cut an stl file and regenerate new mesh on the cutted face

For my project I'm writing a code so that I can cut stl files in half and determine the volume of the 2 halves to check if the volume is the same and thereby confirm how symetrical the object is.

So far I managed to crop the code but then the object appears with an open side wall as it can be seen in the picture.

Any kind of help would be welcome, thanks in advance.

    %% Crop stl file
            
            F1 = cropPatch(ba, [0 3000], [0 3000], [0 3000])
            
            D = patch(F1,'FaceColor',       [0.8 0.8 1.0], ...
                     'EdgeColor',       'none',        ...
                     'FaceLighting',    'gouraud',     ...
                     'AmbientStrength', 0.15);
                 
            % Add a camera light, and tone down the specular highlighting
            camlight('headlight');
            material('dull');
             
            % Fix the axes scaling, and set a nice view angle
            axis('image');
                view([-135 35]);

[Rik]

How are you closing the surface of your cropped object?

[Dirk Snoeijen]

I don‘t know how to close it, that was also my question.

[Rik]

You are cutting the object along a set of nodes. You need to connect all those vertices with faces in order to close the object. Since you don't show the contents of cropPatch, I can't tell you how easy or hard it will be to implement that.

[Jianxun Xie]

Im also waiting for the answer to this.

[Rik]

@Jianxun Xie You can ask Dirk for the code he's using and make the required edits yourself. Posting your comment as an answer doesn't do anything.

[Dirk Snoeijen]

I've found an other way to do it.

First I'll make a plot with only the mesh with the function below.

%% make a mesh plot

X = ba.Points;
Y = ba.ConnectivityList;
plot_mesh(X,Y) %plot mesh with the "plot_mesh" toolbox

That will make our sphere like in the picture below.

Then I'll us a filter function where all the data points above my given limit will be given the same value as my limit value which can be done using the code below.

% vectorize the matrix into x,y,z axis
Ex =  X(:,1); %vector maken van de x as
Ey =  X(:,2); %vector maken van de y as
Ez =  X(:,3); %vector maken van de z as

first make arrays of the vector

%% filter all points back to the limit value

Ey2 = filtermeetfout_ondergrens(Ey, 10, 0)';

E2 = [Ex, Ey2, Ez];
plot_mesh(E2,Y)

with the function "filtermeetfout_ondergrens" I've filterd all the data points back to y = 10 what results in only a part of the sphere.

[DGM]

This does not work generally. A simple counterexample:

%a file
unzip squarefrust.stl.zip
T = stlread('squarefrust.stl');
F0 = T.ConnectivityList;
V0 = T.Points;

% cut it in "half"
zrange = imrange(V0(:,3)); % the part spans z = [0 1]
cutheight = mean(zrange); % the midpoint
mask = V0(:,3) > cutheight; % make a mask
V1 = V0;
V1(mask,3) = cutheight; % move those points to the cut line

% display it using patch()
patch('faces',F0,'vertices',V0,'facecolor','w', ...
    'edgecolor','k','facealpha',0.4); hold on
patch('faces',F0,'vertices',V1, ...
    'facecolor','r','edgecolor','k');
view(3); camlight;
axis equal; grid on
xlabel('X'); ylabel('Y'); zlabel('Z')

% measure the volumes
[v0,~] = stlVolume(V0',F0');
[v1,~] = stlVolume(V1',F0');
ratio = v1/v0

ratio = 0.5000

% is the part symmetric?
tol = 0.01;
issymmetric = abs(ratio - 0.5) < tol

issymmetric = logical
   1

The part is obviously not symmetric across Z = 0.5, but because of the simplicity of the mesh, our poor attempt at truncation actually becomes a scaling. The naive test gives us the wrong answer

For finer meshes, it may be a fair approximation, but your volume calculation will need to contend with all the degenerate faces you'll likely be creating when you collapse the mesh like that. If I recall, stlVolume() does not return correct results if there are degenerate faces. I'm pretty sure I modified this copy at some point.

In order to actually cut the model, it needs new vertices and faces added. It can be done, but in general it's not something that you'd readily do with base tools.

https://www.mathworks.com/matlabcentral/answers/2169008-are-there-matlab-functions-or-scripts-to-perform-boolean-operations-on-triangulated-solids

https://www.mathworks.com/matlabcentral/answers/462934-make-valid-stl-file-from-overlapping-spheres