Electronic – Video overlay / on screen display architecture

The fundamental question here is how fast you need to update the sign's image.

If you don't need to update it more often than once every few seconds, DVI is WAAAAAY overkill. (And placing something that shows video next to a highway seems like a very bad idea)

You're probably best off using a frame-buffer in your hardware, and using something like a FT232 or FT245 to dump images to it. The FT2** IC Gives you ~8 MBps of interface bandwith over USB, and a dead-easy software driver you can talk to.

Alternatively, depending on your sign's resolution, you could use a simple serial interface.

I would assume you would be sending raw bitmaps to the sign (it makes the software end easier). Then you just cache them in the hardware, and then serve them up to whatever interface the sign implements from your hardware.

This is probably off-topic, because the easiest way to achieve this is to use the tools already provided by your graphics card.

Mirror your display, adjust the resolution of your screen to that of the window, and pan the screen until the desired area is displayed.

If you want to be able to select an arbitrary window to be mirrored, you need to add this functionality to your window manager, which is impossible on Windows (but should be doable for most Linux window managers). If you want to do this with an app you're developing (or an open-source terminal), just use a second display.