I want to create a fullscreen camera preview where the middle of the view is a rectangle. Everything inside the rectangle should be opaque and everything outside should be semi-transparent:

The solid lines indicates opaque parts of the preview (the grey rectangle) and the dotted lines are the semi-transparent parts.

Currently, I’m using the standard AVCaptureVideoPreviewLayer but I don’t see a way to apply any alpha to the output let alone applying an alpha to only some areas. Is this possible?