How to create a boundary along the edges for 3d plot?

Hello MATLAB Community,

I am having a problem creating boundary line for the point cloud as shown in the below image in RED colour.

(Note: the below image is taken from MATLAB blogs)

here is my code and excel document consisting of around 14000 points:

A = readmatrix('points435');

plot3(A(:,1),A(:,2),A(:,3),'.')

My code runs a part of the problem I am working on.

The image shown above is different from the results of the code attached.

I only need to trace a line along the boundary of the point cloud which is 3d.

Can anyone please help me with any suggestions.

Thank you in advance!!

I really appreciate your help.

Kind regards,

Shiv

Answer 1

[Benjamin Thompson]

There is a function called "boundary" that should work for you. Type "help boundary" or "doc boundary" for more information.

[Shiv Karpoor]

Hi Benjamin, it worked for 2d plot but its not working for 3d plots.

I checked the details on boundary function, they say for 3d we have to use trisurf but it is not what I want.

I require the boundary for a 3d plot.

[Benjamin Thompson]

Boundary supports 3D problems also. In that case the return value specifies the set of triangles enclosing your shape. But you problem is really 2D since the third dimension is constrained to 435. Here is how to construct the boundary of your 2D data set in 3D space:

>> data = xlsread('points435.xlsx');

>> figure, plot3(data(:,1), data(:,2), data(:,3));

>> min(data(:,1))

ans =

-7.926606985093684

>> min(data(:,2))

ans =

-6.864643014906309

>> min(data(:,3))

ans =

435

>> max(data(:,3))

ans =

435

>> k = boundary(data(:,1), data(:,2));

>> hold on, plot3(data(k,1), data(k,2), 435*ones(size(k,1),1), 'r-')

>> grid on;

>>

If you are looking for something different look over all the options that boundary offers. Adjusting the shrink factor parameter "s" may help you tweak the output some.

[Shiv Karpoor]

Hi Benjamin,

I tried your method on the actual problem I am working on and I am glad to inform you that it worked.

Thanks for your help! I really appreciate it.

Answer 2

[Max Alger-Meyer]

So this is actually a pretty complicated problem. Luckily for you, someone already wrote and posted the code for this exact situation: https://www.mathworks.com/matlabcentral/fileexchange/60690-boundary-extraction-identification-and-tracing-from-point-cloud-data

[Shiv Karpoor]

Hi Max,

I had a look at the link, its quite complicated.

But since my Z values are constant all the time, Benjamin provided a solution to trace the boundary.

Anyways, thanks for the suggestion.