[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 #32 from Nicolas Dufresne (ndufresne) <nicolas at ndufresne.ca> ---
Ok, I wonder how many time I'll need to read this, this code is hard to follow.
So reading through find_first_field() correct, before you hack:

0. Get the previous frame (for current view)

1. If the previous frame is missing a field, and current slice is progressive
(!slice_hdr->field_pic_flag), fill the gap in previous frame and add it to the
DPB.

2. If the previous frame is a frame, and the first field of the previous frame
is different parity then this "new frame" slice parity, assume there was a
missing picture.

3. If the slice if from the same frame_num as previous frame, then take the
previous frame as current to complete it, unless the parity is the same (looks
like this case is a bitstream error though), in which case, just finish
previous frame and start a new one.

4. I don't fully understand this one, but basically we pick the first field of
previous frame, fill_picture_other_field_gap() which adds it to the DPB. That
is very odd. If the previous frame first field parity is different then current
slice parity, assume there was a missing first field.

So basically two 2 implements what I had in mind already. And that's why it
mostly work with your hack. But case 4 seems to mess-up a little bit. I'm very
surprised we just assume field was missing, if some bitstream is passed twice,
it would endup overriding pictures ... Is that normal thing to do ?

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