| Ellipse fitting [message #72472] |
Thu, 09 September 2010 04:18 |
Oriol Güell Riera
Messages: 7
Registered: September 2010
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Junior Member |
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Hi all,
I've got a problem when I try to find the orientation of a fitted
ellipse of a region of interest (http://www.dfanning.com/ip_tips/
fit_ellipse.html).
I've got an stack of images of an ellipse rotating. My problem arises
when I try to find the eigenvectors of the covariance matrix, the
signs of the components of them change randomly. In other words, I
start with my ellipse in the 3rd quadrant. The first image gives the
correct signs for the components of the principal eigenvector, but the
second image doesn't give the signs of the components well, so in this
case the eigenvector is located in the 4th quadrant, but my ellipse
still stands in the 3rd quadrant!
I don't know if I have explained my problem correctly. The problem
comes from the indeterminacy of the signs of the eigenvectors. If they
had the correct signs, I would use the atan2 function, which will give
the correct orientation. However, the signs aren't correct, so I can't
get the orientation of the ellipse after a whole rotation.
Thank you very much
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