How to trim the non-manifold triangle off the stl file?

Hi everyone

I have a question about the 3D stl file processing.

Suppose that I have the 3D geometry (stl file).

I use stlread and trimesh command to plot it, then I found that some of elements are non-manifold.

I try to directly remove these two triangule faces for about entire afternoon, but I haven't done it succefully yet.

Could any one give me some suggestions?

Attached is the stl file, to view the stl file, with the codes as below:

fv=stlread('test.stl')
figure();
trimesh(fv);
axis equal

Answer 1

[Bruno Luong]

There is SW that can detect topology issue and attempt to correct it, such as

https://www.meshlab.net/

Answer 2

[DGM]

I'd agree with Bruno on this. Especially if it's just a small fix on a single file, it's easier and quicker to just throw it in Meshlab.

That said, I figured I'd try to do it. Those two triangles are actually four. They're coincident faces, so if you just go looking for a 3-connected edge, you won't find one. You'll find two 4-connected edges. I decided that it was least ambiguous to start by removing coincident faces first. Assuming the model is a closed surface, any remaining open edges belong to flags. Just erode them away until you're left with a closed volume.

unzip test.zip

% an object we assume is a closed surface
T = stlread('test.stl');

% get rid of coincident faces
[~,ia] = unique(sort(T.ConnectivityList,2),'rows','stable');
T = triangulation(T.ConnectivityList(ia,:),T.Points);

% get rid of open edges
% this will get rid of t-connected faces, so long as they're not coincident.
% it will also destroy a surface with a hole or open edges
E = freeBoundary(T);
while ~isempty(E) % this could probably be simplified
    EA = edgeAttachments(T,E(:,1),E(:,2));
    F = T.ConnectivityList;
    F(unique(cell2mat(EA)),:) = [];
    T = triangulation(F,T.Points);
    E = freeBoundary(T);
end

Warning: Some input points are not referenced by the triangulation.

% get rid of those unused vertices and remap the face list
[F,V] = pruneunusedverts(T.ConnectivityList,T.Points);
T = triangulation(F,V);

% write it
stlwrite(T,'test2.stl')

% display it using patch()
patch('faces',T.ConnectivityList,'vertices',T.Points, ...
    'facecolor','w','edgecolor','k');
view(3); camlight;
axis equal; grid on
xlabel('X'); ylabel('Y'); zlabel('Z')

●

This whole approach relies on some assumptions based on observing the model. There are plenty of other things that aren't being checked.