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.