[Bug 796169] vaapidec: Sample video that renders fine with FFMPEG but not in GStreamer

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=796169

--- Comment #20 from sreerenj <bsreerenj at gmail.com> ---
Current code tries to identify missing first field based on the assumption that
we keep the same field sequence, that is, if previous frames were in
top-field-first (TFF) order, then so are subsequent frames.
Unfortunately H264 spec doesn't have a TFF syntax element and only field parity
is communicating through slice_hdr syntax elements.
So gstreamer-vaapi took an assumption that within the same SPS activated
sequence always have a same first-field parity.
See this bug report: https://bugzilla.gnome.org/show_bug.cgi?id=745048


In this stream, 49th frame has a FRAME which has TFF parity order.
50 th frame (two separate fields with two slice headers) has a parity change,
Bottom field came first.
But since gstreaamer-vaapi took the assumption of TFF in a sequence of frames
activated by SPS, it guess the first field is missing and tries to add ghost
field and eventually everything get messed-up.

I'm not sure how to fix this scenario.

One option could be to remove the assumption we took in #745048 may cause
issues with other streams.

The second option is to cache 2 fields (all its internal state structures)
before making the assumption "missing-first-field",  unfortunately implementing
this requires significant effort!

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