MATLAB: Sum particular adjacent elements

conv2MATLAB

I learned about conv2 from the the post:
https://www.mathworks.com/matlabcentral/answers/357208-how-to-sum-all-adjacent-elements-in-a-matrix-with-1-s-and-0-s
My original goal was to sum all the adjacent elements in a matrix. The solution using conv2, from the post above, with the mask:
msk_0 = [1 1 1; 1 0 1; 1 1 1]
worked well.
Using conv2, with a similar logic, now I want to sum the elements above and adjacent to a given element. In this case my mask becomes:
msk_1 = [1 1 1; 0 0 0; 0 0 0] %top
I want to run this on a matrix f, indentical to the mask. 
f = [1 1 1; 0 0 0; 0 0 0]
The answer produced is 0
>> conv2(f, msk_1, 'same')
ans =
     0     0     0
     0     0     0
     0     0     0
My expectation was that the mask would result in the sum of the elements above and adjacent to a given element in f. The result in this case would be:
     0     0     0
     2     3     2
     0     0     0
What is it I am missing about the functionality of conv2?
How do I achieve my goal of summing only the specific elements in a given mask?
Thanks for your input.

Best Answer

No, you don't understand what convolution is. With convolution, the kernel gets flipped top-to-bottom, and left-to-right. There are mathematical reasons for this based on linear systems theory. So if you want the sum of the above row, your kernel would have to be
msk_1 = [0 0 0; 0 0 0; 1 1 1] %top
Or you can use imfilter() and not flip the mask because imfilter() does not flip the mask.
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