Fig. 2.
This figure depicts the neighbor count algorithm. The inner and outer gray rings represent the lower and upper bounds respectively. The small circles represent the 
atoms of amino acids. The black circle represents the amino acid of interest. Amino acids a and f are assigned a neighbor weight of 0.0 because they are outside of the upper bound. Amino acids b and e are assigned a weight between 0.0 and 1.0 because they lie between the upper and lower bounds. Amino acids c and d are counted as one complete neighbor each because they lie within the lower bound