Introduction - If you have any usage issues, please Google them yourself
ch4_1_1: rectangular continuous function of the Fourier transform (§ 4.1.1) ch4_1_2: to build a rectangular function (§ 4.1.2) ch4_1_3: of f for two-dimensional fast Fourier transform (§ 4.1.2) ch4_1_4: for f to zero (region size of 256 × 256), and then carry out two-dimensional fast Fourier transform (§ 4.1.2) ch4_1_5: Transform the results so that the zero-frequency component at the center, call the function fftshift (§ 4.1.2)