Step 1. Choose the pair of pixels farthest from one another (denoted
by xt and xb in Fig. 4A, respectively) in the boundary. Find the
point xc that divides line xt–xb into equal segments.
Step 2. Select two boundary regions (depicted by red in Fig. 4A) that
are within 20% of the distance of line xt–xb above and below
the line that contains the point xc and is perpendicular to line
xt–xb.
Step 3. Choose a pair of pixels closest to one another, xl and xr,
between the two boundaries selected in step 2. Find the point
xo that divides the line xl–xr into equal segments.