Remove centroid locations of objects that are too close

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Jason on 8 Aug 2022
Commented: Jason on 9 Aug 2022
Hi, I have a list of centroid locations of spots in an image (xf, yf). The spots are randomly distributed. I want to remove the centroid locations that are too close to each other, or say within a distance of d. I've got to the following but not sure what to do next. Also is there a better way to do this ratrher than use loops?
%using a loop
M=[];
for j=1:length(xf)
for i=1:length(xf)
dx=xf(j)-xf(i);
dy=yf(j)-yf(i);
dr=sqrt(power(dx,2)+power(dy,2));
M(j,1)=xf(j);
M(j,2)=yf(j);
M(j,3)=xf(i);
M(j,4)=yf(i);
M(j,5)=dr;
end
end
Matt J on 9 Aug 2022
If two spots are less than d apart should both be removed? If not, by what criterion do you decide which spot remains?
Image Analyst on 9 Aug 2022
To me it sounds like both are to be removed. This is easily done with pdist2.

Matt J on 9 Aug 2022
Edited: Matt J on 9 Aug 2022
A=[xf(:) yf(:)];
D=pdist2(A,A);
D(D==0)=inf; %retroactive EDIT
keep = all(D>d,1);
xf=xf(keep);
yf=yf(keep);
Jason on 9 Aug 2022
Of course, its clear now, thankyou
Jason on 9 Aug 2022
Just to add, when I had over 400 objects, akthough i do other operations, using the for loop took 110s.using the pdist2 approach took just under 2s. Remarkable!

yanqi liu on 9 Aug 2022
yes，sir，may be use pdist2， such as
xf = rand(1, 10);
yf = rand(1, 10);
%using a loop
M=[];
for j=1:length(xf)
for i=1:length(xf)
dx=xf(j)-xf(i);
dy=yf(j)-yf(i);
dr=sqrt(power(dx,2)+power(dy,2));
M(end+1,1)=xf(j);
M(end,2)=yf(j);
M(end,3)=xf(i);
M(end,4)=yf(i);
M(end,5)=dr;
end
end
% second method
A=[xf(:) yf(:)];
B=pdist2(A,A);
C=B';
C=C(:);
isequal(M(:,end),C)
ans = logical
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