How to make a 3D matrix from 3 vectors?

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John
John on 13 Jan 2016
Commented: John on 13 Jan 2016
3 vectors, a b c, to be used as factor or filter for a 3D matrix V. For 2D it's easy and elegant:
d=a'*b;
f=M.*d; % M 2D matrix
The 3D filter can be done this way:
f=V*0.0;
for n=1:n3
f(:,:,n)=V(:,:,n)*c(n);
end
Is there a more elegant way for this? For example make a 3D matrix D from the 3 vector and then
f=D.*V
Thanks

Accepted Answer

Matt J
Matt J on 13 Jan 2016
Edited: Matt J on 13 Jan 2016
For filtering, it is probably not a good idea to convolve with d or D. Instead, you should filter separably. For example, in 2D, you could do,
f = conv2(a,b,M);
For element-wise multiplication in 3D, I similarly don't think building D is the way to go, as it creates an unnecessary extra array. I would probably do,
f=bsxfun(@times,V, a(:)*b(:).');
f=bsxfun(@times,f, reshape(c,1,1,[]));
But, if you really insist on building D, you could do
D = bsxfun(@times, a(:)*b(:).', reshape(c,1,1,[]));
Or, you could do
d=a(:)*b(:).';
D=reshape(d(:)*c(:).', size(V));

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