The sphere-based patch will change size and shape a little when you map it back to the folded surface due to metric distortion. So you could refine it some, eg, run your Dijstra algorithm on just the patch. I would not expect the distortion to make much of a difference, so it's probably not worth the trouble.
dougx 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 > > > > -- > Daniel Goldenholz > ----------------------------------------------------- > Cell: 617-935-9421 http://people.bu.edu/danielg/ -- Douglas N. Greve, Ph.D. MGH-NMR Center [EMAIL PROTECTED] Phone Number: 617-724-2358 Fax: 617-726-7422 _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
