Centroid of two arrays

Hi,

I have a code that computes the centroids of two arrays as:

for ij=1:size(v1,1)
    cent(ij)=centroid(v2,v1(ij,:));
end 

What is the physical meaning of doing this operation? The help of 'centroid' explains the calculation of centroids for polygon shapes and I can not understand how the data in the two arrays is affected by centroid through that. The attached .mat file contains the two variables v1 and v2

Answer 1

[Adam Danz]

I agree that the documentation on describing the inputs could be more specific. Look at the examples in the documentation for centroid and it may fill in the gaps. Plotting the results may help to interpret the results, too.

ij = 1; % sample the first iteration
plot(v2, v1(ij,:), 'ko')

ps = polyshape(v2,v1(ij,:));
[xCnt, yCnt] = centroid(ps);
hold on
plot(xCnt, yCnt, 'rx', 'MarkerSize', 15)

% Warning: Polyshape has duplicate vertices, intersections, or other inconsistencies that may produce inaccurate or unexpected results. Input data has been
% modified to create a well-defined polyshape. 
% > In polyshape/checkAndSimplify (line 517)
% In polyshape (line 175)

[Ayesha Zaka]

Thank you for your answer. I was able to understand things from this. However, I have one question related to resulting xCnt value using

ij = 1;
cent(ij)=centroid(v2,v1(ij,:));

and

ps = polyshape(v2,v1(ij,:));
[xCnt, yCnt] = centroid(ps);

The resulting value is not the same from both of these. Is it because of the warning that we get from the method that you mentioned in your answer?

[Adam Danz]

What centroid function are you using?

which centroid

should return

...toolbox\matlab\polyfun\@polyshape\centroid.m  % polyshape method

When I run your first example I get an error:

ij = 1;
cent(ij)=centroid(v2,v1(ij,:));

% ERROR:
% Check for missing argument or incorrect argument data type in call to function
% 'centroid'. 

The documentation needs significant improvement. The initial description in correct,

centroid(polyin) returns the x-coordinates and the y-coordinates of the centroid of a polyshape.

But the desciption of input 1 lists options: scalar | vector | matrix | multidimensional array which does not include the polyshape class as is shown in their examples.

You can provide a rating and feedback for that page at the bottom of the page:

https://www.mathworks.com/help/matlab/ref/polyshape.centroid.html

[Ayesha Zaka]

I am using the same function as I mentioned in the code and it is working for me. But the results are a bit offset from the ones that I am getting from your implementation.

At least following code is working for me:

xcent = centroid(v2,v1(1,:))

[Adam Danz]

I realize it's working for you which is why I'm suspicious becuase it's not working for me using the same data. It generates and error. This is why I suggested using the which function to verify that you are not working with another function with the same name. Also, knowing your Matlab release would be helpful to elimit the possiblity of version differences from r2020b.

[Ayesha Zaka]

You were right! The user had written another function for centroid that was being used instead of the matlab's built-in function.

Thanks for your help.

[Adam Danz]

Ahhhh... that's what I thought. If the differences between the built-in function and the 3rd party function are very small, it might just be roundoff error but if you're unsure which to use and you're sure that the built-in version does what you want, use that one.

[Image Analyst]

So which centroid function do you want to use?

Sounds like you don't want the built-in one that works on polygons and that you're wondering how/why your colleague's code works.

But what we'd really like to know is, given the x and y vectors you attached (I'm assuming that's what Adam plotted), what do you expect to get from it and why? The "centroid" of a Gaussian, like the red cross Adam plotted, doesn't seem useful but maybe I'm wrong. What are you going to do next, assuming you can get whatever it is you need?