Determine the coordinates of the nodes forming the outermost circle

Hi! How can I extract only the coordinates of the nodes forming the outermost circle? The nodes are located on radius R = 4!

nodes = importdata("nodes.txt");
figure
plot3(nodes(:,1),nodes(:,2),nodes(:,3),'b.','Markersize',15);
axis equal
xlabel('x')
ylabel('y')
zlabel('z')

[Dyuman Joshi]

If you need a general answer (considering the cases where the data points are not uniformaly spaced), using convhull would be the most fitting.

[Alberto Acri]

I will consider it!

Answer 1

[Hassaan]

% Load the nodes data
nodes = importdata('nodes.mat');

% Define the desired radius
R = 4;

% Calculate the distance of each node from the origin in the x-y plane
distances = sqrt(nodes(:,1).^2 + nodes(:,2).^2);

% Find the indices of nodes that are close to the radius R
% You may need to adjust the tolerance depending on the precision of your data
tolerance = 0.1; % Tolerance for the radius match
outermost_indices = find(abs(distances - R) < tolerance);

% Extract the coordinates of the outermost nodes
outermost_nodes = nodes(outermost_indices, :);

% Plot all nodes
figure;
plot3(nodes(:,1), nodes(:,2), nodes(:,3), 'b.', 'Markersize', 15);
hold on;

% Highlight the outermost nodes in red
plot3(outermost_nodes(:,1), outermost_nodes(:,2), outermost_nodes(:,3), 'r.', 'Markersize', 15);

% Set axis properties
axis equal;
xlabel('x');
ylabel('y');
zlabel('z');

% Print the coordinates of the outermost nodes
disp('Coordinates of the nodes forming the outermost circle:');
disp(outermost_nodes);

% If you want to print coordinates with a label:
for i = 1:size(outermost_nodes, 1)
    fprintf('Node %d: (x, y, z) = (%.4f, %.4f, %.4f)\n', i, outermost_nodes(i, 1), outermost_nodes(i, 2), outermost_nodes(i, 3));
end

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Answer 2

[Image Analyst]

Can you get the coordinates as a list of (x,y) locations? If so, get the convex hull with convexHull or convhull

[Dyuman Joshi]

+1 for convhull as I mentioned in my comment.

Answer 3

[Alan Stevens]

Here's an alternative way:

nodes = importdata("nodes.mat");
figure
plot3(nodes(:,1),nodes(:,2),nodes(:,3),'b.','Markersize',15);
axis equal
xlabel('x')
ylabel('y')
zlabel('z')
r = 4;
theta = 0:2*pi/20:2*pi;
for i = 1:numel(theta)
    x(i) = r*cos(theta(i));
    y(i) = r*sin(theta(i));
    z(i) = 3; 
end
hold on
plot3(x,y,z,'ro','markersize',15)
view(0,90)