I was visiting a museum today. One of the exhibits featured a circular projection against the wall.
To be clear the projector they used was a perfectly ordinary projector which protects a rectangular image. But a mask was installed near the lens, blocking out all light outside of a soft-edged disk shaped region.
The result is surprising effective. The region within registered not so much as a projection, but as a magical round window into another world.
I am delighted that a technique which is so simple can produce something that is so effective. Somehow this makes me very happy.