ffmpeg: add option -isync

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 either input lacks starting timestamps, then no sync adjustment is made.

doc/ffmpeg.texi

@@ -518,6 +518,21 @@ 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 reference 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 either of the inputs has no starting timestamp
+then no sync adjustment is made.
+
+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.
 

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;
@@ -410,6 +411,7 @@ typedef struct InputFile {
                              at the moment when looping happens */
     AVRational time_base; /* time base of the duration */
     int64_t input_ts_offset;
+    int input_sync_ref;
 
     int64_t ts_offset;
     int64_t last_ts;

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 +288,58 @@ 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 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 {
+            start_times_set = 0;
+        }
+
+        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.\n", i, adjustment, self->input_sync_ref);
+        } 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 +1358,7 @@ 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->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 +3543,8 @@ 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 +3659,9 @@ 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" },