How to select, or filter, the external border/boundary in a set of (x,y)-points?

Question

Sim 2023-7-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1994013-how-to-select-or-filter-the-external-border-boundary-in-a-set-of-x-y-points

评论： MarKf 2023-8-7

borders.mat

Given two or more set of points (please see attached, as example, two sets of points called "b1" and "b2", that are stored in "borders.mat"), how can I select or filter the common external border/boundary from the union of those point datasets?

After joining the two datasets, I would like to keep only the points that represent the total common external border (i.e. removing any common set of points or any "internal" point). Here, in this example, I show only two set of points, but I have many of them.

load("borders.mat")
hold on
plot(b1(:,1),b1(:,2))
plot(b2(:,1),b2(:,2))

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Answer 1

Dyuman Joshi 2023-7-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1994013-how-to-select-or-filter-the-external-border-boundary-in-a-set-of-x-y-points#answer_1270433

在 MATLAB Online 中打开

borders.mat

load("borders.mat");

%Original figure

plot(b1(:,1),b1(:,2))

hold on

plot(b2(:,1),b2(:,2))

%Obtain coordinates corresponding to outside border by performing

%exclusive OR on the data

z=setxor(b1,b2,'rows','stable');

%'stable' option is to maintain the order of the coordinates

%plot the boundary

figure

plot(z(:,1),z(:,2),'k')

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Answer 2

MarKf 2023-7-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1994013-how-to-select-or-filter-the-external-border-boundary-in-a-set-of-x-y-points#answer_1270398

在 MATLAB Online 中打开

ismember

As in

borders = load(websave('rd', "https://nl.mathworks.com/matlabcentral/answers/uploaded_files/1430803/borders.mat"));
b1 = borders.b1; b2 = borders.b2; 
idx_common = ismember(b1,b2,'rows');
coordsincommon = b1(idx_common,:); %169

It seems that they are the exact same coordinates, so this works, tho I haven't checked if it captures them all, otherwise there is also ismembertol

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Sim 2023-7-11

在 MATLAB Online 中打开

Hi @MarKf! Thanks for your reply! I tried to compare both methods here proposed, but, unless I misinterpreted your method or I did something wrong, I do not get the desired result.... :-)

% Load data

borders = load(websave('rd', "https://nl.mathworks.com/matlabcentral/answers/uploaded_files/1430803/borders.mat"));

b1 = borders.b1; b2 = borders.b2;

% Dyuman Joshi's Method

c=setxor(b1,b2,'rows','stable');

plot(c(:,1),c(:,2),'r')

% MarKf's Method

d=b1(ismember(b1,b2,'rows'),:);

plot(d(:,1),d(:,2),'b','linewidth',2)

MarKf 2023-8-7

Yeah the accepted answer is what you want more straightfowardly, setxor being the opposite of ismember (very useful but there is no tolerance version).

I showed how you can get those common points, answering your specific "how can I select [or filter] the common external border/boundary from the union of those point datasets?" query, you could've then used the index provided (idx_common) to filter them out. You plotted only the common boundary points directly (last point seem to get captured first or viceversa).

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