[MPlayer-dev-eng] libmpeg2 and lavc don't agree on PTS values

[Trent Piepho]

I've spent hours trying to get a 2-pass filter working correctly until I
finally figured out it's not my fault.  It's either lavc's fault or
libmpeg2's fault!

This is the first few frames of an ATSC ts stream as decoded with -vc
mpeg12, listing pts value, frame type, and mean qscale value:

85354.2812 B 3.976389     # -2
85354.3047 B 4.143333     # -1
85354.3203 I 2.543056	  # 0
85354.3359 B 4.448056     # 1
85354.3516 B 4.578333     # 2
85354.3672 P 3.290556     # 3

This is the same file with -vc ffmpeg2:
85354.3047 I 2.543056     # 0
85354.3203 B 4.448056     # 1
85354.3359 B 4.578333     # 2
85354.3516 P 3.290556     # 3
85354.3672 B 4.040556     # 4

You see the frame type and mean qscale match exactly, but the pts values
have been shifted down by one.

If you look at frame #0, the I frame, the qscale is the same for both
decoders (and visually, the same frame is displayed), but ffmpeg2 has the
pts value that libmpeg2 has given to frame -1.  It's the same for the whole
file.  For any frame mpeg12 will assign it the pts value that ffmpeg2
assigned to the following frame.  Or you could say ffmpeg2 assigns a frame
the pts value that that mpeg12 has given to the previous frame.

So... which one is right?  And which one has audio sync off by one frame?