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16.4 Details of the Territory Valuation

This section explains how GNU Go assigns a territorial value to an intersection once the white and black influence have been computed. The intention is that an intersection that has a chance of xx% of becoming white territory is counted as 0.xx points of territory for white, and similar for black.

The algorithm in the function new_value_territory goes roughly as follows:

If wi is the white influence at a point, and bi the black influence, then value = ( (wi-bi)/ (wi+bi) )^3 (positive values indicates likley white territory, negative stand for black territory) turns out to be very simple first guess that is still far off, but reasonable enough to be useful.

This value is then suspect a number of corrections. Assume that this first guess resulted in a positive value.

If both bi and wi are small, it gets reduced. What exactly is "small" depends on whether the intersection is close to a corner or an edge of the board, since it is easier to claim territory in the corner than in the center.

Then the value at each intersection is degraded to the minimum value of its neighbors. This can be seen as a second implementation of the proverb saying that there is no territory in the center of the board. This step substantially reduces the size of spheres of territory that are open at several sides.

Finally, there are a number of patterns that explicitly forbid GNU Go to count territory at some intersections. This is used e. g. for false eyes that will eventually have to be filled in. Also, points for prisoners are added.

To fine tune this scheme, some revisions have been made to the influence computations that are relevant for territorial evaluation. This includes a reduced default attenuation and some revised pattern handling.


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