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16.7 Permeability

The permeability at the different points is initially one at all empty intersections and zero at occupied intersections. To get a useful influence function we need to modify this, however. Consider the following position:

 
|......
|OOOO..
|...O..
|...a.X   ('a' empty intersection)
|...O..
|...OOO
|.....O
+------

The corner is of course secure territory for `O' and clearly the `X' stone has negligible effect inside this position. To stop `X' influence from leaking into the corner we use pattern matching (pattern Barrier1/Barrier2 in `barriers.db') to modify the permeability for `X' at this intersection to zero. `O' can still spread influence through this connection.

Another case that needs to be mentioned is how the permeability damping is computed for diagonal influence radiation. For horizontal and vertical radiation we just use the permeability (for the relevant color) at the intersection we are radiating from. In the diagonal case we additionally multiply with the maximum permeability at the two intersections we are trying to squeeze between. The reason for this can be found in the diagram below:

 
|...X    |...X    
|OO..    |Oda.
|..O.    |.bc.
|..O.    |..O.
+----    +----

We don't want `X' influence to be spread from `a' to `b', and since the permeability at both c and d is zero, the rule above stops this.


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