mov->min_fragment_duration)
-                mov_flush_fragment(s);
+                mov_auto_flush_fragment(s);
         }
 
         return ff_mov_write_packet(s, pkt);
@@ -3636,6 +3674,9 @@ static int mov_write_header(AVFormatContext *s)
         else if (!strcmp("f4v", s->oformat->name)) mov->mode = MODE_F4V;
     }
 
+    if (mov->flags & FF_MOV_FLAG_DELAY_MOOV)
+        mov->flags |= FF_MOV_FLAG_EMPTY_MOOV;
+
     /* Set the FRAGMENT flag if any of the fragmentation methods are
      * enabled. */
     if (mov->max_fragment_duration || mov->max_fragment_size ||
@@ -3663,8 +3704,9 @@ static int mov_write_header(AVFormatContext *s)
                 mov->use_editlist = 0;
         }
     }
-    if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV && mov->use_editlist)
+    if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV &&
-        av_log(s, AV_LOG_WARNING, "No meaningful edit list will be written when using empty_moov\n");
+        !(mov->flags & FF_MOV_FLAG_DELAY_MOOV) && mov->use_editlist)
+        av_log(s, AV_LOG_WARNING, "No meaningful edit list will be written when using empty_moov without delay_moov\n");
 
     if (!mov->use_editlist && s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_AUTO)
         s->avoid_negative_ts = AVFMT_AVOID_NEG_TS_MAKE_ZERO;
@@ -3678,8 +3720,10 @@ static int mov_write_header(AVFormatContext *s)
     }
 
 
-    if ((ret = mov_write_identification(pb, s)) < 0)
+    if (!(mov->flags & FF_MOV_FLAG_DELAY_MOOV)) {
-        return ret;
+        if ((ret = mov_write_identification(pb, s)) < 0)
+            return ret;
+    }
 
     mov->nb_streams = s->nb_streams;
     if (mov->mode & (MODE_MP4|MODE_MOV|MODE_IPOD) && s->nb_chapters)
@@ -3828,7 +3872,8 @@ static int mov_write_header(AVFormatContext *s)
     if (mov->flags & FF_MOV_FLAG_ISML)
         mov_write_isml_manifest(pb, mov);
 
-    if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV) {
+    if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV &&
+        !(mov->flags & FF_MOV_FLAG_DELAY_MOOV)) {
         mov_write_moov_tag(pb, mov, s);
         mov->fragments++;
         if (mov->flags & FF_MOV_FLAG_FASTSTART)
@@ -4025,7 +4070,7 @@ static int mov_write_trailer(AVFormatContext *s)
             mov_write_moov_tag(pb, mov, s);
         }
     } else {
-        mov_flush_fragment(s);
+        mov_auto_flush_fragment(s);
         for (i = 0; i < mov->nb_streams; i++)
            mov->tracks[i].data_offset = 0;
         if (mov->flags & FF_MOV_FLAG_FASTSTART) {

libavformat/movenc.h

@@ -185,6 +185,7 @@ typedef struct MOVMuxContext {
 #define FF_MOV_FLAG_DEFAULT_BASE_MOOF     (1 << 10)
 #define FF_MOV_FLAG_DASH                  (1 << 11)
 #define FF_MOV_FLAG_FRAG_DISCONT          (1 << 12)
+#define FF_MOV_FLAG_DELAY_MOOV            (1 << 13)
 
 int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt);
 

libavformat/version.h

@@ -30,7 +30,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVFORMAT_VERSION_MAJOR 56
-#define LIBAVFORMAT_VERSION_MINOR 10
+#define LIBAVFORMAT_VERSION_MINOR 11
 #define LIBAVFORMAT_VERSION_MICRO  0
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \