Output of the function conv2 is not the size I expected?

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John Thress
John Thress 2019-7-17
评论： Star Strider 2019-7-17
Hello,
I am trying to use the conv2 function to convolute a matrix with a filter for the creation of a convolutional neural network. Take the following 3 by 3 matrix:
A = [1 2 3; 4 5 6; 7 8 9];
and the filter:
B = [1 0; 0 1];
For a stride of 1, my understanding of convolution from the examples I have seen is that the output should be (1*1) + (0*2) + (0*4) + (5*1) = 6 for the first element in the resulting matrix and (1*2) + (0*3) + (0*5) + (1*6) = 8 for the second element and so on until the filter reaches the bottom lower half of A for a final result of
ans = [6 8; 12 14];
when I use the conv2 function my results is:
conv2(A,B) = [1 2 3 0
4 6 8 3
7 12 14 6
0 7 8 9];
where my expected answer is in the middle of the output. Where do these extra numbers come from?

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Star Strider
Star Strider 2019-7-17
Reverse the order of the arguments (so that the smaller size matrix is first), then use the 'same' shape argument:
C = conv2(B,A,'same')
produces:
C =
6 8
12 14
  2 个评论
John Thress
John Thress 2019-7-17
Thank you so much!
What was the previous operation doing? Or rather what was the reason for those extra numbers?
Star Strider
Star Strider 2019-7-17
As always, my pleasure!
It was giving the default ‘shape’ result of conv2, which is to say it produced the 'full' convolution. Using the 'same' argument with the arguments presented as you originally did:
C = conv2(A,B,'same')
would produce a (3x3) result.

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