[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 #31 from sreerenj <bsreerenj at gmail.com> ---
(In reply to Nicolas Dufresne (ndufresne) from comment #29)
> It's flawed, because it does not adhere to the spec. No other decoder I've
> read seems to do that. It's also flawed because if you have two frames,
> placed like this t0 b0 t1 b1 and loose b0 and t1, it will gently create one
> mixed frame t0 b1.


It loses fields only if the order is "t0 b0 t1 b1" and no dpb reset after b0.

copy&pasting the commit message:
"Try to identify missing first fields too, thus disregarding any
    intermediate gaps in frames. We also assume that we keep the same
    field sequence, i.e. if previous frames were in top-field-first
    (TFF) order, then so are subsequent frames.

    Note that insertion of dummy first fields need to operate in two
    steps: (i) create the original first field that the current field
    will inherit from, and (ii) submit that field into the DPB prior
    to initializing the current (other) field POC values but after any
    reference flag was set. i.e. copy reference flags from the child
    (other field) to the parent (first field).
"

> 
> The frame num check would solve this (JM does check this). But then we need
> late concealment, as this decode call will produce two pictures.

Of course, Checking dpb for missing fields seems to be a viable option for me.
A patch might be more tricky. Waiting for the fix :)

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