Square tiling

The square grid, square tiling, square tessellation or square lattice (Schläfli symbol: {4,4}) is one of the three possible proper Euclidean tilings of 2D space, in which four squares are placed at each vertex.

The square grid is the most familiar of all 2D tilings, with cell positions being easily and uniquely defined via Cartesian coordinates, and is the grid on which cellular automata are simulated most frequently.

Neighbourhoods on the square grid

Range-1 cases

Most of the cellular automata which are investigated are isotropic and use neighbourhoods of range 1. These constraints permit two neighbourhoods of interest; the 4-cell von Neumann neighbourhood, consisting of the four cells orthogonally adjacent to a central cell, and the 8-cell Moore neighbourhood, consisting of the four cells orthogonally adjacent to a central cell as well as the four cells diagonally adjacent to said center cell. (A third neighbourhood, referred to on this wiki as the "saltire" neighbourhood, which consists only of the four cells diagonally adjacent to the central cell, functionally duplicates the von Neumann neighbourhood and is disregarded in this section for the sake of brevity.)

Whether the central cell is counted as being part of the neighbourhood commonly depends on the situation.

von Neumann	Moore	saltire (trivial)

Due to its simplicity, the von Neumann neighbourhood does not lend itself to particularly many interesting 2-state outer-totalistic rules; it does, however, see much more use in more complicated anisotropic rules, and outer-totalistic rules based on it can often be sighted in more complex rules; for example, 2D replicators tend to follow the rule B13/S13V to an extent.

The Moore neighbourhood is the neighbourhood most commonly used in naturalistic rules, with many of the most famous rules such as Life and HighLife being based on it. Its larger size permits a much wider array of rules, many with their own interesting behaviours, in contrast to the more oppressive von Neumann neighbourhood, while not being so large as to include an unthinkably large amount of possibilities which makes finding interesting rules difficult.

Higher ranges

It is entirely possible to extend the aforementioned von Neumann, Moore and even saltire neighbourhoods to higher ranges, such that cells which are not immediately orthogonally or diagonally adjacent to the central cell are also included in the neighbourhood.

There are two immediately obvious ways in which the von Neumann neighbourhood can be extended to a range of 2:

• by counting all of the cells in the von Neumann neighbourhood of the cells which are in turn already in the von Neumann neighbourhood of a designated central cell, or
• by counting all of the cells which are at an orthogonal distance 1 or 2 from a central cell.

The former is usually referred to as the von Neumann neighbourhood of range 2 (depending on how much this process is repeated), and gives a square shape which is rotated 45 degrees from its constituent cells. The latter is referred to as the cross neighbourhood of range 2, giving a figure that resembles a plus shape.

A third, more subtle way in which the range-1 von Neumann neighbourhood can be extended to higher ranges is by considering the grid to be a checkerboard, and only considering the squares of opposite parity to the center cell out to a distance of 2 both orthogonally and diagonally. The resulting neighbourhood is therefore referred to as the checkerboard neighbourhood of range 2.

These three methods can be used to generate neighbourhoods of any desired positive integer range, not just 1 or 2. Examples up to a range of 5 can be seen below.

Range \ Type	von Neumann neighbourhood	cross neighbourhood	checkerboard neighbourhood
1
2
3
4
5

The Moore neighbourhood can also be generalized to higher ranges in much the same way. Two obvious examples follow:

• by counting all of the cells in the Moore neighbourhood of the cells which are in turn already in the Moore neighbourhood of a designated central cell, or
• by counting all of the cells which are at an orthogonal or diagonal distance 1 or 2 from a central cell.

In these cases, the former neighbourhood is considered to be the Moore neighbourhood of range 2, whereas the latter is referred to as the star neighbourhood of range 2. The former is a solid square, five cells to a side, and the latter generates a figure composed of a superimposed plus and X.

Both of these processes can again be used to create neighbourhoods of any desired range, with ranges up to 5 shown below.

Range \ Type	Moore neighbourhood	star neighbourhood
1
2
3
4
5

Symmetries

Isotropic rules on the square grid will always confirm to one of 16 different static symmetries. Its fourfold rotational and reflectional symmetry permits asymmetry, twofold cyclic symmetry, fourfold cyclic symmetry, twofold mirror symmetry, fourfold mirror symmetry and eightfold mirror symmetry, with further variants depending on the positioning of the center of rotation or the positioning and/or rotation of the planes of reflection.

Asymmetry	Cyclic symmetries
Onefold	Twofold			Fourfold
	Cell-centered	Edge-centered	Vertex-centered	Cell-centered	Vertex-centered
C1	C2_1	C2_2	C2_4	C4_1	C4_4

Mirror symmetries
Twofold			Fourfold					Eightfold
Orthogonal		Diagonal	Orthogonal			Diagonal		Odd	Even
Odd	Even		Odd-odd	Odd-even	Even-even	Odd	Even
D2_+1	D2_+2	D2_x	D4_+1	D4_+2	D4_+4	D4_x1	D4_x4	D8_1	D8_4