Extract contour from obj 3d object in Matlab

Question

I have an .obj file representing a 3D object.

I need to extract from this 3D object the contour that is obtained by intersection with a plane. So for example, I have an object representing a cylinder oriented with vertical axis, then I want to extract a circle contour when the intersecting plane is horizontal or a rectangular contour when the intersection plane is vertical. Any suggestion about how to do it?

Since I didn't know how to visualise this obj file, I have converted to a patch with the following code (some function taken from loadawobj from Matlab file exchange).

modelname='file.obj';
S=loadawobj(modelname);
mtl=loadawmtl(['obj/' S.mtllib]);
p3=patch('Vertices',S.v','Faces',S.f3');

for ii=1:length(S.umat3)
    mtlnum=S.umat3(ii);
    fvcd3(ii,:)=mtl(1).Kd';
end

p3.FaceVertexCData=fvcd3;
p3.FaceColor='flat';

But I don't necessarily need to extract the contour from the resulting patch if this is too complex to accomplish. If there is an easier procedure starting from the obj file, it's also fine and acceptable. Thank you!

Answer 1

That's the way I solved the problem, after collecting information all around the web. I couldn't find anything ready on line so I had to implement an algorithm on my own. The basic idea is very simple but there are many steps required. I start from two info: one array containing the coordinates of the cloud point and another array containing a bunch of tuples about how the 3 vertex are connected to form a triangle.

• First of all you need to find a representation of the plane you want to use for your cutting. That means you just use one point and the normal to the plane to represent it. That plane is required in order to identify the cutting point on the structure.
• Second step is to identify the triangles on the plane. In few words you just need to scroll over all the triangles of the structure and find those having one corner above the cutting plane and another corner below the cutting plane. Also don't forget to account for the condition where one corner is on the plane or two are on the plane. All the other triangles are not needed, since they are totally above or below the cutting plane.
• Now you have a subset of all your triangles. You need to extract points of the contour. So for each triangle you have 3 vertex: in general case you can imagine that one vertex is above the plane and the other two are below. Then you have two lines cutting the plane. You can extract two point by simply intersecting these lines with the cutting plane.
• By repeating this operation you get a series of points on 2D space. But they have no order and if you plot them as a continuous plot, you get lines jumping up and down since the points you have extracted are randomly located in the array. So, it's required to order them in a proper way. The method I used is the very simple: start from one point and connect to the closest one. There are some bad situations where that doesn't work but you can probably avoid it by adding some more rules on the algorithm.