. "The apparatus according to claim 3, wherein when i???N???2 is established, said first operation means calculates differences between an output level of said notable pixel i and that of respective ones of a plurality of neighboring pixels ranging from (i+1)th pixel to (i+k)th pixel for 2???k???N???1, where k is a natural number, which adjoins said notable pixel i, and thereafter calculates an avera" . . .