of the target pixel and the two adjacent pixels with the predetermined value when the differences between the gray level of the target pixel and the gray level of the adjacent pixels are greater than the predetermined values k 1 k 2 the comparators 550 560 outputs logic 1 when the difference s between the gray level of the target pixel and the gray level of the adjacent pixels is smaller than predetermined value k 1 k 2 the comparators 550 560 outputs logic 0 the and gate 570 coupled to the outputs of the two comparators 550 560 and the output of xor 540 the accumulation unit 580 coupled to the output of the and gate 570 comprises an adder 582 and a buffer 584 for accumulating the non smooth changes in this embodiment if the difference of the gray level of the target pixel and each gray level of the two adjacent pixels is greater than the predetermined value and if the gray level of the target pixel is either greatest or smallest of the three values the comparator 510 will output logic 1 to the accumulation unit 580 which represents a zigzag exists at the target pixel otherwise the pixel comparator 510 will output the logic 0 to the accumulation unit 580 therefore each pixel of the first field sequentially acts as the target pixel and by comparing with the adjacent pixels of the second field the pixel comparator 510 outputs logic 1 or 0 to the accumulation unit 580 the similarity decision unit 590 is coupled to the output of the accumulation unit 580 when the accumulation result of the accumulation unit 580 is greater than.