where 1.4 are threshold values of t

where 1.4 are threshold values of the four nearest reference
points to p(x,y), and R1.4 are the distances from p(x,
y) to I1. . .4, respectively, as shown in Fig. 7d. The interpolated
threshold value is calculated based on the distances
of p(x, y) to its four nearest neighbor grid points. A complete
threshold value map is finally formed by determining
the threshold values for all pixels p(x, y) in all interpolation
grids. Compared with conventional bilinear interpolation
method, this algorithm is more efficient in terms of computational
effort.