Hexagonal LatticeWritten by Paul BourkeDecember 1997, Updated February 2004 
Of the three 2 dimensional shapes (equilateral triangle, rectangle, and hexagon) that can be used to tile the plane without holes, the hexagon is the most complex and has many interesting properties. In what follows, an indexing system will be described for a hexagonal tiling called a Spiral Honeycomb Mosaic (SHM). A SHM consists of groups of 7^{n} hexagons (n > 0) called superhexagons. It uses a base 7 numbering system for the hexagonal mesh, this is illustrated below for n = 1, 2, and 3.
An application has been developed to explore operations in SHM space. It is based upon XWindows and OpenGL and is currently available for Mac OSX and Linux (by request). Note that before running the Mac OSX version check that XWindows is running. Download: macosx.tar.gz
The interface is straightforward, to find out about any command line options type "hex h". The left mouse button pans the image, the middle button rolls, and the right button presents a list of menu options. In order to position/rotate/scale the hexagon to the image see the image mapping dialog box. To experiment with SHM operations see the transformation dialog box.
>hex h Usage: hex [options] Options h this text (help) i s load input TGA file v verbose mode Key commands ,+ zoom in/out r reset w write TGA image of window contents 1..6 set resolution ESC quit Mouse buttons left translate middle rotate right menus
Addition and Multiplication
The two basic arithmetic operations can be defined for the SMH, addition and multiplication. These operations act on the addresses of the SHM and result in translation (addition) and rotation/scaling (multiplication) when applied to images represented with the "pixels" of the SHM. The following two C snippets (HexAdd() and HexMul()) implement addition and multiplication. An important characteristics of the operations defined in this way is they are bijective, that is, it is a one to one mapping of each address. Every address (hexagonal pixel) maps uniquely to another address so that no information is lost.
Examples
This is an example of repeatedly multiplying an image on an order 5 SHM by 10_{hex} or 7_{dec}
This is the same example at two powers of 7 greater resolution showing the "superduck".
This example shows repeated addition by 6666_{hex}.
The curve through powers of 7 (10_{hex}) is an equiangular spiral described by
a = sqrt(3)
b = log_{e}(sqrt(7)) /
arctan(sqrt(3) / 2) = 1.3632084.
Since the angle in the above case is taken from the negative y (imaginary) axis, the curve would be traced by
The angle between each successive multiples of 10_{hex} is arctan(sqrt(3) / 2) = 40.893395^{o}, the ratio of two successive radii is sqrt(7) = 2.6457513.
Online SHM Calculator
Base Conversion Addition/multiplication Other Functions Cartesian

References
Alexander, D. and Sheridan, P.
Proceedings Australian Neuroscience Society (1995)
Algebraic Geometric Model of the Receptive Fields Properties
of the Macaque Striate Cortex.
Sheridan, P.
PhD Thesis, University of Technology, Sydney (1996)
Spiral Architecture for Machine Vision
Sheridan, P., Alexander, D.M.
Proceedings of Vision, Recognition, Action: Boston. (1997)
Invariant transformations on a spacevariant hexagonal grid
Sheridan, P., Hintz, T., Alexander, D.
Image and Vision Computing, 18 (2000)
Pseudoinvariant image transformations on a hexagonal lattice