Fingerprint Ridge Thinning

Ridge thinning is to eliminate the redundant pixels of ridges till the ridges are just one pixel wide.  In each scan of the full fingerprint image, the algorithm marks down redundant pixels in each small image window (3x3) and finally removes all those marked pixels after several scans. The thinned ridge map is then filtered by other Morphological operations to remove some H breaks, isolated points and spikes. In this step, any single points, whether they are single-point ridges or single-point breaks in a ridge are eliminated and considered processing noise. In previous testing, the parallel thinning algorithm had bad efficiency, but it got an ideal thinned ridge map after enough scans.
Below is an image of a thinned version of a fingerprint image: