MATLAB: ¿How to sum determinated values from an image

Question

Hi! I want to create a "median filtering" without medfilt2(). I created a random matrix just to visualize how the program works, but I'm suposed to apply this to any gray image (uint8).

I have this code right now

if true
  % code
>> f=uint8(round(255*rand(10,9)));
>> [M,N]=size(f);
>> g=uint8(zeros(M+2,N+2));
>> g(2:end-1,2:end-1)=f
g =
    0    0    0    0    0    0    0    0    0    0    0
    0  252   35  241  105   90  200   57   28   80    0
    0   17   56  173  154  248  177   69   46   46    0
    0  240   46  252  191   88    2  172   25   86    0
    0    5   11  196  149  226  215  122  125   54    0
    0  174   27   86  141  116  235  159   49  130    0
    0  200  157  169  149  105  197   60  228  231    0
    0  136  240   62  131   56   11   45   25  160    0
    0  226   90   75   21   32   96  212   11   26    0
    0  229  105  173  183   79  180  196  142  100    0
    0  160  251  135  254  185  186  238  197   14    0
    0    0    0    0    0    0    0    0    0    0    0
>> for m=1:M
for n=1:N
h(m,n)=(g(m,n)+g(m,n+1)+g(m,n+2)+g(m+1,n)+g(m+1,n+2)+g(m+2,n)+g(m+2,n+1)+g(m+2,n+2))/8;
end
end
>> h
h =
   14   32   32   32   32   32   32   32   15
   32   32   32   32   32   32   32   32   32
   17   32   32   32   32   32   32   32   32
   32   32   32   32   32   32   32   32   32
   32   32   32   32   32   32   32   32   32
   32   32   32   32   32   32   32   32   32
   32   32   32   32   32   32   32   32   32
   32   32   32   32   32   32   32   32   32
   32   32   32   32   32   32   32   32   32
   32   32   32   32   32   32   32   32   32
>> 
  end

So I was wondering why the values of h were almost always 32, And I found that if I sum for example 14+18, it is 22, but if I sum 127+128 it gives me (in matlab, in the upper code) 255, if doesn't matter if the number is really bigger than 255. That's why when I do the median (/8) it gives me almost always 32 (255/8=31.87)

So, how can I avoid this? how can I sum the values of the images and how can matlab gives me the real result?

Thanks (And sorry if my english is so bad) 🙂

Answer 1

When you add two uint8 values the result will be a uint8 value and so will be at most 255. You need to convert the values to a type that can hold the maximum sum. Or use mean()