Calculate the length of the major axis of an ellipse using its perimeter
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Vahid Esmaeili
am 26 Aug. 2020
Beantwortet: Image Analyst
am 26 Aug. 2020
Hello,
I am wondering how to calculate the length of the major axis of an ellipse using its perimeter in MATLAB? In fact, I have the perimeter of the forefoot (which is like an ellipse) but I need its major axis.
Thank you so much,
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Image Analyst
am 26 Aug. 2020
Vahid, if you have the Statistics and Machine Learning toolbox you can use pdist2(). If you have an N-by-2 array of (x,y) coordinates, you can do
distances = pdist2(xy, xy);
majorAxisLength = max(distances(:)) % Twice the semi-major radius value.
If you don't have that toolbox, you can easily compute the distances of every point to every other point using a pair of for loops.
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John D'Errico
am 26 Aug. 2020
Bearbeitet: John D'Errico
am 26 Aug. 2020
You can't.
That is, knowing ONLY the perimeter of an ellipse, you CANNOT know the major axis. That is, I can give you two ellipses, one of which is a perfect circle, and the other an infinitely narrow ellipse, yet both of them will have the same perimeter. Of course in the two cases, they will have very different shapes.
So case closed, you cannot do what you have asked to compute. You need to provide more information. And perhaps do some reading.
For example, if you are willing to provide some other parameter, such as the minor axis length, or the eccentricity, then it is possible to compute the major axis length.
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