ffmpeg: Rescale DTS better during FPS passthrough

This mostly ensures that non-B frames have the same dts/pts.

The PTS/DTS from the encoder can be "squashed" a bit during rescaling
back to the source timebase if it is used directly, due to the lower
resolution of the encoder timebase. We avoid this problem with the
PTS in in FPS passthrough mode by reusing the source pts, but only
rescale the encoder-provided DTS back to the source timebase for some
semblance of timestamp consistency. Because the DTS values are
squashed, they can differ from the PTS even with non-B frames.

The DTS values are still monotonic, so the exact numbers are not really
important. However, some tools use `dts == pts` as a heuristic to check
for B-frames ... so help them out to avoid spurious B-frame detections.

To fix the DTS/PTS mismatch, take the difference between the
encoder-provided dts/pts, rescale that difference back to the source
time base, and re-calculate the dts using the source pts.

Also see #405

ffmpeg/encoder.c

@@ -373,8 +373,9 @@ static int encode(AVCodecContext* encoder, AVFrame *frame, struct output_ctx* oc
     if (AVMEDIA_TYPE_VIDEO == ost->codecpar->codec_type && !octx->fps.den && octx->vf.active) {
       // try to preserve source timestamps for fps passthrough.
       time_base = octx->vf.time_base;
+      int64_t pts_dts_diff = pkt->pts - pkt->dts;
       pkt->pts = (int64_t)pkt->opaque; // already in filter timebase
-      pkt->dts = av_rescale_q(pkt->dts, encoder->time_base, time_base);
+      pkt->dts = pkt->pts - av_rescale_q(pts_dts_diff, encoder->time_base, time_base);
     }
     ret = mux(pkt, time_base, octx, ost);
     if (ret < 0) goto encode_cleanup;

ffmpeg/ffmpeg_test.go

@@ -1516,6 +1516,75 @@ func TestTranscoder_PassthroughFPS(t *testing.T) {
 	run(cmd)
 }
 
+func TestTranscoder_PassthroughFPS_AdjustTimestamps(t *testing.T) {
+	// check timestamp adjustments for fps passthrough
+
+	run, dir := setupTest(t)
+	defer os.RemoveAll(dir)
+
+	cmd := `
+		ffmpeg -i "$1/../transcoder/test.ts" -an -c:v copy -t 0.5 test-short.ts
+		ffprobe -loglevel warning -show_entries frame=pts,duration -of csv=p=0 test-short.ts | grep -v '^$' > expected-frame-pts.out
+		wc -l expected-frame-pts.out | grep "32 expected-frame-pts.out"
+		cat << EXPECTED_TS_EOF > expected-pkt-ts.out
+pts,dts,duration
+128970,125970,1500
+134910,127410,1500
+131940,128940,1500
+130500,130500,1500
+133380,131880,1500
+137970,133470,1500
+136440,134940,1500
+143910,136410,1500
+140940,137940,1500
+139410,139410,1500
+142470,140970,1500
+149940,142440,1500
+146970,143970,1500
+145440,145440,1500
+148500,147000,1500
+155970,148470,1500
+152910,149910,1500
+151380,151380,1500
+154440,152940,1500
+161910,154410,1500
+158940,155940,1500
+157410,157410,1500
+160470,158970,1500
+167940,160440,1500
+164970,161970,1500
+163440,163440,1500
+166500,165000,1500
+173970,166470,1500
+170910,167910,1500
+169380,169380,1500
+172440,170940,1500
+176940,172440,1500
+EXPECTED_TS_EOF
+	`
+	run(cmd)
+
+	in := &TranscodeOptionsIn{Fname: dir + "/test-short.ts"}
+	out := []TranscodeOptions{{Profile: P144p30fps16x9}}
+	out[0].Profile.Framerate = 0 // Passthrough!
+	out[0].Profile.Profile = ProfileH264High
+	out[0].Oname = dir + "/out-0.ts"
+	_, err := Transcode3(in, out)
+	require.Nil(t, err)
+	cmd = `
+		echo "pts,dts,duration" > received-pkt-ts.out
+		ffprobe -loglevel warning -show_entries packet=pts,dts,duration,pict_type -of csv=p=0 out-0.ts | grep -v '^$' | sed 's/,*$//g' >> received-pkt-ts.out
+		ffprobe -loglevel warning -show_entries frame=pts,duration -of csv=p=0 test-short.ts | grep -v '^$' > received-frame-pts.out
+
+		# ensure packet pts+dts matches what is expected
+		diff -u expected-pkt-ts.out received-pkt-ts.out
+
+		# ensure all pts are accounted for from original
+		diff -u expected-frame-pts.out received-frame-pts.out
+	`
+	run(cmd)
+}
+
 func TestTranscoder_FormatOptions(t *testing.T) {
 	// Test combinations of VideoProfile.Format and TranscodeOptions.Muxer
 	// The former takes precedence over the latter if set