How to export 3D sphere at stl format

Hi all!

I created an ellipsoid from 3 matrixes of x,y, and z. The ellipsoid is tilted upward and eastward if it changes something. I'm trying to export the ellipsoid in STL format. I've tried using the write stl function but without success.

Any help in this problem would be amazing, attached is my code.

Thanks,

Yaad

% Clear the workspace

clear

clc

% Define the dimensions of the sphere

semiMajorAxis = 1; % Semi-major axis

semiMinorAxis = 0.5; % Semi-minor axis (ratio of 0.9)

% Define the number of points for sphere meshing

numPoints = 100;

% Create a meshgrid for the sphere

phi = linspace(0, pi, numPoints);

theta = linspace(0, 2*pi, numPoints);

[phi, theta] = meshgrid(phi, theta);

% Calculate the coordinates of the points on the ellipsoid

x = semiMajorAxis * sin(phi) .* cos(theta);

y = semiMajorAxis * sin(phi) .* sin(theta);

z = semiMinorAxis * cos(phi);

%%

% Define the tilt angle in radians (45 degrees upward)

tiltAngle = deg2rad(90);

% Define the rotation matrix for the tilt

R = [1, 0, 0; 0, cos(tiltAngle), -sin(tiltAngle); 0, sin(tiltAngle), cos(tiltAngle)];

% Apply the rotation matrix to the coordinates

rotatedCoords = R * [x(:)'; y(:)'; z(:)'];

% Reshape the rotated coordinates back to a grid

xRotated = reshape(rotatedCoords(1, :), size(x));

yRotated = reshape(rotatedCoords(2, :), size(y));

zRotated = reshape(rotatedCoords(3, :), size(z));

%%

% Create a 3D plot of the tilted ellipsoid

subplot(1,2,1)

surf(xRotated, yRotated, zRotated);

axis equal; % Equal aspect ratio

xlabel('X');

ylabel('Y');

zlabel('Z');

title('3D Sphere with Ratio 1:0.9 (45-Degree Upward Tilt)');

grid on;

%%

% Clear the workspace

% Define the dimensions of the sphere

semiMajorAxis = 1; % Semi-major axis

semiMinorAxis = 0.5; % Semi-minor axis (ratio of 0.9)

% Define the number of points for sphere meshing

numPoints = 100;

% Create a meshgrid for the sphere

phi = linspace(0, pi, numPoints);

theta = linspace(0, 2*pi, numPoints);

[phi, theta] = meshgrid(phi, theta);

% Calculate the coordinates of the points on the ellipsoid

x = semiMajorAxis * sin(phi) .* cos(theta);

y = semiMajorAxis * sin(phi) .* sin(theta);

z = semiMinorAxis * cos(phi);

% Just one sample ;

% Define the tilt angles in radians (45 degrees upward and 30 degrees east)

upwardTiltAngle = deg2rad(45);

eastTiltAngle = deg2rad(90);

% Define the rotation matrices for the tilts

R_upward = [1, 0, 0; 0, cos(upwardTiltAngle), -sin(upwardTiltAngle); 0, sin(upwardTiltAngle), cos(upwardTiltAngle)];

R_east = [cos(eastTiltAngle), 0, sin(eastTiltAngle); 0, 1, 0; -sin(eastTiltAngle), 0, cos(eastTiltAngle)];

% Combine the rotation matrices to apply both tilts

R_combined = R_upward * R_east;

% Apply the combined rotation matrix to the coordinates

rotatedCoords = R_combined * [x(:)'; y(:)'; z(:)'];

% Reshape the rotated coordinates back to a grid

xRotated = reshape(rotatedCoords(1, :), size(x));

yRotated = reshape(rotatedCoords(2, :), size(y));

zRotated = reshape(rotatedCoords(3, :), size(z));

% Create a 3D plot of the tilted ellipsoid

subplot(1,2,2)

surf(xRotated, yRotated, zRotated);

axis equal; % Equal aspect ratio

xlabel('X');

ylabel('Y');

zlabel('Z');

title('3D Sphere with Ratio 1:0.9 (45-Degree Upward Tilt and 30-Degree East Tilt)');

filename= ['Sphere_dxf/' 'UPaE']

writedxf(filename, xRotated, yRotated, zRotated);

grid on;

Answer 1

[Chris]

You need a triangulation.

stlwrite only works with triangles (vs tetrahedrons), which maybe has to do with an stl only defining the surface of an object.

To get triangles defining the ellipsoid, I used convhull() .

points = rotatedCoords.';
ConnectivityList = convhull(points);
TR = triangulation(ConnectivityList,points);
filename = 'Sphere_stl/UPaE.stl';
stlwrite(TR,filename)

% To view the triangulation:
figure
trisurf(TR)
axis equal

[Yaad Ben Lulu]

First of all, Thanks for your answer. But actually, I didn't understand the meaning of "points = rotatedCoords.';" What should it do?

[Chris]

@Yaad Ben Lulu that's simply transposing the rotatedcoords matrix to be compatible with the convhull and triangulation functions (the functions expect an n x 3 matrix)

I use .' out of habit, since the transpose for a complex matrix is different than simply moving numbers around (it gives the complex conjugate). The dot tells Matlab to leave the values alone. In your case, I don't think it will make a difference.

points = rotatedCoords';

should work just as well.