the output only works in full screen.
OSX does a lot of stuff behind the scenes to manage window layers - and this needs to be minimised.

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You cannot use the decklink outputs because all the graphics processing is done on the graphics card - decklink card doesnt have any opengl graphics.

computer architecture does not make it possible to send the screens from the graphics card to the decklink card.

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also the design of the decklink cards restricts you to using either input or output - when using its I/O. not both at the same time.