3 5 x 1 21 x 7 8 July 8, 2019 16 32 64 128 15 3 8 x 21 7 8 x 5 1 4 filter 15 3 8 x 21 7 8 x 5 1 4 5 x 7 = 35 5 3/5 x 1 4/21
