[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 #23 from sreerenj <bsreerenj at gmail.com> ---
(In reply to Nicolas Dufresne (ndufresne) from comment #21)
> I'm trying to understand what to do. What I see from FFMPEG code (assuming
> I'm reading it properly), whenever it starts a new frame, it will first go
> over the DPB to find all frame with unpaired fields. When one is found,
> there is two venue, pair it with the new frame, otherwise it's a lost field,
> and it must be concealed. In the software decoder, they copy the previous
> field of the same parity, I don't see any concealment on the VAAPI side
> there.
> 
> Is the software FFMPEG decoder method what you had in mind when you say
> "cache 2 fields" ?


I was thinking to wait for the next field too before making an assumption. But
I afraid that this could break the current design of gstreamer-vaapi. Again, it
won't work well if there is two immediate field-parity order change in
consecutive frames.

What we need is some heuristics to find whether the field was missing or not. I
mean, how to distinguish field-parity change from "missing the first field"
scenario.

As you said checking in dpb for the unpaired field could be an option to try.

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