[edit: I simplified the calculation of the normalized normal vectors]
I ran your code. It runs without error. It makes an STL file (attached). I can render the STL file at https://www.viewstl.com without getting an error. What is the problem?
The rendered object looks like two 2-D sheets, rather than a 3-D object. I don;t know what you expect your object to look like. I recommend that you start with a simple set of points. I re-ran your code with 4 points and 4 triangles, to make a tetrahedron. It works. The STL file produces a tetrahedron when viewed at https://www.viewstl.com.
Nodes = [0 0 0;
1 0 0;
0 1 0;
0 0 1];
singletriangle = [1 3 2;
1 2 4;
1 4 3;
2 3 4];
% Find the point coordinates according to the triangle index
points_triangles = [Nodes(singletriangle,1),Nodes(singletriangle,2),Nodes(singletriangle,3)];
% Find the point coordinates of each column (three-dimensional)
points_one=points_triangles(1:length(points_triangles)/3,:);
points_two=points_triangles(length(points_triangles)/3+1:length(points_triangles)/3*2,:);
points_three=points_triangles(length(points_triangles)/3*2+1:length(points_triangles),:);
% Subtract the matrix and compute the vector
vectors_one=points_two-points_one;
vectors_two=points_three-points_one;
% Take the cross product of two vectors
normal_vectors=cross(vectors_one,vectors_two);
% Normalizes the 3D vector so that its modulus is equal to 1
normalized_normal_vectors=normal_vectors./norm(normal_vectors);
% Initializes the empty array
output=zeros(length(points_one)*4,3);
% Write output data
for i=1:length(points_one)
output(i*4-3,:)=normalized_normal_vectors(i,:);
output(i*4-2,:)=points_one(i,:);
output(i*4-1,:)=points_two(i,:);
output(i*4,:)=points_three(i,:);
end
% Output content
output=output';
STL_file_name=['tetra.stl'] ;
STL_file = fopen (STL_file_name,'wt');
fprintf (STL_file,'solid %s\n','solid');
fprintf (STL_file, ' facet normal %14e %14e %14e\n outer loop\n vertex %14e %14e %14e\n vertex %14e %14e %14e\n vertex %14e %14e %14e\n endloop\n endfacet\n',output);
fprintf (STL_file,'endsolid %s\n','solid' );
fclose (STL_file);
The code above makes a tetrahedron. Maybe you are not happy with the normal vectors. Let us inspect them:
disp(normal_vectors)
These normal vectors point in the expected directions: outward from each face.
However, the normalized normal vectors do not have unit length:
disp(normalized_normal_vectors)
That is a problem. Modify the normalization command. You did
normalized_normal_vectors=normal_vectors./norm(normal_vectors);
That does not give the desired result, because norm(X) returns a scalar when X is a matrix. Use vecnorm() instead:
nnv=normal_vectors./(vecnorm(normal_vectors'))';
disp(nnv)
nnv looks like the correct set of normal vectors with unit length.
Good luck.
