I've used VLC running behind OpenLP to display a live video feed before. Basically, I attached a HD webcam to the computer. Then opened VLC and then Media / Open Capture Device from the menu, select the webcam from the "Video Device Name" drop-down, and enter the desired webcam resolution in "Video Size". Set VLC to full screen behind the OpenLP display screen and the webcam feed is displaying.
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Then go back to OpenLP and either create a theme with a transparent background or just tap the blue "Show Desktop" icon and ta da, you're displaying a live feed for the cost of a cheap webcam and a long usb cable.
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Now, I did find there was enough lag in the system that I couldn't truly show a "live" feed. If you showed someone talking, the half-second or even full-second delay was terribly distracting, so the times I've used it are to show closeups of things that would otherwise be hard for the congregation to see -- everyone dropping commitment cards into a basket on the altar, the baptism water, people pinning notes to a cross up front... those sorts of things.
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Oh, and of course, you've gotta mute VLC or the mic on the webcam creates a terrible feedback loop!