[FFmpeg-devel] [PATCH] fftools/ffmpeg: Added ability to set a input burst time before readrate is enforced

[Davy Durham <ddurham@davyandbeth.com>]

Implemented is the -irb <seconds> flag (i.e. "initial read burst") that
causes ffmpeg to read the specified number of seconds of input before a
given readrate starts to be enforced.  All inputs have to reach this
point before the readrate is enforced.  The reason for this change is,
in the scenario of live-streaming by transcoding from an pre-recorded or
delayed source to an HLS or DASH webroot, we can more quickly prime the
system of the first few seconds before the real-world read rate is
imposed.  Else, one must wait for an entire segment length before the
data can be fetched from the web server.

Signed-off-by: Davy Durham <ddurham@users.sourceforge.net>
---
  doc/ffmpeg.texi      |  2 ++
  fftools/ffmpeg.c     |  3 ++-
  fftools/ffmpeg.h     |  2 ++
  fftools/ffmpeg_opt.c | 13 +++++++++++++
  4 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
index 767df69b7f..16ab1a336f 100644
--- a/doc/ffmpeg.texi
+++ b/doc/ffmpeg.texi
@@ -1633,6 +1633,8 @@ it may cause packet loss.
  It is useful for when flow speed of output packets is important, such 
as live streaming.
  @item -re (@emph{input})
  Read input at native frame rate. This is equivalent to setting 
@code{-readrate 1}.
+@item -irb @var{seconds}
+Set an initial read burst time, in seconds, after which any specified 
readrate will be enforced.
  @item -vsync @var{parameter} (@emph{global})
  @itemx -fps_mode[:@var{stream_specifier}] @var{parameter} 
(@emph{output,per-stream})
  Set video sync method / framerate mode. vsync is applied to all output 
video streams
diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index e7384f052a..e4697c8772 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -3768,6 +3768,7 @@ static int get_input_packet(InputFile *f, AVPacket 
**pkt)
                                (f->start_time != AV_NOPTS_VALUE ? 
f->start_time : 0)
                               );
          float scale = f->rate_emu ? 1.0 : f->readrate;
+        int64_t burst_until = AV_TIME_BASE * f->initial_read_burst;
          for (i = 0; i < f->nb_streams; i++) {
              InputStream *ist = input_streams[f->ist_index + i];
              int64_t stream_ts_offset, pts, now;
@@ -3775,7 +3776,7 @@ static int get_input_packet(InputFile *f, AVPacket 
**pkt)
              stream_ts_offset = FFMAX(ist->first_dts != AV_NOPTS_VALUE 
? ist->first_dts : 0, file_start);
              pts = av_rescale(ist->dts, 1000000, AV_TIME_BASE);
              now = (av_gettime_relative() - ist->start) * scale + 
stream_ts_offset;
-            if (pts > now)
+            if (pts - burst_until > now)
                  return AVERROR(EAGAIN);
          }
      }
diff --git a/fftools/ffmpeg.h b/fftools/ffmpeg.h
index 391a35cf50..aa079ab3e9 100644
--- a/fftools/ffmpeg.h
+++ b/fftools/ffmpeg.h
@@ -116,6 +116,7 @@ typedef struct OptionsContext {
      int loop;
      int rate_emu;
      float readrate;
+    double initial_read_burst;
      int accurate_seek;
      int thread_queue_size;
      int input_sync_ref;
@@ -422,6 +423,7 @@ typedef struct InputFile {
      int nb_streams_warn;  /* number of streams that the user was 
warned of */
      int rate_emu;
      float readrate;
+    double initial_read_burst;
      int accurate_seek;
       AVPacket *pkt;
diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
index 6e18a4a23e..57aa4929fa 100644
--- a/fftools/ffmpeg_opt.c
+++ b/fftools/ffmpeg_opt.c
@@ -1378,6 +1378,16 @@ static int open_input_file(OptionsContext *o, 
const char *filename)
          f->rate_emu = 0;
      }
  +    f->initial_read_burst = o->initial_read_burst ? 
o->initial_read_burst : 0.0;
+    if (f->initial_read_burst < 0.0) {
+        av_log(NULL, AV_LOG_ERROR, "Option -irb for Input #%d is %0.3f; 
it must be non-negative.\n", nb_input_files, f->initial_read_burst);
+        exit_program(1);
+    }
+    if ((!f->readrate && !f->rate_emu) && f->initial_read_burst) {
+        av_log(NULL, AV_LOG_WARNING, "Option -irb ignored since neither 
-readrate nor -re were given\n");
+        f->initial_read_burst = 0;
+    }
+
      f->pkt = av_packet_alloc();
      if (!f->pkt)
          exit_program(1);
@@ -3734,6 +3744,9 @@ const OptionDef options[] = {
      { "readrate",       HAS_ARG | OPT_FLOAT | OPT_OFFSET |
                          OPT_EXPERT | OPT_INPUT,                      { 
.off = OFFSET(readrate) },
          "read input at specified rate", "speed" },
+    { "irb",            HAS_ARG | OPT_DOUBLE | OPT_OFFSET |
+                        OPT_EXPERT | OPT_INPUT,                      { 
.off = OFFSET(initial_read_burst) },
+        "The initial amount of input to burst read before imposing any 
readrate", "seconds" },
      { "target",         HAS_ARG | OPT_PERFILE | OPT_OUTPUT,          { 
.func_arg = opt_target },
          "specify target file type (\"vcd\", \"svcd\", \"dvd\", \"dv\" 
or \"dv50\" "
          "with optional prefixes \"pal-\", \"ntsc-\" or \"film-\")", 
"type" },
-- 
2.25.1

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