Get the middle point of a matrix
30 views (last 30 days)
Show older comments
Xin on 23 Jun 2020
Commented: Xin on 26 Jun 2020
Hi everyone. I have an interesting situation. So the idea is to calculate the value of middle points of a 1D, 2D or 3D matrix.
For example, A is a 100*100 matrix. What I want to obtain is an 99*99 matrix which simply represent the averaged value of the 3D matrix A.
I could easily do it by following:
B = (A(2:end,2:end)+A(2:end,1:end-1)+A(1:end-1,2:end)+A(1:end-1,1:end-1))/4;
However, this is rather slow as I need to reference A by 4 times. The situation is worse for 3D or large A. I am just wondering if there is a faster way to do this task by avoiding the multiple referencing or if there is any build-in matlab function that could do it in a faster way?
Thank you very much!
Tommy on 23 Jun 2020
Edited: Tommy on 23 Jun 2020
Possibly MATLAB's convolution functions will be faster:
% 1D case:
B = conv(A, ones(2,1), 'valid') / 2;
% 2D case:
B = conv2(A, ones(2), 'valid') / 4;
% 3D case:
B = convn(A, ones(2,2,2), 'valid') / 8;
Generalized for dimension n (I think) by something like this:
B = convn(A, ones([2*ones(1,n),1]), 'valid') / 2^n;
Seems to be faster for this case at least:
A = rand(100,100,100);
f1 = @() (A(2:end,2:end,2:end)+A(2:end,2:end,1:end-1)+A(2:end,1:end-1,2:end)+A(2:end,1:end-1,1:end-1)+...
f2 = @() convn(A, ones(2,2,2), 'valid') / 8;
>> all(abs(f1() - f2()) < 0.000000001, 'all')
More Answers (0)
Find more on Logical in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!Start Hunting!