AH Physics - Polarisation Question

Basically I’m doing a question where they’re on about how old 3D cinema equipment worked by using polarisers infront of two protectors that lets each eye see a different image though the glasses (also polarisers or analysers cause they’re the second ones...).

So “during the film the student looks at the screen through only one of the filters in the glasses. The student then rotates the filter through 90° and notices no change in brightness.
Explain this observation”

So obviously he shouldn’t be able to see anything at both 0 and 90° because of how polarisers work and cancelling out oscillations in all planes...

EDIT: I figured it out because there’s both a horizontal and vertical image on the screen so as he turns the lens from horizontal to vertical, the other picture begins to account for the lost brightness and eventually is the only component of the brightness...