delete the top and bottom triangular faces of the 3D object

Question

Alberto Acri 2023-10-5

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2029819-delete-the-top-and-bottom-triangular-faces-of-the-3d-object

评论： Alberto Acri 2023-10-9

Hi! Is there a way to delete the top and bottom triangular faces of the tube?

nodes = importdata("NODES.mat");
faces = importdata("FACES.mat");
figure
plot3(nodes(:,1),nodes(:,2),nodes(:,3),'k.','Markersize',2);
hold on
trimesh(faces(:,:),nodes(:,1),nodes(:,2),nodes(:,3),'EdgeColor','k','Linewidth',0.1,'Facecolor','b','FaceAlpha',1)
hold off
axis equal

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Answer 1

Fabio Freschi 2023-10-5

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https://ww2.mathworks.cn/matlabcentral/answers/2029819-delete-the-top-and-bottom-triangular-faces-of-the-3d-object#answer_1326999

在 MATLAB Online 中打开

By looking at your data I see that the top and bottom triangles have a predominant z direction of the face normals. So I play with this

% your code
nodes = importdata("NODES.mat");
faces = importdata("FACES.mat");
figure
plot3(nodes(:,1),nodes(:,2),nodes(:,3),'k.','Markersize',2);
hold on
trimesh(faces(:,:),nodes(:,1),nodes(:,2),nodes(:,3),'EdgeColor','k','Linewidth',0.1,'Facecolor','b','FaceAlpha',1)
hold off
axis equal
% create triangulation object to easily calculate face normals
TR = triangulation(faces,nodes);
P = incenter(TR);
F = faceNormal(TR);
% plot on the previous plot to see what's happening
hold on  
quiver3(P(:,1),P(:,2),P(:,3), ...
     F(:,1),F(:,2),F(:,3),0.5,'color','r');
% dot product with z directed unit vector
proj = F*[0 0 1].';
% threshold (play with this value)
idx = abs(proj) > 0.6;
% remove faces
faces(idx,:) = [];
% new plot without faces
figure
trimesh(faces(:,:),nodes(:,1),nodes(:,2),nodes(:,3),'EdgeColor','k','Linewidth',0.1,'Facecolor','b','FaceAlpha',1)

Hope it helps

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Alberto Acri 2023-10-6

编辑：Alberto Acri 2023-10-6

在 MATLAB Online 中打开

Thank you for your answer @Fabio Freschi. I have several geometries and your code doesn't always work. Is it possible to apply your code in restricted areas?

For example, for this new geometry, I would like to be able to delete only the triangles that lie on the magenta planes (or possibly in the area between the green/red planes).

I thank you if you can help me!

nodes = importdata("nodes_geometry.txt");
faces = importdata("faces_geometry.txt");
P1 = [-24.4053 -9.7273 39.6897];
P1_sup = [-24.24285 -9.75115 38.98785];
P1_inf = [-24.56465 -9.73375 40.36425];
V1 = importdata("V1_geometry.mat");
V1_sup = importdata("V1_geometry_SUP.mat");
V1_inf = importdata("V1_geometry_INF.mat");
P2 = [-25.0631 -13.8062 50.2981];
P2_sup = [-24.8931 -14.06965 50.88635];
P2_inf = [-25.19075 -13.4698 49.7241];
V2 = importdata("V2_geometry.mat");
V2_sup = importdata("V2_geometry_SUP.mat");
V2_inf = importdata("V2_geometry_INF.mat");
figure
plot3(nodes(:,1),nodes(:,2),nodes(:,3),'k.','Markersize',2);
hold on
trimesh(faces(:,:),nodes(:,1),nodes(:,2),nodes(:,3),'EdgeColor','k','Linewidth',0.1,'Facecolor','b','FaceAlpha',0.5)
plot3(P1(:,1),P1(:,2),P1(:,3),'m.','Markersize',20);
patch(V1(:,1), V1(:,2),V1(:,3),'m','FaceAlpha',1);
plot3(P1_sup(:,1),P1_sup(:,2),P1_sup(:,3),'g.','Markersize',10);
patch(V1_sup(:,1), V1_sup(:,2),V1_sup(:,3),'g','FaceAlpha',0.5);
plot3(P1_inf(:,1),P1_inf(:,2),P1_inf(:,3),'r.','Markersize',10);
patch(V1_inf(:,1), V1_inf(:,2),V1_inf(:,3),'r','FaceAlpha',0.5);
plot3(P2(:,1),P2(:,2),P2(:,3),'m.','Markersize',20);
patch(V2(:,1), V2(:,2),V2(:,3),'m','FaceAlpha',1);
plot3(P2_sup(:,1),P2_sup(:,2),P2_sup(:,3),'g.','Markersize',10);
patch(V2_sup(:,1), V2_sup(:,2),V2_sup(:,3),'g','FaceAlpha',0.5);
plot3(P2_inf(:,1),P2_inf(:,2),P2_inf(:,3),'r.','Markersize',10);
patch(V2_inf(:,1), V2_inf(:,2),V2_inf(:,3),'r','FaceAlpha',0.5);
hold off
axis equal

