Blue Max and my own game, Momentum, are both played on a hex grid. Working with a hex grid raises some problems not present in a square grid, not the least of which is how do you identify a particular hex in the grid when they do not line up neatly into rows and columns.

The coordinates I use in Blue Max is a three axis system. Each axis runs through the centers of opposite sides of the hexagon. The positive directions of each axis are adjacent. On a clock face the positive axis directions are found at 12, 2 and 4. The negative axis run in the opposite direction at 6, 8 and 10.

Hex system axises

Coordinates are then represented by a set of three numbers, one for each axis. I write these coordinates in square brackets as that is the syntax for arrays in both Javascript and PHP, my scripting languages of choice.

When you start assigning coordinates to a hex grid in this system you soon discover the same hex can be represented with more than one set of coordinates. For example [1,0,1] is the same as [0,1,0].

These coordinates are equivalent

The later of those two coordinates is the simpler, it involves one direction and less movement. Reducing a set of coordinates to its simplest state is called normalisation.

By using three coordinates to represent six directions there is some normalisation to start with. The next step of normalisation is to take travel in non-adjacent directions and convert it to travel in the direction between the two. In the above example travel in the X and Z directions is converted to travel in the Y direction. If the two non-adjacent directions are not equal convert as much as possible and leave the rest – effectively reducing one of the non-adjacent coordinates to zero.

In the end a normalised coordinate can contain at most two non-zero values, and those two values must be for adjacent directions.

When coordinates are normalised in this way hexes have one unique coordinate each.

Hex coordinate normalisation

This is the system I am use in Blue Max. I have implemented a series of functions for it in Javascript as a jQuery plugin and in PHP.

Pros

• This coordinate system works well for getting the distance and direction between two discrete hexes.

Cons

• Coordinates are not easily human readable.
• Areas of hexes cannot be easily described.
• There are “holes” in the coordinate system – not all combinations of coordinates are valid.

### An alternate 2 coordinate system

This system uses two of the three directions described above. For the ease of conversion between the systems directions X and Y are used. This coordinate system places all hexes on two oblique axises.

To convert from two to three coordinates, a Z value of zero is added.

[1,2] = [1,2,0]

The resulting three coordinates would then need to be normalised as described above.

To convert three coordinates to two coordinates the Z value needs to be converted to an X,Y equivalent. In this case:

[0,0,1] = [-1,1]

[0,0,-1] = [1,-1]

So

[0,1,2] = [(0-2), (1+2)] = [-2,3]

Pros

• Coordinates are normalised by their very nature, there can be no ambiguity.
• All coordinate combinations are valid, there are no holes.

Cons

• Because of the oblique axises of the system two hexes can be closer together or further apart then a simple inspection of their coordinates would imply.