[FFmpeg-devel] [PATCH v3] ffmpeg: add option -isync

[Gyan Doshi <ffmpeg@gyani.pro>]

This is a per-file input option that adjusts an input's timestamps
with reference to another input, so that emitted packet timestamps
account for the difference between the start times of the two inputs.

Typical use case is to sync two or more live inputs such as from capture
devices. Both the target and reference input source timestamps should be
based on the same clock source.

If not all inputs have timestamps, the wallclock times at the time of
reception of inputs shall be used. FFmpeg must have been compiled with
thread support for this last case.
---
This is a single patch with the reception wallclock stored only in
fftools. I find this sub-optimal because find_stream_info will have
demuxed an indeterminate number of frames by the time the wallclock is
stored. But it's better than nothing.

 doc/ffmpeg.texi      | 16 ++++++++++
 fftools/ffmpeg.c     |  4 +++
 fftools/ffmpeg.h     |  3 ++
 fftools/ffmpeg_opt.c | 69 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 92 insertions(+)

diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
index 1a534ff1cc..1fe0a559c0 100644
--- a/doc/ffmpeg.texi
+++ b/doc/ffmpeg.texi
@@ -518,6 +518,22 @@ see @ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1)
 Like the @code{-ss} option but relative to the "end of file". That is negative
 values are earlier in the file, 0 is at EOF.
 
+@item -isync @var{input_index} (@emph{input})
+Assign an input as a sync source.
+
+This will take the difference between the start times of the target and referenced inputs and
+offset the timestamps of the target file by that difference. The source timestamps of the two
+inputs should derive from the same clock source for expected results. If @code{copyts} is set
+then @code{start_at_zero} must also be set. If at least one of the inputs has no starting
+timestamp then the wallclock time at time of reception of the inputs is used as a best-effort
+sync basis.
+
+Acceptable values are those that refer to a valid ffmpeg input index. If the sync reference is
+the target index itself or @var{-1}, then no adjustment is made to target timestamps. A sync
+reference may not itself be synced to any other input.
+
+Default value is @var{-1}.
+
 @item -itsoffset @var{offset} (@emph{input})
 Set the input time offset.
 
diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index e7384f052a..290b8228b8 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -3658,6 +3658,10 @@ static void *input_thread(void *arg)
             av_thread_message_queue_set_err_recv(f->in_thread_queue, ret);
             break;
         }
+
+        if (f->first_pkt_wallclock == AV_NOPTS_VALUE)
+            f->first_pkt_wallclock = av_gettime();
+
         queue_pkt = av_packet_alloc();
         if (!queue_pkt) {
             av_packet_unref(pkt);
diff --git a/fftools/ffmpeg.h b/fftools/ffmpeg.h
index 99d31c346e..760e94d1e5 100644
--- a/fftools/ffmpeg.h
+++ b/fftools/ffmpeg.h
@@ -118,6 +118,7 @@ typedef struct OptionsContext {
     float readrate;
     int accurate_seek;
     int thread_queue_size;
+    int input_sync_ref;
 
     SpecifierOpt *ts_scale;
     int        nb_ts_scale;
@@ -409,7 +410,9 @@ typedef struct InputFile {
     int64_t duration;     /* actual duration of the longest stream in a file
                              at the moment when looping happens */
     AVRational time_base; /* time base of the duration */
+    int64_t first_pkt_wallclock;
     int64_t input_ts_offset;
+    int input_sync_ref;
 
     int64_t ts_offset;
     int64_t last_ts;
diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
index e08455478f..b586f3e6b0 100644
--- a/fftools/ffmpeg_opt.c
+++ b/fftools/ffmpeg_opt.c
@@ -235,6 +235,7 @@ static void init_options(OptionsContext *o)
     o->chapters_input_file = INT_MAX;
     o->accurate_seek  = 1;
     o->thread_queue_size = -1;
+    o->input_sync_ref = -1;
 }
 
 static int show_hwaccels(void *optctx, const char *opt, const char *arg)
@@ -287,6 +288,67 @@ static int parse_and_set_vsync(const char *arg, int *vsync_var, int file_idx, in
     return 0;
 }
 
