Backdrops Overlays

Backdrops Overlays

Can i shoot with my camera and see it on the backdrop?

i shoot with my camera hooked up to either my computer or a tv with a plain backdrop. is there any way i can bring up my digital back drop on tv on computer and overlay the images so i can line them up with digital backdrop items later...(for example if i have a bridge in back drop i want her hand on the rail. i want to line up right and take picture so when i insert later it is accurate and right?!)
sorry if this is confusing. i don't know how else to explain
i did looked up chroma key and its all replaceing the background later...i want to be able to see the person in digital background before pics taken....how??????

yeah, there is a way which is quite simple, i don't really remember how without doing it but learnt it in college, ideally need a green screen to add computer generated backdrops accurately, have a look around the net at some photography tips sites im sure you will find the exact step by step somewhere