Just to be clear i quickly made these images:
-the first image is a corridor full screen with the vanishing point at the center
-the second image is exactly like the ishar image i posted, the corridor is resized to fit a smaller window and the blue area is the ui
-the third image is what i wanted, the corridor is fullscreen but with a shifted vanishing point and the ui that only in part covers the corridor.
Maybe there's a way i ignore but with they way you suggest i can only get the second result, while what i want, and got with my method, is the third