# How to divide a triangulation

8 views (last 30 days)
Wang Jack on 16 Aug 2020
Answered: Jun Matsubayashi on 8 Apr 2022
There is a triangulation, which is two parts that are not connected, how to distinguish them?
For example, a triangulation TR, which connection matrix is TR.ConnectivityList
[3 4 5;
4 6 3;
5 3 6;
8 9 2;
9 8 2]
As we can see that 3 4 5 6 are connected and 2 8 9 are connected, how to distinguish them to two triangulations, Tr1 and TR2
I tried to use dual for loop or dual while. Although the function can be realized, but the efficiency is very low, and the actual data is thousands of points, (3D point)
Now I use undirected graph to deal with them, it is faster than for or while, but the process is very complicated,,,
Is there a simpler way? or some matlab build-in function/statement?
connection=[TR.ConnectivityList(:,[1 2]);TR.ConnectivityList(:,[2 3]);TR.ConnectivityList(:,[1 3])];
connection=unique(sort(connection,2),'rows');
G=graph(connection(:,1),connection(:,2));%node connectivity, has lot of noise points
%plot(G)
bins = conncomp(G);
for i=1:max(bins)
p_index=find(bins==i);
m=ismember(TR.ConnectivityList,p_index);
t_ind=sum(m,2)>0;
TR_sorted{i,:}=triangulation(TR.ConnectivityList(t_ind,:),TR.Points);
end

Jun Matsubayashi on 8 Apr 2022
I am facing the same problem as you. It may be too late to reply, but I suggest one solution. This code does not involve any for loop.
tmp=TR.ConnectivityList;
groupnumber=1;
subTR=cell(1,1);
while ~isempty(tmp)
fprintf('Finding group #%d', groupnumber);
poolvec=tmp(1,:);
poolvecsize=3;
while length(poolvecsize)==1 || poolvecsize(end)>poolvecsize(end-1)
ind=any(ismember(tmp,poolvec),2);
poolvec=unique([reshape(tmp(ind,:),1,[]) poolvec]);
poolvecsize=[poolvecsize length(poolvec)];
fprintf('.');
end
fprintf('\nCumulative number of faces: %d\n', sum(ind));
subTR{groupnumber}=tmp(ind,:);
tmp=tmp(~ind,:);
groupnumber=groupnumber+1;
end

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