How do extract boundary nodes in matlab?

I am trying to extract all of the boundary notes for an ellipse from PDEModeler (pictured below). I know that the boundary nodes are 1:24 but when I use the following code I get extra entries. My nodes for the whole system are as follows:

gd =

    4.0000
    0.0033
   -0.0082
    0.9967
    0.4704
         0

Then I wish to find the boudnary nodes. To do this I follow a similar appraoch to Larson's Finite Element book:

   fixed=boundary(nodes(:,1),nodes(:,2),1)%Finds boundary points for complex geometries 
T=sort(fixed);
T' =

  Columns 1 through 12

     1     1     2     3     4     5     6     7     8     9    10    11

  Columns 13 through 24

    12    13    14    15    16    17    18    19    20    21    22    23

  Columns 25 through 36

    24    26    26    28    33    38    41    42    49    52    53    65

But we know that the only entries should be 1:24. If anyone could help, I'd greaty appreciate it.

[Javad Bagheri]

hello my friend

im trying to extract the boundry nodes from pdemodeler and i couldn't yet

please help me,

this is my email adress:

[e-mail]

would you please send me whole code, thanks a lot

Answer 1

[Image Analyst]

If you have the (x,y) coordinates of all those points, call convhull() to get the indexes of those that are on the convex hull.

[KieranSQ]

Many thanks, I hadn't heard of that function!

Answer 2

[Mohammed Abdallatif]

I do this using the neighbour function implemented in Matlab.: N = neighbors(G,nodeID)

Imagine you clustered the data in x clusters, you can identify the boundary nodes easily as those nodes who are member (you might use the ismember function as well) of a cluster: , and have nighbours which are mbers of an other cluster

Applysing this for a 9 Node graph as below, yiels to the following list of boundary nodes BN:

BN = [4 6 5 7 8 9]