Re: Efficient submatrix computation or matrix index looping

[fuankarion]

Ok, this is the context:

I'm interested in the elements around every element of a matrix (and the
element itself), say for every element al position i,j I want to get a
submatrix of the elements at positions:

i-1,j-1    i,j-1    i+1,j-1

i-1,j       i,j      i+1,j

i-1,j+1   i,j+1   i+1,j+1

This is not restricted to 3x3 submatrices, actually I want to get 8x8
submatrices and 16x16 submatrices arround every element, part of the problem
is that I have to do this for a large number of matrices.

My question is simple: How can I do this without looping through all the
possible combinations of the indexes i,j as this nested loop runs very slow
in octave.

I think that any of the folling might help me.

A function in octave API that automatically retrieves all the submatrices
A quick way to get all the indexes i,j, and build submatrices upon this
indexes
If I have to stick to the nested loop, I want to know how to use more than
one core to process a matrix, or to be able to process some matrices at the
same time.

Thanks
Fuanka

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