always get 255 in 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.

[KSSV]

Hello....you need not to run a loop to get sum...read the documentation of sum carefully. You can specify the dimensions and get the sum at once.

[Roger Stafford]

There is a Matlab function by the name of 'sum' and therefore you should not be using that name for your variable. It can confuse Matlab.

[Sneha]

I know about the function sum. But in my work there is an equation as given below. I was trying to implement the same. Thats why i tried that loop.

[Stephen23]

Do NOT use the variable name sum, as sum is the name of a very important inbuilt function.

[Sneha]

ok got it sir

Answer 1

[James Tursa]

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.