[Bug 753257] avenc_mjpeg does not play well with appsrc do-timestamp=true

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=753257

--- Comment #13 from Tim-Philipp Müller <t.i.m at zen.co.uk> ---
The problem here is the transformation of GStreamer timestamps in ffmpeg
timestamps/ticks. The scaling is based on the framerate (or some notional
framerate) in such a way that you're likely get the same result in case your
timestamps are much closer together.

In your example code you basically just push/pull in a tight loop without
sleeping, so the timestamps are only a fraction of a millisecond apart, but
things are set up for 30fps.

This fixes it for me, but it might need other changes, like adjustment of
ticks_per_frame or such, to make sure the bitrate estimation is not affected:


diff --git a/ext/libav/gstavutils.h b/ext/libav/gstavutils.h
index 6d111a2..8485908 100644
--- a/ext/libav/gstavutils.h
+++ b/ext/libav/gstavutils.h
@@ -57,7 +57,7 @@ gst_ffmpeg_time_ff_to_gst (gint64 pts, AVRational base)
   if (pts == AV_NOPTS_VALUE){
     out = GST_CLOCK_TIME_NONE;
   } else {
-    AVRational bq = { 1, GST_SECOND };
+    AVRational bq = { 1000, GST_SECOND };
     out = av_rescale_q (pts, base, bq);
   }

@@ -72,7 +72,7 @@ gst_ffmpeg_time_gst_to_ff (guint64 time, AVRational base)
   if (!GST_CLOCK_TIME_IS_VALID (time) || base.num == 0) {
     out = AV_NOPTS_VALUE;
   } else {
-    AVRational bq = { 1, GST_SECOND };
+    AVRational bq = { 1000, GST_SECOND };
     out = av_rescale_q (time, bq, base);
   }


In your code, you should:

1) set appsrc to is-live=true, because that's more appropriate for your case,
and because without the first buffer will not have a valid timestamp (since
it's produced before the pipeline goes to playing, so no clock yet at that
point.

2) set appsrc to format=time, which is the right thing to do.

3) maybe set framerate=0/1 which means variable framerate (could cause other
issues though, only one way to find out)

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