movenc: Add an option for enabling negative CTS offsets

This reduces the need for an edit list; streams that start with
e.g. dts=-1, pts=0 can be encoded as dts=0, pts=0 (which is valid
in mov/mp4) by shifting the dts values of all packets forward.
This avoids the need for edit lists for such streams (while they
still are needed for audio streams with encoder delay).

This eases conformance with the DASH-IF interoperability guidelines.

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

2 files changed, 26 insertions, 4 deletions

diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index 840190d..356d1d6 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -62,6 +62,7 @@ static const AVOption options[] = {
     { "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" },
     { "global_sidx", "Write a global sidx index at the start of the file", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_GLOBAL_SIDX}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
     { "skip_trailer", "Skip writing the mfra/tfra/mfro trailer for fragmented files", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_SKIP_TRAILER}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
+    { "negative_cts_offsets", "Use negative CTS offsets (reducing the need for edit lists)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS}, 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},
@@ -1163,8 +1164,9 @@ static int mov_write_stsd_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
     return update_size(pb, pos);
 }
 
-static int mov_write_ctts_tag(AVIOContext *pb, MOVTrack *track)
+static int mov_write_ctts_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *track)
 {
+    MOVMuxContext *mov = s->priv_data;
     MOVStts *ctts_entries;
     uint32_t entries = 0;
     uint32_t atom_size;
@@ -1188,7 +1190,11 @@ static int mov_write_ctts_tag(AVIOContext *pb, MOVTrack *track)
     atom_size = 16 + (entries * 8);
     avio_wb32(pb, atom_size); /* size */
     ffio_wfourcc(pb, "ctts");
-    avio_wb32(pb, 0); /* version & flags */
+    if (mov->flags & FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS)
+        avio_w8(pb, 1); /* version */
+    else
+        avio_w8(pb, 0); /* version */
+    avio_wb24(pb, 0); /* flags */
     avio_wb32(pb, entries); /* entry count */
     for (i = 0; i < entries; i++) {
         avio_wb32(pb, ctts_entries[i].count);
@@ -1273,7 +1279,7 @@ static int mov_write_stbl_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
         mov_write_stss_tag(pb, track, MOV_PARTIAL_SYNC_SAMPLE);
     if (track->par->codec_type == AVMEDIA_TYPE_VIDEO &&
         track->flags & MOV_TRACK_CTTS && track->entry)
-        mov_write_ctts_tag(pb, track);
+        mov_write_ctts_tag(s, pb, track);
     mov_write_stsc_tag(pb, track);
     mov_write_stsz_tag(pb, track);
     mov_write_stco_tag(pb, track);
@@ -2594,7 +2600,10 @@ static int mov_write_trun_tag(AVIOContext *pb, MOVMuxContext *mov,
 
     avio_wb32(pb, 0); /* size placeholder */
     ffio_wfourcc(pb, "trun");
-    avio_w8(pb, 0); /* version */
+    if (mov->flags & FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS)
+        avio_w8(pb, 1); /* version */
+    else
+        avio_w8(pb, 0); /* version */
     avio_wb24(pb, flags);
 
     avio_wb32(pb, end - first); /* sample count */
@@ -3040,6 +3049,8 @@ static int mov_write_ftyp_tag(AVIOContext *pb, AVFormatContext *s)
         ffio_wfourcc(pb, "MSNV");
     else if (mov->mode == MODE_MP4 && mov->flags & FF_MOV_FLAG_DEFAULT_BASE_MOOF)
         ffio_wfourcc(pb, "iso5"); // Required when using default-base-is-moof
+    else if (mov->mode == MODE_MP4 && mov->flags & FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS)
+        ffio_wfourcc(pb, "iso4");
     else if (mov->mode == MODE_MP4)
         ffio_wfourcc(pb, "isom");
     else if (mov->mode == MODE_IPOD)
@@ -3288,6 +3299,8 @@ static int mov_flush_fragment(AVFormatContext *s, int force)
         if (!track->end_reliable) {
             AVPacket pkt;
             if (!ff_interleaved_peek(s, i, &pkt, 1)) {
+                if (track->dts_shift != AV_NOPTS_VALUE)
+                    pkt.dts += track->dts_shift;
                 track->track_duration = pkt.dts - track->start_dts;
                 if (pkt.pts != AV_NOPTS_VALUE)
                     track->end_pts = pkt.pts;
@@ -3729,6 +3742,12 @@ static int mov_write_packet(AVFormatContext *s, AVPacket *pkt)
             mov->flags &= ~FF_MOV_FLAG_FRAG_DISCONT;
         }
 
+        if (mov->flags & FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS) {
+            if (trk->dts_shift == AV_NOPTS_VALUE)
+                trk->dts_shift = pkt->pts - pkt->dts;
+            pkt->dts += trk->dts_shift;
+        }
+
         if (!pkt->size) {
             if (trk->start_dts == AV_NOPTS_VALUE && trk->frag_discont) {
                 trk->start_dts = pkt->dts;
@@ -4095,6 +4114,7 @@ static int mov_write_header(AVFormatContext *s)
         track->start_dts  = AV_NOPTS_VALUE;
         track->start_cts  = AV_NOPTS_VALUE;
         track->end_pts    = AV_NOPTS_VALUE;
+        track->dts_shift  = AV_NOPTS_VALUE;
         if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
             if (track->tag == MKTAG('m','x','3','p') || track->tag == MKTAG('m','x','3','n') ||
                 track->tag == MKTAG('m','x','4','p') || track->tag == MKTAG('m','x','4','n') ||
diff --git a/libavformat/movenc.h b/libavformat/movenc.h
index f4ed188..008f467 100644
--- a/libavformat/movenc.h
+++ b/libavformat/movenc.h
@@ -107,6 +107,7 @@ typedef struct MOVTrack {
     int64_t     start_cts;
     int64_t     end_pts;
     int         end_reliable;
+    int64_t     dts_shift;
 
     int         hint_track;   ///< the track that hints this track, -1 if no hint track is set
     int         src_track;    ///< the track that this hint track describes
@@ -195,6 +196,7 @@ typedef struct MOVMuxContext {
 #define FF_MOV_FLAG_DELAY_MOOV            (1 << 13)
 #define FF_MOV_FLAG_GLOBAL_SIDX           (1 << 14)
 #define FF_MOV_FLAG_SKIP_TRAILER          (1 << 15)
+#define FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS  (1 << 16)
 
 int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt);