Motion blur image not working

I want to simulate motion blur kernel as in Gonzalez book.My code below works perfectly well for horizontal and vertical direction but doesn't work in diagonal direction.

Does anyone know where is the problem?

% Degrade the image to produce motion blurred images
% shown in Figures 5.26 of text book

clear; clc; 

f=imread('cover.jpg');
[M N] = size(f);

%[u,v]=freqspace([M N]);
%[u,v]=meshgrid(u,v);
T=1;a=0.1;b=0.1;r=(-1)^.5;
%center=ceil(N/2);
for u=1:M
   for v=1:N
      s=(pi*(a*u+b*v));
      H(u,v)=(T/s)*sin(s)*exp(-r*s);
   end
end


F=(fft2(f));

%Degrading the original image by multiplying H by F in frequency domain.
G=F.*H;
g=abs(ifft2(G));

%Displaying results
subplot(2,2,1);imshow(mat2gray(f)); title('f: Original'); 
subplot(2,2,2);imshow(mat2gray(g));title('Degraded Image by motion');

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Vertical Motion

for u=1:M
      for v=1:N
      
      H1(u,v)=(T/(pi*(a*u)))*sin(pi*(a*u))*exp(-r*pi*(a*u));
      
      end
end


%Degrading the original image by multiplying H by F in frequency domain.
G1=F.*H1;
g1=abs(ifft2(G1));

%Displaying results
subplot(2,2,3);imshow(g1,[]); title('Vertical Motion'); 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Horizontal Motion

for u=1:M
      for v=1:N
      
      H2(u,v)=(T/(pi*(b*v)))*sin(pi*(b*v))*exp(-r*pi*(b*v));
      
      end
end


%Degrading the original image by multiplying H by F in frequency domain.
G2=F.*H2;
g2=abs(ifft2(G2));

%Displaying results
subplot(2,2,4);imshow(g2,[]); title('Horizontal Motion'); 

[Eirik Kvernevik]

I have the same problem here, any advice ?