
Layout of shared edges and corners for a 3x3 grid of tiles

  1----16----2----17----3----18----4
  |          |          |          |
  |          |          |          |
 19    NW   20    N    21    NE   22
  |          |          |          |
  |          |          |          |
  5----23----6----24----7----25----8
  |          |          |          |
  |          |          |          |
 26     W   27    C    28     E   29
  |          |          |          |
  |          |          |          |
  9----30---10----31---11----32---12
  |          |          |          |
  |          |          |          |
 33    SW   34     S   35    SE   36
  |          |          |          |
  |          |          |          |
 13----37---14----38---15----39---16

Each tile requires 8 seeds to be specified - and these need to match
those of the neighbouring tiles in order that the grid is tiled seamlessly.
So, to construct the 9 images, at most 39 different seeds are required.
For example, the NorthEastern tile, the one marked NE in the diagram, has
corners 3, 4, 7 and 8, and edges 18, 21, 22 and 25. The corners 3, 7 and 8
are shared with neighbouring tiles N, C and E, whereas the edges 21 and 25
are shared with tiles N and E.

Hence, when the images are constructed, each tile can refer to a look-up
table of 39 seeds.