Hi Daniel,
don't use Euclidean distance. Use the length of the great circle connecting each point to the central vertex on the ?h.sphere surface.
Bruce
On Tue, 8 Mar 2005, Daniel Goldenholz wrote:
Doug
I was thinking of trying something like that.
If I took all vertices which are a radius M or less from my centriod, aren't I basically done?
(M is defined as the maximum euclidean distance between the centroid on the sphere and the ring of outer vertices. If my surface distance max is called R, I can use some geometry to figure out the relation between R and M)
What tweaks would I need to do at the folded surface level?
On Tue, 8 Mar 2005, Doug Greve wrote:
You might try starting off with the ?h.sphere. It's easy to parameterize these things on the sphere. The sphere, of course, has some metric distortion, but if the patch is small, it might not be to bad. Once you've identified the vertices in the patch (or maybe a slightly bigger patch), then you can refine it in the folded space.
doug
Daniel Goldenholz wrote:
Hi. I have a graph theory question:
I would like to make a definition of a particular patch of cortex, that has a centroid at position (x,y,z) and has a total area of A.
I thought that this was going to be somewhat hard, but I soon learned that
it is very hard.
First, I found some code for the Dijkstra algorithm, which can quickly
tell me what is the minimum number of jumps needed from one vertex to
another. IF the freesurfer surface triangles were all of EXACTLY the same
euclidean distances on each side, then this alone could solve my problem.
I could simply take all vertices which have a jump distance less than
some number.
However, because the side lengths of the triangles in a freesurfer surface
are NOT the same, I need to get more sophisticated.
Ideally, what I would like is the following: If I were an ant, walking along on the surface of the folded brain, I could walk from the centroid in any direction for a certain distance, and still be within my "patch." Knowing that distance (radius), I could just use pi*r^2 and call that the area of the patch.
But how do I find this idealized patch in real life?
Thanks Daniel Goldenholz _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
-- Douglas N. Greve, Ph.D. MGH-NMR Center [EMAIL PROTECTED] Phone Number: 617-724-2358 Fax: 617-726-7422
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