What does this refers to?

What does this refers to? And how can be implemented in matlab?

Where S refers to an Image.

Answer 1

[Wayne King]

I'm assuming this means the average (or mean). In that usage, it is a case of the bra-ket notation introduced by Dirac.

For a matrix, just use mean() with an optional dimension input to tell whether you want to take the mean columnwise, or by rows.

 A = randn(10,10);
 mean(A) % gives a 1x10 vector of column means
 mean(A,2) % gives a 10x1 vector row means
 mean(A(:)) % or mean(mean(A)) gives you the mean of all elements