Output of the function conv2 is not the size I expected?
6 views (last 30 days)
Show older comments
John Thress
on 17 Jul 2019
Commented: Star Strider
on 17 Jul 2019
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?
0 Comments
Accepted Answer
Star Strider
on 17 Jul 2019
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 Comments
Star Strider
on 17 Jul 2019
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.
More Answers (0)
See Also
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!