function hist_vector = calc_hist_vector(input_img)
    hist_vector = zeros(1,256); 
    for i1 = 1:size(input_img,1)
            for i2 = 1:size(input_img,2)
                for idx = 1:256
                    if input_img(i1, i2) == idx-1
                         hist_vector(idx) = hist_vector(idx) + 1;
                    end
                end
            end
    end
end