How to fit an ellipse to an image in matlab.

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diskimage1.jpgI have a image like this and I want to fit an ellipse around this disk image and get the info about the ellipse like its axis, center point, etc. I am unsure how to use the image proccessing toolbox to draw an ellipse to it and get info about the ellipse.

Accepted Answer

Akira Agata
Akira Agata on 11 Dec 2019
How about the following?
% Read image
I = imread('diskimage1.jpeg');
% Binarize
Igray = rgb2gray(I);
BW = imbinarize(Igray);
% Extract the maximum area
BW = imclearborder(BW);
BW = bwareafilt(BW,1);
% Calculate centroid, orientation and major/minor axis length of the ellipse
s = regionprops(BW,{'Centroid','Orientation','MajorAxisLength','MinorAxisLength'});
% Calculate the ellipse line
theta = linspace(0,2*pi);
col = (s.MajorAxisLength/2)*cos(theta);
row = (s.MinorAxisLength/2)*sin(theta);
M = makehgtform('translate',[s.Centroid, 0],'zrotate',deg2rad(-1*s.Orientation));
D = M*[col;row;zeros(1,numel(row));ones(1,numel(row))];
% Visualize the result
figure
imshow(Igray)
hold on
plot(D(1,:),D(2,:),'r','LineWidth',2)
untitled.png
  11 Comments
Image Analyst
Image Analyst on 10 Nov 2023
@Connor that is not a blob. Well it is but it's a very snake-like, line-like blob. So the fit is accurate. But I suspect what you want is to fit the (x,y) coordinates of the blob to an ellipse. So you need to get x and y from the binary image like this:
[y, x] = find(binaryImage);

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More Answers (2)

Image Analyst
Image Analyst on 11 Dec 2019

Matt J
Matt J on 10 Nov 2023
Edited: Matt J on 10 Nov 2023
You can use ellipticalFit from this FEX download
processImage('Connors_image.png')
ans =
ellipticalFit with properties: center: [196.4747 218.0069] a: 130.0274 b: 119.1008 angle: 3.0398
processImage('Hardits_image.png')
ans =
ellipticalFit with properties: center: [171.3735 173.6428] a: 112.7502 b: 104.1636 angle: 47.0771
function e=processImage(filename)
I = imread(filename);
% Binarize
I = bwareafilt(imclearborder(imbinarize(rgb2gray(I))),1);
b = bwboundaries(I);
e=ellipticalFit(flipud(b{1}'));
imshow(I,[]); hold on
showfit(e,'LineWidth',1,'Color','r','LineStyle','--'); hold off
end

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