In short: Use capture sdk's to pair time-aligned SDI inputs as a single key-and-fill input for external sources with alpha
For capturing sources with alpha, at least in my testing with BMD decklinks, capturing key and fill as paired SDI inputs is much more performant than NDI, especially at 4K. These key and fill inputs originate from a server with time-aligned outputs (eg on another PC with Decklink configured as key and fill outputs).
For slow or still content, Frz's "External Alpha" effect can achieve this, but once there's motion you can see the artifacts due to the mask and image's temporal misalignment. I believe there are (at least BMD) sdk functions to achieve this, could madmapper please support it? Key and fill outputs would also open up some system design options too.
Many thanks