I want to use 5*5 filter mask on my image in order to detect edges but my image is 3*3 how can i make my image 5*5? Please help me

I want to use 5*5 filter mask on my image in order to detect edges but my image is 3*3 how can i make my image 5*5? Please help me, my code is

%Input Image
A=imread('cameraman.jpg');
imshow(A);
A=rgb2gray(A);

%Preallocate the matrices with zeros
I=zeros(size(A));

%Filter Masks
F1=[5 8 10 8 5;4 10 20 10 4;0 0 0 0 0;-4 -10 -20 -10 -4;-5 -8 -10 -8 -5];
F2=[-5 -4 0 4 5;-8 -10 0 10 8;-10 -20 0 20 10;-8 -10 0 10 8;-5 -4 0 4 5];

A=double(A);

for i=1:size(A,1)
    for j=1:size(A,2)
        %Gradient operations
        Gx=sum(sum(F1.*A(i:i+2,j:j+2)));
        Gy=sum(sum(F2.*A(i:i+2,j:j+2)));

          %Magnitude of vector
           I(i+1,j+1)=sqrt(Gx.^2+Gy.^2);

      end
  end

I=uint8(I);
figure,imshow(I);title('Filtered Image');
%Define a threshold value
Thresh=100;
B=max(I,Thresh);
B(B==round(Thresh))=0;
B=uint8(B);

%B=im2bw(B);
figure,imshow(~B);title('Edge detected Image');
|edgePixels = nnz(B)

error shows- error using .* "matrix dimension must agree"|

Answer 1

[Adam]

Why are you extracting a 3x3 subset of your image in the first place if you have a 5x5 filter?

I would assume you need

Gx=sum(sum(F1.*A(i-2:i+2,j-2:j+2)));
Gy=sum(sum(F2.*A(i-2:i+2,j-2:j+2)));

and your loops will have to run from 3 up to size-2 instead unless you want to do special handling for the edge pixels where you don't have a full neighbourhood.

you should be able to use

doc nlfilter

or similar functions for this though too, although last time I used it it did annoyingly force a progress bar on you.

[rachita katoch]

this still give me error. which is

Subscript indices must either be real positive integers or logicals.

Please help me, i am new for this matlab and my programming is also not good.

[Adam]

Did you change your for loops to

for i=3:size(A,1)-2
    for j=3:size(A,2)-2

?

[rachita katoch]

Thank you so much sir, it worked. Sir can you do another favor for me? Please sir can you help me to find out pratt figure of merit for edge images using matlab. I will be very thankful if you could help?