Can someone guide me how to compute Hd to be used in fwind2 as an input argument?

example: Use fwind2 to design an approximately circularly symmetric two-dimensional bandpass filter with passband between 0.1 and 0.5 (normalized frequency, where 1.0 corresponds to half the sampling frequency, or π radians): Create a matrix Hd that contains the desired bandpass response. Use freqspace to create the frequency range vectors f1 and f2. [f1,f2] = freqspace(21,'meshgrid'); Hd = ones(21); (how and on what basis this statement has been written?) r = sqrt(f1.^2 + f2.^2); Hd((r<0.1)|(r>0.5)) = 0;(how and on what basis this statement has been written?)