And what is delight? For me, delight is born from a tool’s intuitiveness. Things just working without much thought or fiddling. Delight is a simple menu system you almost never have to use. Delight is a well-balanced weight on the shoulder, in the hand. Delight is the just-right tension on the aperture ring between stops. Delight is a single battery lasting all day. Delight is being able to knock out a 10,000 iso image and know it'll be usable. Delight is extracting gorgeous details from the cloak of shadows. Delight is firing off a number of shots without having to wait for the buffer to catch up. Delight is constraints, joyfully embraced.
Wang tiles (Hao Wang, 1961) are a class of formal systems. They are modelled visually by square tiles with a color on each side. A set of such tiles is selected, and copies of the tiles are arranged side by side with matching colors, without rotating or reflecting them.
The basic question about a set of Wang tiles is whether it can tile the plane or not, i.e., whether an entire infinite plane can be filled this way. The next question is whether this can be done in a periodic pattern.
In 1966, Wang's student Robert Berger solved the problem in the negative. He proved that no algorithm for the problem can exist, by showing how to translate any Turing machine into a set of Wang tiles that tiles the plane if and only if the Turing machine does not halt. The undecidability of the halting problem then implies the undecidability of Wang's tiling problem.