According to Wong's method, the coordinates of the center of a target were computed by the following formulas:
where 
is the grey intensity of a pixel located at row i and
column j, and has a value between 0 and 255, and n, m are the
image's rows and columns, respectively.
A validity check was performed on each target using the criteria of
shape and size. Because the targets were circular or symmetry in
shape, the ratio of the second moments ( 
and 
) about the two principal
axes should be approximately equal to 1. The following computation
formulas were used:
A rejection value of the roundness 
was used. If is below a
given threshold, it was rejected as a fiducial mark.