Find Concave Edges in STL-File

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]

Hi, I have the same problem. Did you find any solution?