Georges Rousse (also see this page) constructs elaborate optical illusions by selectively painting parts of buildings in such a way that, when viewed from a certain direction, a pattern emerges. (Isn’t this description a fine example of how a description can be very precise, yet not too helpful? Have a look at this image: Here, the white circle is formed by those parts of the building that the artist painted white - because you look at it from the direction where all the different white bits of the building form a circle).
Update (10 May): This page documents how one of those rooms is prepared. Note how the tedious work is done by students!