# Find Concave Edges in STL-File

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Timo Schmid on 19 Jan 2021
Commented: Alfredo Bagalà on 14 Sep 2022
Hello all,
I am looking for a method to identify concave edges of an STL model which is not too computationally intensive..
The stlread function provides the vertex-coordinates (mx3), faces (nx3) and normals of the faces (nx3)... Where m is the number of all triangle vertices and n is the number of corresponding face-indices.
The edges I want to identify are the ones I marked red (as an example):
I tried different methods which all took ages too compute.
For example I created an Triangulation object using the triangulate function and compared the angle between normals of adjacent faces to further on store their common edge in case of exceeding a treshold - stopped working on this idea because of the sheer amount of calculations needed.
Pseudo Code:
TR = triangulation(faces,x,y,z);
% Compare all faces if they are neighbors
for queryId=1:(size(TR.ConnectivityList,1))
for compId = 1:(size(TR.ConnectivityList,1))
% ID for comparison is not equal to query ID
if queryId ~= compId
% If faces are connected calculate angle between face
% normals
if (TR.isConnected(queryId,compId))
n1 = TR.faceNormal(queryId);
n2 = TR.faceNormal(compId);
theta = acos(dot(n1,n2) / (norm(n1)*norm(n2)));
% Do further calculations here if theta > treshold
end
end
end
end
I also tried using the Triangulation objects property "featureEdges" to extract these edges, but unfortunately it doesn't yield the result I expected (right picture), although it works quite fine for the example (left picture) provided in the function file.
Hope anybody can help me. Thanks!
Alfredo Bagalà on 14 Sep 2022
Hi, I have the same problem. Did you find any solution?

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