Fabio Freschi 2023-10-8

编辑：Fabio Freschi 2023-10-8

在 MATLAB Online 中打开

You can use the method I previpusly suggested, using the info regarding the unit vector orthogonal to the magenta planes. Maybe you can do this in a smarter way, starting from the knowledge of the uquations of magenta planes.

clear variables, close all

nodes = importdata("nodes_geometry_1.mat");

faces = importdata("faces_geometry_1.mat");

P1 = [-24.4053 -9.7273 39.6897];

V1 = importdata("V1_geometry.mat");

P2 = [-25.0631 -13.8062 50.2981];

V2 = importdata("V2_geometry.mat");

figure, axis equal, hold on

plot3(nodes(:,1),nodes(:,2),nodes(:,3),'k.','Markersize',2);

trimesh(faces(:,:),nodes(:,1),nodes(:,2),nodes(:,3),'EdgeColor','k','Linewidth',0.1,'Facecolor','b','FaceAlpha',0.5)

plot3(P1(:,1),P1(:,2),P1(:,3),'m.','Markersize',20);

patch(V1(:,1), V1(:,2),V1(:,3),'m','FaceAlpha',1);

plot3(P2(:,1),P2(:,2),P2(:,3),'m.','Markersize',20);

patch(V2(:,1), V2(:,2),V2(:,3),'m','FaceAlpha',1);

% create triangulation object to easily calculate face normals

TR = triangulation(faces,nodes);

P = incenter(TR);

F = faceNormal(TR);

% get unit vector from V1

u1 = cross(V1(1,:)-P1,V1(2,:)-P1);

u1 = u1/sqrt(sum(u1.^2,2));

quiver3(P1(1),P1(2),P1(3),u1(1),u1(2),u1(3));

% get unit vector from V2

u2 = cross(V2(1,:)-P2,V2(2,:)-P2);

u2 = u2/sqrt(sum(u2.^2,2));

quiver3(P2(1),P2(2),P2(3),u2(1),u2(2),u2(3));

% find normals oriented like u1 (dot product)

idx1 = abs(F*u1.') > 0.9999;

% find normals oriented like u2 (dot product)

idx2 = abs(F*u2.') > 0.9999;

% remove bottom lid

faces(idx1 | idx2,:) = [];

% new plot without faces

figure, axis equal

trimesh(faces(:,:),nodes(:,1),nodes(:,2),nodes(:,3),'EdgeColor','k','Linewidth',0.1,'Facecolor','b','FaceAlpha',1)

Fabio Freschi 2023-10-9

在 MATLAB Online 中打开

% find normals oriented like u1 (dot product)
idx1 = abs(F*u1.') > 0.9999;
% find normals oriented like u2 (dot product)
idx2 = abs(F*u2.') > 0.9999;
% find barycenters laying on the magenta plane
ipln1 = abs(sum(P.*u1,2)-sum(u1.*P1,2)) < 0.001;
% find barycenters laying on the magenta plane
ipln2 = abs(sum(P.*u2,2)-sum(u2.*P2,2)) < 0.001;
% remove bottom lid
faces((idx1 & ipln1) | (idx2 & ipln2),:) = [];

Alberto Acri 2023-10-9

Thank you!

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delete the top and bottom triangular faces of the 3D object

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