1. A method comprising: receiving, at one or more computer systems, information specifying a first polytope, the first polytope including a plurality of vertices defining a closed concave boundary; determining, with one or more processors associated with the one or more computer systems, a value for each vertex in the plurality of vertices of the first polytope representing relative influence of the vertex at a first location within the boundary as measured using a path within the boundary that respects the closed concave boundary; interpolating, with one or more processors associated with the one or more computer systems, at least one value for the first location based on at least the value determined for each vertex in the plurality of vertices defining the closed concave boundary; and storing, in a storage device associated with the one or more computer systems, information associated with the at least one interpolated value for the first location.