+static int apply_sync_offsets(void)
+{
+    for (int i = 0; i < nb_input_files; i++) {
+        InputFile *ref, *self = input_files[i];
+        int64_t adjustment;
+        int64_t self_start_time, ref_start_time, self_seek_start, ref_seek_start;
+        int sync_fpw = 0, start_times_set = 1;
+
+        if (self->input_sync_ref == -1 || self->input_sync_ref == i) continue;
+        if (self->input_sync_ref >= nb_input_files || self->input_sync_ref < -1) {
+            av_log(NULL, AV_LOG_FATAL, "-isync for input %d references non-existent input %d.\n", i, self->input_sync_ref);
+            exit_program(1);
+        }
+
+        if (copy_ts && !start_at_zero) {
+            av_log(NULL, AV_LOG_FATAL, "Use of -isync requires that start_at_zero be set if copyts is set.\n");
+            exit_program(1);
+        }
+
+        ref = input_files[self->input_sync_ref];
+        if (ref->input_sync_ref != -1 && ref->input_sync_ref != self->input_sync_ref) {
+            av_log(NULL, AV_LOG_ERROR, "-isync for input %d references a resynced input %d. Sync not set.\n", i, self->input_sync_ref);
+            continue;
+        }
+
+        if (self->ctx->start_time_realtime != AV_NOPTS_VALUE && ref->ctx->start_time_realtime != AV_NOPTS_VALUE) {
+            self_start_time = self->ctx->start_time_realtime;
+            ref_start_time  =  ref->ctx->start_time_realtime;
+        } else if (self->ctx->start_time != AV_NOPTS_VALUE && ref->ctx->start_time != AV_NOPTS_VALUE) {
+            self_start_time = self->ctx->start_time;
+            ref_start_time  =  ref->ctx->start_time;
+        } else {
+#if HAVE_THREADS
+            self_start_time = self->first_pkt_wallclock;
+            ref_start_time  =  ref->first_pkt_wallclock;
+            sync_fpw = 1;
+#else
+            start_times_set = 0;
+#endif
+        }
+
+        if (start_times_set) {
+            self_seek_start = self->start_time == AV_NOPTS_VALUE ? 0 : self->start_time;
+            ref_seek_start  =  ref->start_time == AV_NOPTS_VALUE ? 0 :  ref->start_time;
+
+            adjustment = (self_start_time - ref_start_time) + !copy_ts*(self_seek_start - ref_seek_start) + ref->input_ts_offset;
+
+            self->ts_offset += adjustment;
+
+            av_log(NULL, AV_LOG_INFO, "Adjusted ts offset for Input #%d by %"PRId64" us to sync with Input #%d", i, adjustment, self->input_sync_ref);
+            if (sync_fpw) av_log(NULL, AV_LOG_INFO, " using reception wallclock time. Sync may not be obtained");
+            av_log(NULL, AV_LOG_INFO, ".\n");
+        } else {
+            av_log(NULL, AV_LOG_INFO, "Unable to identify start times for Inputs #%d and %d both. No sync adjustment made.\n",
+                   i, self->input_sync_ref);
+       }
+    }
+
+    return 0;
+}
+
 static int opt_filter_threads(void *optctx, const char *opt, const char *arg)
 {
     av_free(filter_nbthreads);
@@ -1305,6 +1367,8 @@ static int open_input_file(OptionsContext *o, const char *filename)
     f->ist_index  = nb_input_streams - ic->nb_streams;
     f->start_time = o->start_time;
     f->recording_time = o->recording_time;
+    f->first_pkt_wallclock = AV_NOPTS_VALUE;
+    f->input_sync_ref = o->input_sync_ref;
     f->input_ts_offset = o->input_ts_offset;
     f->ts_offset  = o->input_ts_offset - (copy_ts ? (start_at_zero && ic->start_time != AV_NOPTS_VALUE ? ic->start_time : 0) : timestamp);
     f->nb_streams = ic->nb_streams;
@@ -3489,6 +3553,8 @@ int ffmpeg_parse_options(int argc, char **argv)
         goto fail;
     }
 
+    apply_sync_offsets();
+
     /* create the complex filtergraphs */
     ret = init_complex_filters();
     if (ret < 0) {
@@ -3603,6 +3669,9 @@ const OptionDef options[] = {
     { "accurate_seek",  OPT_BOOL | OPT_OFFSET | OPT_EXPERT |
                         OPT_INPUT,                                   { .off = OFFSET(accurate_seek) },
         "enable/disable accurate seeking with -ss" },
+    { "isync",          HAS_ARG | OPT_INT | OPT_OFFSET |
+                        OPT_EXPERT | OPT_INPUT,                      { .off = OFFSET(input_sync_ref) },
+        "Indicate the input index for sync reference", "sync ref" },
     { "itsoffset",      HAS_ARG | OPT_TIME | OPT_OFFSET |
                         OPT_EXPERT | OPT_INPUT,                      { .off = OFFSET(input_ts_offset) },
         "set the input ts offset", "time_off" },
-- 
2.36.1

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