[FFmpeg-devel] [PATCH 0/2] Interpret a degenerate SAR as 1/1.

[Giovanni Mascellani <gmascellani@codeweavers.com>]

I came across some H.264 files in the wild whose SAR is set to
20480/0. The files are provided by the Ubisoft game OddBallers, and
are seemingly accepted and played properly when the game is played on
Windows (thus using the Microsoft Media Foundations
implementation).

When running the game with Wine the files are ultimately decoded by
libavcodec (via GStreamer), and playback is broken. It seems that only
a frame each second or so (maybe the key frames? I didn't check) is
decoded and presented, the others are discarded. After dumping the
video, I ran it with ffplay and it has the same problem, and the
following message is emitted many times:

  [h264 @ 0x7fd7301ef440] ignoring invalid SAR: 20480/1

Interestingly the invalid SAR is dumped as 20480/1 (which would be
strange, but in princple legal), while the file has 20480/0 (which
doesn't make sense at all). Equally interestingly, the frames that are
presented are indeed presented with SAR 1/1, like they are on Windows.

The H.264 standard says that "When aspect_ratio_idc is equal to 0 or
sar_width is equal to 0 or sar_height is equal to 0, the sample aspect
ratio shall be considered unspecified by this Recommendation |
International Standard". Given the behavior on Windows it seems that
the de facto standard way to solve the missing specification is to
assume that SAR is 1/1, which is what my patches seek to do.

With these patches both Wine and ffplay replay the broken file
correctly.

Since this is my first contributing to ffmpeg and I am not very
experienced in the project architecture, I'm not sure this is the
right abstraction level at which this problem should be handled. If
it's not, I'd be grateful if you could direct me to the appropriate
place.

Giovanni Mascellani (2):
  avcodec/h2645_vui: Interpret a degenerate SAR as 1/1.
  avcodec/evc_ps: Interpret a degenerate SAR as 1/1.

 libavcodec/evc_ps.c    | 4 ++++
 libavcodec/h2645_vui.c | 4 ++++
 2 files changed, 8 insertions(+)

-- 
2.45.2

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