After the automatic segmentation of fiducial marks, a more accurate (sub-pixel) target location method was developed. There are two processing steps in the location procedure: 1) finding a rectangular window which just covers the complete fiducial mark and 2) precisely locating the center of the fiducial into a sub-pixel accuracy.
After segmentation processing, the non-fiducial regions have been removed. A simple routine is used to find a rectangle which covers the complete fiducial mark.
For most of the fiducial marks, they are circular or cross shaped. To locate the center of such targets, two algorithms are employed, they are Wong location operator [14] and Förstner location operator [15]. The optimal location is obtained after integrating the two location operators' results.