PAFF sample shows wrong estimated fps

[Carl Eugen Hoyos]

​http://ffmpeg.org/pipermail/ffmpeg-user/2016-August/033231.html
A user provided a mov file containing 25fps PAFF encoded H264 stream, FFmpeg reports 50fps.

$ ffmpeg -i H264_1080i_cut.mp4 -f null -t 2 -
ffmpeg version N-81447-gc1de1f0 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      55. 29.100 / 55. 29.100
  libavcodec     57. 54.100 / 57. 54.100
  libavformat    57. 48.100 / 57. 48.100
  libavdevice    57.  0.102 / 57.  0.102
  libavfilter     6. 54.100 /  6. 54.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  1.100 /  2.  1.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'H264_1080i_cut.mp4':
  Metadata:
    major_brand     : avc1
    minor_version   : 0
    compatible_brands: avc1mp42
    creation_time   : 2016-07-28 00:48:45
  Duration: 00:03:10.28, start: 0.000000, bitrate: 107 kb/s
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 6987 kb/s, 50 fps, 50 tbr, 50 tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2016-07-28 00:48:45
      handler_name    : Video Media Handler
      encoder         : AVC Coding
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 192 kb/s (default)
    Metadata:
      creation_time   : 2016-07-28 00:48:45
      handler_name    : Sound Media Handler
[null @ 0x2f1cf00] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
    Last message repeated 1 times
Output #0, null, to 'pipe:':
  Metadata:
    major_brand     : avc1
    minor_version   : 0
    compatible_brands: avc1mp42
    encoder         : Lavf57.48.100
    Stream #0:0(eng): Video: wrapped_avframe, yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 50 fps, 50 tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2016-07-28 00:48:45
      handler_name    : Video Media Handler
      encoder         : Lavc57.54.100 wrapped_avframe
    Stream #0:1(eng): Audio: pcm_s16le, 48000 Hz, stereo, s16, 1536 kb/s (default)
    Metadata:
      creation_time   : 2016-07-28 00:48:45
      handler_name    : Sound Media Handler
      encoder         : Lavc57.54.100 pcm_s16le
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> wrapped_avframe (native))
  Stream #0:1 -> #0:1 (aac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
frame=   50 fps=0.0 q=-0.0 Lsize=N/A time=00:00:02.00 bitrate=N/A speed=7.62x
video:24kB audio:375kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

[Carl Eugen Hoyos]

Not reproducible with Cam_PAL_D1_inter_Field_25fps_2.5MBS.mp4 and DVD500k.mp4 (which do not contain B-frames).