我是个matlab菜鸟，请大家指导一下，谢谢！请帮我解释一下下列函数中各个语句的意思，最好加注释。

[gravin77]

clear all
clc
II=imread('b.bmp');
II=double(II(:,:,1));
FF=abs(fftshift(fft2(II)));
[r,c]=size(II);
s1=100;
H=zeros(r,c);
H(300-s1:300+s1,400-s1:400+s1)=1;
IIFF=fftshift(fft2(II));
HFF=H.*IIFF;
HFF2=zeros(r,c);
HFF2(r/2-s1:r/2+s1,c/2-s1:c/2+s1)=IIFF(211-s1:211+s1,438-s1:438+s1);
IIyp=ifft2(HFF2);
OR=(IIyp);
phyp=atan(imag(OR)./(real(OR)+eps));
figure,imshow(phyp,[0,10]);