Calculate radius from scatter plot

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yonatan s
yonatan s on 22 Jun 2020
Edited: yonatan s on 9 Jul 2020
Hi,
I need to calculate the radius of a circle, ignoring all surrounding particles (image attached). The circle itself consists of many particles (over 100,000). Note that the center of the circle is not at the origin.
Thanks.
EDIT: here is how I solved it:
posn=normal(poscm); %poscm is the coordinates matrix
[k,edges]=histcounts(posn,1e5);
[~,imaxk]=max(k);
R=edges(imaxk+find(k(imaxk:end)==0,1)); %find the first zero value after the maximum
  5 Comments
yonatan s
yonatan s on 23 Jun 2020
It's not perfect, but it seems isoutlier is the right approach.
pdist consumes too much memory.
I tried using some image processing tools, the problem is that axes change when transforming the plot to an image.
Thanks.
Adam Danz
Adam Danz on 23 Jun 2020
If you're still looking for a solution, attach the fig file containing the data, or attach a mat file containing the (x,y) coordinates.

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Answers (3)

Matt J
Matt J on 22 Jun 2020
You could try using clusterdata to find the big concentration of points. Then minboundcircle from the File Exchange to get the radius,
  1 Comment
yonatan s
yonatan s on 23 Jun 2020
I have too many particles for that, it requires a lot of memory

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darova
darova on 23 Jun 2020
Try density function hist3
r = rand(500,1)/5;
t = rand(500,1)*2*pi;
x = [rand(50,1); r.*cos(t)+0.5];
y = [rand(50,1); r.*sin(t)+0.5];
n = 20;
k = hist3([x y],[n n]);
k(k<2) = nan;
pcolor((0:n-1)/n,(0:n-1)/n,k)
hold on
plot(x,y,'.r')
hold off
  4 Comments
Matt J
Matt J on 23 Jun 2020
[centers,radii] = imfindcircles(A,radiusRange)
yonatan s
yonatan s on 23 Jun 2020
the axes change as I transform a plot to an image.

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darova
darova on 24 Jun 2020
Here is the idea
  • scale your data
  • create an image (fill pixels)
  • dilate image to create solid round object (circle)
  • use imfindcircles
  5 Comments
darova
darova on 25 Jun 2020
There is no need for it
yonatan s
yonatan s on 9 Jul 2020
Edited: yonatan s on 9 Jul 2020
Hi darova, went over it again. How do I scale the radius back? If I simply multiply it by the scaling factor it doesnt come out right.

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