Decklinkvideosrc sync inputs fill and key

General Discussion
1

I have 2 sdi inputs in my decklink (in my case quad2) . It’s “Fill OUT” (FILL) and “Mate OUT” (KEY) of Ultimatte 12 4K.
Can i somehow make them frame accurate synced to use it with alphacombine plugin?

sample pipeline not work frame accurated:

gst-launch-1.0 decklinkvideosrc device-number=0 connection=sdi drop-no-signal-frames=true skip-first-time=0 num-buffers=20 ! queue leaky=2 ! identity sync=true ! videoconvert ! videobalance brightness=0.1 contrast=1 ! combine. decklinkvideosrc device-number=4 connection=sdi drop-no-signal-frames=true skip-first-time=0 num-buffers=20 ! queue leaky=2 ! identity sync=true ! videoconvert ! videobalance brightness=0.1 contrast=1 ! combine. alphacombine name=combine ! queue ! compositor name=comp force-live=true ! queue ! fakesink

2

It will always be fragile to use alphacombine, since the element is not an aggregator to avoid introducing latency in its intended use case (webm alpha and hopefully HEVC and H.264 alpha in the future).

You may need to implement a new one, using aggregator base class, which will introduce a thread.