Threshold selection using second derivatives of the gray scale image

It is known that when a bilevel image is blurred, the intensity of the original pixels is related with the sign of the curvature of the pixels of the blurred image. A technique for threshold selection is presented where a partial histogram is constructed solely from the pixels where curvature achieves extrema values. The method is most suitable when low-contrast images with textured backgrounds (but not sparse dot matrices) are a large fraction of the input population