Number of encoded frames does not match number of decoded frames

[j0sh]

Under particular conditions, the number of encoded frames does not match number of decoded frames.

• Reproducible case that encodes 61 "frames" (61 frames encoded and 61 packets muxed) but only decodes 60, as shown below.
• FFProbe mirrors our numbers: it reads the same 61 packets in the bitstream, but only decodes 60 frames.
• This only seems to happen with the MP4 muxer. The mpegts muxer works as expected.
• Possible that this is a separate slice / field / etc but this doesn't seem likely.
• If this is indeed a non-display frame of some sort : is there a way to detect this?
  

  lpms/ffmpeg/ffmpeg_test.go

  Lines 1117 to 1209 in d722afc

  	func TestTranscoder_MismatchedEncodeDecode(t *testing.T) {
  	// Encoded frame count does not match decoded frame count for mp4
  	// Note this is not an issue for mpegts! (this is sanity checked)
  	run, dir := setupTest(t)
  	defer os.RemoveAll(dir)
  	p144p60fps := P144p30fps16x9
  	p144p60fps.Framerate = 60
  	cmd := `
  	set -eux
  	cd $0
  	# prepare 1-second input
  	cp "$1/../transcoder/test.ts" inp.ts
  	ffmpeg -loglevel warning -i inp.ts -c:a copy -c:v copy -t 1 test.ts
  	`
  	run(cmd)
  	in := &TranscodeOptionsIn{Fname: dir + "/test.ts"}
  	out := []TranscodeOptions{TranscodeOptions{Oname: dir + "/out.mp4", Profile: p144p60fps}}
  	res, err := Transcode3(in, out)
  	if err != nil {
  	t.Error(err)
  	}
  	in.Fname = dir + "/out.mp4"
  	res2, err := Transcode3(in, nil)
  	if err != nil {
  	t.Error(err)
  	}
  	// TODO Ideally these two should match. As far as I can tell it is due
  	// to timestamp rounding around EOF. Note this does not happen with
  	// mpegts formatted output!
  	if res2.Decoded.Frames != 60 || res.Encoded[0].Frames != 61 {
  	t.Error("Did not get expected frame counts: is this fixed?!? ",
  	res2.Decoded.Frames, res.Encoded[0].Frames)
  	}
  	// Interestingly enough, ffmpeg shows that we have 61 packets but 60 frames.
  	// That indicates we are receving, encoding and muxing 61 *things* but one
  	// of them isn't a complete frame.
  	cmd = `
  	ffprobe -count_frames -show_packets -show_streams -select_streams v out.mp4 2>&1 > mp4.out
  	grep nb_read_frames=60 mp4.out
  	grep nb_read_packets=61 mp4.out
  	`
  	run(cmd)
  	// Sanity check mpegts works as expected
  	in.Fname = dir + "/test.ts"
  	out[0].Oname = dir + "/out.ts"
  	res, err = Transcode3(in, out)
  	if err != nil {
  	t.Error(err)
  	}
  	in.Fname = dir + "/out.ts"
  	res2, err = Transcode3(in, nil)
  	if err != nil {
  	t.Error(err)
  	}
  	if res2.Decoded.Frames != 61 || res.Encoded[0].Frames != 61 {
  	t.Error("Did not get expected frame counts for mpegts ",
  	res2.Decoded.Frames, res.Encoded[0].Frames)
  	}
  	// Sanity check we still get the same results with multiple outputs?
  	in.Fname = dir + "/test.ts"
  	out = []TranscodeOptions{
  	TranscodeOptions{Oname: dir + "/out2.ts", Profile: p144p60fps},
  	TranscodeOptions{Oname: dir + "/out2.mp4", Profile: p144p60fps},
  	}
  	res, err = Transcode3(in, out)
  	if err != nil {
  	t.Error(err)
  	}
  	in.Fname = dir + "/out2.ts"
  	res2, err = Transcode3(in, nil)
  	if err != nil {
  	t.Error(err)
  	}
  	in.Fname = dir + "/out2.mp4"
  	res3, err := Transcode3(in, nil)
  	if err != nil {
  	t.Error(err)
  	}
  	// first output is mpegts
  	if res2.Decoded.Frames != 61 || res.Encoded[0].Frames != 61 {
  	t.Error("Sanity check of mpegts failed ", res2.Decoded.Frames, res.Encoded[0].Frames)
  	}
  	if res3.Decoded.Frames != 60 || res.Encoded[1].Frames != 61 {
  	t.Error("Sanity check of mp4 failed ", res2.Decoded.Frames, res.Encoded[1].Frames)
  	}
  	}