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?

[ET]

Me too, any solutions?

Answer 1

[ET]

featureEdges isn't working because the stl files have unique vertices defined for every triangle. This means that every triangle is all alone with no neighbours - so no features to find.

To fix this, you need to remap faces so that neighbouring triangles share vertices.

F = faces;

V = [x,y,z];

% find shared vertices and remap faces

[V2,~,IC] = uniquetol(V,'ByRows',true);

F2 = IC(F);

TR = triangulation(F2,V2);

trisurf(TR,'linestyle','none');

axis equal

grid on

hold all

% Find and plot feature edges

F = featureEdges(TR,pi/4);

x = HR.Points(:,1);

y = HR.Points(:,2);

z = HR.Points(:,3);

plot3(x(F)',y(F)',z(F)','-r','LineWidth',1.5)

[Chris Hooper]

Fantastic thanks!