It is very clever. They also seem to have figured out how to get the same perspex block to display several different images depending on the angle the light hits it from.
One application seems to be for use in buildings. The milling really is very light so apparently a block would look like a window, with little obvious distortion, but it can project images.
I think they were going to mill glass - I did not know you could do that.
Also interesting that the ancient Chinese had managed something similar without the befit of computers.