A pair of on-center and off-center filters derived from the first absolute central moment: a pair of on-center and off-center filters

Due to the absolute value involved, the first absolute central moment can be divided into two complementary filters: a positive deviation e_p and a negative deviation e_n. The architectures of e_p and e_n and their responses to luminous stimuli recall the separation of the ganglion cells into on-center and off-center cells. Once e_p and e_n are computed separately both a first derivative operator and a second derivative operator can be obtained with the same filtering stage. A ridge map similar to the ridge map provided by a GoG filter is obtained by subtracting the negative deviation en from the positive deviation e_p. Instead, the sum of e_p and e_n provides the same map which is obtained with a DoG filter.