I am trying to get my head around setting up something like this:

The idea came from a potential client who wants to pull off something similiar but with a more interactive setup, where People can go to an pre-installed Ipad and "launch" the comet (for example) through the push on a button.

Here's what I've got so far:

LED screen stands on street. Pre-rendered rgba clips sit on an Ipad which is connected to an ATEM unit through Lightning->HDMI Adapter (setup as secondary Screen). A camera behind the LED screen feeds the "window content" live through HDMI to ATEM. In ATEM software the live feed is overlayed permanently with whatever comes from the Ipad. ATEM output goes to LED screen.

So when a user goes to the Ipad and chooses a clip to launch it gets played over the Ipad's output and mixed live in ATEM and then displayed on the screen in front of the user.

There are a few possible problems which I can already think of: Will the Ipad output "black" (which would then black out my live feed) when nothing is being played? Can ATEM overlay two vieo feeds, one with alpha? Will alpha even be transferred via HDMI? EDIT: No it's not. So instead of rgba I could use greenscreen footage and let ATEM do a live key?

I would very much appreciate any of your thoughts on this.

THANK YOU!