[Mne_analysis] regarding morphing in matlab

[Alexandre Gramfort]

Hi Eric,

here is a matlab function that takes the faces (ie. the triangles)
of a mesh and returns a sparse adjacency matrix.

hope this helps
Alex

On Tue, Mar 29, 2011 at 9:51 PM, Eric Larson <larson.eric.d at gmail.com> wrote:
> Hello all,
>
> I have been using a nearest-neighbor mapping between subjects based on
> the morphing maps, but now I am trying to implement the smoothing
> operator in MATLAB as Matti suggested in an earlier email. However, I
> have encountered a problem at step 2 ("On each row, insert ones to the
> columns corresponding to the vertices adjacent to the vertex
> corresponding to the row."): I am unsure how to determine which
> vertices are adjacent to other vertices in the original
> (~100,000-element) source space, which I think is where Matti is
> suggesting the spreading operator needs to be applied.
>
> I tried using mne_add_patch_info with a distance argument, but that
> only seems to determine:
>
> 1) distances between elements in the decimated source space (thus why
> it shows up as dist_limit=-0.01 for 10mm distance calculation when
> using "mne_read_source_space(fname,true)" in MATLAB), and
>
> 2) the nearest vertex in the decimated source space for each vertex in
> the original source space.
>
> Is there some function I can use to determine which vertices in the
> original, un-decimated source space are neighbors? Or am I missing
> something about how to calculate the smoothing operator?
>
> Eric
>
> On Tue, Mar 15, 2011 at 5:17 PM,
> <mne_analysis-request at nmr.mgh.harvard.edu> wrote:
>> Here is the recipe we came up with Alex Gramfort during lunch to make
>> the spreading operator matrix. The C code uses direct iteration
>> instead of forming this matrix explicitly. By "valid neigboring
>> vertex" I mean a vertex which is adjacent to the vertex of interest
>> AND belongs to the source space OR has been defined during previous
>> iterations of the spreading operator.
>>
>> This is what you need to do:
>>
>> 0. Initialize the spreading operator as an nvert x nvert identity
>> matrix (S_0)
>> 1. Create an nvert x nvert empty matrix (S)
>> 2. On each row, insert ones to the columns corresponding to the
>> vertices adjacent to the vertex corresponding to the row.
>> 3. Zero columns corresponding to vertices that are not valid.
>> 4. Divide each row by the number of non-zero entries on that row.
>> 4. Compute the spreading operator corresponding to the k'th smoothstep
>> as S_k = S*S_{k-1}
>>
>> I hope this helps.
>>
>> - Matti
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