I'd guess you need a little bit of RAM, since you need the last y frames, but there is probably not much computation. Just copy line n from n frames back into c

I'd guess you need a little bit of RAM, since you need the last y frames, but there is probably not much computation. Just copy line n from n frames back into current new frame. Maybe somebody could make an app, which does it in realtime? Current smartphones should be powerful enough.