Next: Territory and Moyo, Up: Influence
We define call stones lively if they cannot be tactically attacked, or if they have a tactical defense and belong to the player whose turn it is. Similarly, stones that cannot be strategically attacked (in the sense of the life-and-death analysis), or that have a strategical defense and belong to the player to move, are called alive. If we want to use the influence function before deciding the strategical status, all lively stones count as alive.
Every alive stone on the board works as an influence source, with influence of its color radiating outwards in all directions. The strength of the influence declines exponentially with the distance from the source.
Influence can only flow unhindered if the board is empty, however. All lively stones (regardless of color) act as influence barriers, as do connections between enemy stones that can't be broken through. For example the one space jump counts as a barrier unless either of the stones can be captured. Notice that it doesn't matter much if the connection between the two stones can be broken, since in that case there would come influence from both directions anyway.
From the influence of both colors we compute a territorial value between -1.0 and +1.0 for each intersection, which can be seen as the likely hood of it becoming territory for either color.
In order to avoid finding bogus territory, we add extra influence sources at places where an invasion can be launched, e.g. at 3-3 under a handicap stone, in the middle of wide edge extensions and in the center of large open spaces anywhere. Similarly we add extra influence sources where intrusions can be made into what otherwise looks as solid territory, e.g. monkey jumps. These intrusions depend on whose turn we assume it to be.
All these extra influence sources, as well as connections, are controlled by a pattern database, which consists of the two files patterns/influence.db and patterns/barriers.db. The details are explained in Influential Patterns.