How to prevent the far away points from connecting with "boundary" command?

Hi Community, i am using this command :

[k,vol_results] = boundary(results, 1); %with shrink factor = 1 (maximum shrink) 
trisurf(k,results(:,1),results(:,2),results(:,3),'FaceColor','yellow','FaceAlpha',0.1)

However, the boundary plot still connect the two point which are far away, and give me the wrong volume. How could i use the boundary function to get a curve plot?

Thank you very much.

Answer 1

[Moksh]

Hi Windel,

I understand that you are generating a boundary from a set of 3D coordinates using the “boundary” function in MATLAB, and you are trying to omit the far-off points to get a better-fitting boundary.

To achieve this, you can follow the below steps:

• Calculate the centroid of the given set of points and determine the distance of each coordinate from this centroid.
• Set a threshold distance and create a set of valid coordinates that will be used in the boundary generation.
• Generate the boundary from this set of valid coordinates and keep varying the threshold distance until the desired results.

Here is an example code for the above logic:

%% Example x, y, z coordinates
x = round(rand(20, 1, "double") * 10);
y = round(rand(20, 1, "double") * 10);
z = round(rand(20, 1, "double") * 10);
points = [x y z];

plot3(x, y, z, '.', "MarkerSize", 10);
grid on
hold on

%% Thresholding logic
% Centroid Coordinates and plotting it
centroid = mean(points);
plot3(centroid(1), centroid(2), centroid(3), 'o', "MarkerFaceColor","green")

% Computing valid coordinates
cent_distance = vecnorm(points - centroid, 2, 2);     % Distances from centroid
threshold_distance = 5;                               % Distance Threshold

val_ind = find(cent_distance <= threshold_distance);  % Applying threshold 
valid_coord = points(val_ind, :);                     % Valid coordinates


%% Computing boundary from these valid coordinates
k = boundary(valid_coord);
trisurf(k, x, y, z, 'Facecolor', 'red', 'FaceAlpha', 0.1)

Please refer to the below documentations for more information regarding the “vecnorm”, “find”, “boundary” and “trisurf” functions respectively:

• https://in.mathworks.com/help/matlab/ref/vecnorm.html
• https://in.mathworks.com/help/matlab/ref/find.html
• https://in.mathworks.com/help/matlab/ref/boundary.html
• https://in.mathworks.com/help/matlab/ref/trisurf.html

Hope this helps!

Best Regards,

Moksh Aggarwal