Identify triangular faces of a mesh

Hi! How can I identify the faces highlighted in red within the "faces_part" matrix?

nodes_part = importdata("nodes_part.mat");
faces_part = importdata("faces_part.mat");
figure
plot3(nodes_part(:,1),nodes_part(:,2),nodes_part(:,3),'b.','Markersize',5)
trimesh(faces_part(:,:),nodes_part(:,1),nodes_part(:,2),nodes_part(:,3),'EdgeColor','b','Linewidth',1,'Facecolor','w')
grid on
xlabel('x')
ylabel('y')
zlabel('z')
view([15,50,30])
axis equal

[Rajeev]

For highlighting the mentioned faces, their indices will be needed. Are you trying to highlight top n faces with the largest areas here?

[Alberto Acri]

In what sense their indices? Possibly one could identify red areas based on the area of the individual triangle (e.g. area_T>3)

Answer 1

[Rajeev]

To highlight the triangle with areas greater than, let's say 'k', we need to first compute all the areas of the triangles given by the 'faces_part' array using the cross product formula.

Logical indexing can be used to find the indices of all the triangles having area greater than 'k'.

These returned indices 'idx' can then be passed to the 'trimesh' function to plot the filtered triangles from the 'nodes_part' coordinates.

I have modified the code that was provided by you to include the required changes. Note that the value of 'k' in the code is 1.

k = 1;

nodes_part = importdata("nodes_part.mat");

faces_part = importdata("faces_part.mat");

figure

% plotting all the data points

plot3(nodes_part(:,1),nodes_part(:,2),nodes_part(:,3),'b.','Markersize',5)

% forming the triangles using faces_part

trimesh(faces_part(:,:),nodes_part(:,1),nodes_part(:,2),nodes_part(:,3),'EdgeColor','b','Linewidth',1,'Facecolor','w')

hold on

% Initialize a vector to store the areas of each triangle

areas = zeros(size(faces_part, 1), 1);

% Compute the area of each triangle

for i = 1:size(faces_part, 1)

% Extract the three vertices of the current triangle

v1 = nodes_part(faces_part(i, 1), :);

v2 = nodes_part(faces_part(i, 2), :);

v3 = nodes_part(faces_part(i, 3), :);

% Compute the area of the triangle using the cross product

areas(i) = 0.5 * norm(cross(v2-v1, v3-v1));

end

% Find the indices of triangles with area greater than 3

idx = find(areas > k);

% Colouring all the trianlges with area greater than 1 (k) to red

trimesh(faces_part(idx,:),nodes_part(:,1),nodes_part(:,2),nodes_part(:,3),'EdgeColor','b','Linewidth',1,'Facecolor','r')

% formatting the plot

grid on

xlabel('x')

ylabel('y')

zlabel('z')

view([15,50,30])

axis equal

[Alberto Acri]

The code you provided is perfectly fine.

May I also ask if you know how I can remove the red triangular faces from the mesh ?