How can I convert jpg file to pbm file through only matrix operation?

Question

I have tried the method as follows, but it didn't work so efficient.

for i = 1:h 
            for j = 1:w 
                Img(1,(i-1)*aligned_w+j) = (Matrix(i,j)&1);
                %fwrite(file,1-Matrix(i,j),'ubit1');
            end
            for j = 1:align
                %fwrite(file,0,'ubit1');
                Img(1,(i-1)*aligned_w+w+j)=0;
            end
        end
        fwrite(file,Img,'ubit1');

the result of above code is off track compared to the imwrite(imread('.jpg','jpg'),'.pbm','pbm'), can anyone tell me how to solve it.