Inks: black translucent background with hole

[David Bovill]

Just wandering if someone who knows about inks can work this out: I want to
make a selection area of an image the only bit that shows up clearly with
the rest of the image darkened. I think I've done this before by using two
button on top of the image - one the whole size of the image and the other
the size of the selection. The combination of the inks results in a
completely translucent area - hope that is clear. I guess the other way is
to have 4 translucent black buttons and position them?