Parallax "windows"

I’m pulling my hair out here (not literally - i’m bald).

I have a problem which i think is a basic parallax problem but is made more difficult due to masking. Any help would be appreciated and i’ll explain it the best i can.

Firstly i want my background image to scroll on page scrolling (which doesn’t seem to want to move right now but i’ll sort that out soon enough).

Secondly and most importantly I want that background image to be completely invisible except for when certain div blocks acting as “windows” scroll over it. Best example is going down an elevator and you can see outside as windows pass by.

I wan’t to avoid uploading a separate image for each divblock. So far i have my “windows” nested in a div which is the background image. How (and can I?) mask the rest of the image?