MATLAB: Always get 255 in sum

sum

I am getting the value 255 all the time in the below loop. P is an lena image of size m*n*3 (m=256,n=256). Actually i need the sum of all pixel values in the entire image. sum=0;
for i=1:3
     for j=1:256
         for k=1:256
             sum = sum+P(j,k,i)
         end
     end
 end
Please help me to solve the problem.

Best Answer

Variable P is likely class uint8. Assuming this is true, the summing is done as uint8 which clips at the highest value of 255 for this class. To get around this, do the sum as double instead. E.g.,
Psum = sum(double(P(:)));
Note that I am calling the MATLAB function sum, and NOT using sum as a variable name.
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