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11.2 Eye spaces

In order that each eyespace be assignable to a dragon, it is necessary that all the dragons surrounding it be amalgamated (see section 10.2 Amalgamation). This is the function of dragon_eye().

An EYE SPACE for a black dragon is a collection of vertices adjacent to a dragon which may not yet be completely closed off, but which can potentially become eyespace. If an open eye space is sufficiently large, it will yield two eyes. Vertices at the edge of the eye space (adjacent to empty vertices outside the eye space) are called MARGINAL.

Here is an example from a game:

 |. X . X X . . X O X O |X . . . . . X X O O O |O X X X X . . X O O O |O O O O X . O X O O O |. . . . O O O O X X O |X O . X X X . . X O O |X O O O O O O O X X O |. X X O . O X O . . X |X . . X . X X X X X X |O X X O X . X O O X O

Here the `O' dragon which is surrounded in the center has open eye space. In the middle of this open eye space are three dead `X' stones. This space is large enough that O cannot be killed. We can abstract the properties of this eye shape as follows. Marking certain vertices as follows:

 |- X - X X - - X O X O |X - - - - - X X O O O |O X X X X - - X O O O |O O O O X - O X O O O |! . . . O O O O X X O |X O . X X X . ! X O O |X O O O O O O O X X O |- X X O - O X O - - X |X - - X - X X X X X X |O X X O X - X O O X O

the shape in question has the form:

 !... .XXX.!

The marginal vertices are marked with an exclamation point (`!'). The captured `X' stones inside the eyespace are naturally marked `X'.

The precise algorithm by which the eye spaces are determined is somewhat complex. Documentation of this algorithm is in the comments in the source to the function make_domains() in `optics.c'.

The eyespaces can be conveniently displayed using a colored ascii diagram by running gnugo -E.

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