[FFmpeg-user] debug qp (debug mb_type) output issues

[Chema Gonzalez]

Hi,

I'm trying to get the per-MB QP list for an h264 (avc) video (Annex B
format, though this should not matter much). I can get the exact list
I'm looking for when using ffprobe:

```
$ ffprobe -v quiet -show_frames -debug qp -print_format flat
input.264  > /tmp/stdout 2> /tmp/stderr
```

The stdout gets the (flat format) per-frame info, and the stderr gets
the QP list:

```
$ cat /tmp/stderr
[h264 @ 0x3bce440] Format h264 probed with size=2048 and score=51
[h264 @ 0x3bce440] Before avformat_find_stream_info() pos: 0 bytes
read:32768 seeks:0 nb_streams:1
[extract_extradata @ 0x3bd1840] nal_unit_type: 7(SPS), nal_ref_idc: 3
[extract_extradata @ 0x3bd1840] nal_unit_type: 8(PPS), nal_ref_idc: 3
[extract_extradata @ 0x3bd1840] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 0x3bcfa80] nal_unit_type: 7(SPS), nal_ref_idc: 3
[h264 @ 0x3bcfa80] nal_unit_type: 8(PPS), nal_ref_idc: 3
[h264 @ 0x3bcfa80] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 0x3bcfa80] Format yuv420p chosen by get_format().
[h264 @ 0x3bcfa80] Reinit context to 1280x720, pix_fmt: yuv420p
[h264 @ 0x3bcfa80] New frame, type: I
[h264 @ 0x3bcfa80] 35353535353535353535353535353535353535353535...
[h264 @ 0x3bcfa80] 35353535353535353535353535353535353535353535...
[h264 @ 0x3bcfa80] ...
[h264 @ 0x3bcfa80] 35353535353535353535353535353535353535353535...
[h264 @ 0x3bcfa80] nal_unit_type: 1(Coded slice of a non-IDR picture),
nal_ref_idc: 2
[h264 @ 0x3bcfa80] New frame, type: P
[h264 @ 0x3bcfa80] 37373737373737373737373737373737373737373737...
[h264 @ 0x3bcfa80] ...
...
[h264 @ 0x3c01500] New frame, type: P
[h264 @ 0x3c01500] 24242424242424242424242424242424242424242424...
[h264 @ 0x3c01500] 24242424242424242424242424242424242424242424...
[h264 @ 0x3c01500] ...
[h264 @ 0x3c01500] 24242424242424242424242424242424242424242424...
[AVIOContext @ 0x3bcf100] Statistics: 6508544 bytes read, 0 seeks
```

Note that the per-frame output is correct all the way to the end.

But if I try to do the same using ffmpeg, the output after a few
frames gets garbled. It seems like there are 2x writers competing for
putting QP values to the stderr:

```
$ ffmpeg  -debug:v qp -i in.264 -an -v 100 -f null - > /tmp/stdout2 >&
/tmp/stderr2
```

Then I get:

```
$ cat /tmp/stderr2
-- first few frames work well
[h264 @ 0x36bb140] nal_unit_type: 7(SPS), nal_ref_idc: 3
[h264 @ 0x36bb140] nal_unit_type: 8(PPS), nal_ref_idc: 3
[h264 @ 0x36bb140] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 0x36bb140] Format yuv420p chosen by get_format().
[h264 @ 0x36bb140] Reinit context to 1280x720, pix_fmt: yuv420p
[h264 @ 0x36bb140] New frame, type: I
[h264 @ 0x36bb140] 353535353535353535353535353535353535...
[h264 @ 0x36bb140] 353535353535353535353535353535353535...
[h264 @ 0x36bb140] ...
[h264 @ 0x36bb140] 353535353535353535353535353535353535...
...
-- but at some point output gets garbled
[h264 @ 0x36d68c0] nal_unit_type: 7(SPS), nal_ref_idc: 3
[h264 @ 0x36d68c0] nal_unit_type: 8(PPS), nal_ref_idc: 3
[h264 @ 0x36d68c0] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 0x36d68c0] Format yuv420p chosen by get_format().
[h264 @ 0x36d68c0] Reinit context to 1280x720, pix_fmt: yuv420p
cur_dts is invalid st:0 (0) [init:0 i_done:0 finish:0] (this is
harmless if it occurs once at the start per stream)
[h264 @ 0x373de40] nal_unit_type: 1(Coded slice of a non-IDR picture),
nal_ref_idc: 2
cur_dts is invalid st:0 (0) [init:0 i_done:0 finish:0] (this is
harmless if it occurs once at the start per stream)
[h264 @ 0x37584c0] nal_unit_type: 1(Coded slice of a non-IDR picture),
nal_ref_idc: 2
cur_dts is invalid st:0 (0) [init:0 i_done:0 finish:0] (this is
harmless if it occurs once at the start per stream)
[h264 @ 0x36fbec0] nal_unit_type: 1(Coded slice of a non-IDR picture),
nal_ref_idc: 2
[h264 @ 0x36d68c0] New frame, type: I
[h264 @ 0x36d68c0]
3535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535
[h264 @ 0x36d68c0]
35353535353535353535353535353535353535353535353535New frame, type: P
[h264 @ 0x373de40] 373737373735New frame, type: P
[h264 @ 0x37584c0]
444444444444444444444444444444444444444444444444444444444444444444444435353535353535353535353535353535374435374435353535353535353535353544373544444444373737373737373535353737373737373737373737373737373737373737cur_dts
is invalid st:0 (0) [init:0 i_done:0 finish:0] (this is harmless if it
occurs once at the start per stream)
[h264 @ 0x36d68c0]
35444444444444373735353535373737373737354444444444443737373737373535353544444444373737373737373737373737373737373744444444443737373737373737373737373737
[h264 @ 0x373de40] 37nal_unit_type: 1(Coded slice of a non-IDR
picture), nal_ref_idc: 2
[h264 @ 0x37584c0] 4444444444444444444444444444444444
[h264 @ 0x37584c0]
44444444444444444444444444444444444444444444444444444444443735444444444444444444444444444444443737373737373737373737373737373737373737373737373735353535353544444444373737373737373535353544444444444444373737373737373737373737373737
[h264 @ 0x37584c0]
4437353535353535353535353535353535444444444435353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535353535374435353535
[h264 @ 0x36d68c0]
35373737373737374435353535353535353535353537373737353535353535353535373737373737373737373737373737373535353535353535353737
[h264 @ 0x373de40]
373737374444444444373737373535353535353535353535353535353535353535353535353535353537444444443535353537373737373737373737373737373737373737373737353535354444444437373737373737373737444444
[h264 @ 0x37584c0]
44353535353535353537373737373744444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444cur_dts
is invalid st:0 (0) [init:0 i_done:0 finish:0] (this is harmless if it
occurs once at the start per stream)
[h264 @ 0x36d68c0] 3537373737374435353535
[h264 @ 0x37584c0]
444444444444443737373737443537444444444437373737373544444444353744444444444435nal_unit_type:
1(Coded slice of a non-IDR picture), nal_ref_idc: 2
...
```

I tried with 3x different files, and all do the same. Wonder whether
this makes sense.

Thanks.
-Chema