Create triangular mesh

Hello, I have a matrice with some edges that i need to conect. This results from a .stl file that has a hole and i need to fix...

 badEdges =

     1     2
     1     4
     2     4

What i need to do is to look for comum vertices in the rows and if there are 2, create a new edge conecting them. For example as the first and the second rows have the vertice 1 in comum, we should creat a new edge [2 4]. The same happens with the second and the third vertices with the vertice 4. Can someone help me with this? Thank´s.

Answer 1

[Eduardo]

Hi,

Can someone give me a hand with this?

Answer 2

[DGM]

In order to fill the hole, you need three vertices and you need them in the correct direction. Your edge segments aren't in a consistent direction, so it's a 50/50 guess what the correct direction is. It's either [1 2 4] or [1 4 2]. Get it backwards, and the face will be inside-out. I suppose we could test the normal vector for intersection with the rest of the mesh, but it seems like an avoidable problem.

I think it would have been better to get the open edge segments as they are ordered in their parent triangles. The easy way would be to use triangulation() -- or since we're in 2012, TriRep(). Likewise, we're assuming that stlread() and stlwrite() don't exist yet.

unzip sphere_20.stl.zip

% a model with some isolated missing triangles
[V F] = stlRead('sphere_20.stl'); % FEX #51200
F([110 120],:) = []; % remove some faces

patch('faces',F,'vertices',V,'facecolor','w','edgecolor','k');
view(3); camlight; view(10,33)
axis equal; grid on
xlabel('X'); ylabel('Y'); zlabel('Z')

% get the open edges of the surface
% between R2009a - R2012b, TriRep() can be used;
% R2013a+, triangulation() can be used.
%T = triangulation(F,V);
T = TriRep(F,V);
BV = freeBoundary(T);

% prior to R2018b, segments of BV will not be sequentially ordered,
% but they will still have consistent winding direction.
% reorder the list if necessary; if there are multiple holes, split them
boundaries = fixboundaries(BV);

% each 3-segment boundary is easy to fill with a single triangle.
% filling larger holes is a further complication.
% neighboring triangles traverse their shared edge in 
% opposite direction, so flip the boundary to get the triangle.
for k = 1:numel(boundaries)
    thisb = boundaries{k}(:,1);
    if numel(thisb) == 3
        F = [F; fliplr(thisb.')]; %#ok<AGROW>
    end
end

% write the new STL
stlWrite('test.stl',F,V) % FEX #20922, #51200

% show the repaired model
figure
patch('faces',F,'vertices',V,'facecolor','w','edgecolor','k');
view(3); camlight; view(10,33)
axis equal; grid on
xlabel('X'); ylabel('Y'); zlabel('Z')

See also:

https://www.mathworks.com/matlabcentral/answers/27886-find-holes-and-gaps-in-stl-files