<br><br>Hi Roman,<br><br>You can find Saddle-like points by using <br>the Hessian image filter:<br><br><a href="http://www.itk.org/Doxygen/html/classitk_1_1HessianRecursiveGaussianImageFilter.html">http://www.itk.org/Doxygen/html/classitk_1_1HessianRecursiveGaussianImageFilter.html</a><br>
<br>and then computing Eigen analysis for the <br>Hessian matrices at every pixels.<br><br>Whenever you find that your two eigen values<br>have opposite signs, then you are in a Saddle-like<br>point.<br><br><br>You can compute the Eigen analysis at every <br>
pixel by using the class<br><a href="http://www.itk.org/Doxygen/html/classitk_1_1SymmetricEigenAnalysis.html">http://www.itk.org/Doxygen/html/classitk_1_1SymmetricEigenAnalysis.html</a><br><br>or you can generate a full image by using the class<br>
<a href="http://www.itk.org/Doxygen/html/classitk_1_1SymmetricEigenAnalysisImageFilter.html">http://www.itk.org/Doxygen/html/classitk_1_1SymmetricEigenAnalysisImageFilter.html</a><br><br>The first option will require less memory.<br>
<br><br> Regards,<br><br><br> Luis<br><br><br>---------------------------------------------------------------------------------------<br><div class="gmail_quote">On Thu, Apr 22, 2010 at 10:09 AM, Roman Grothausmann <span dir="ltr"><<a href="mailto:roman.grothausmann@helmholtz-berlin.de">roman.grothausmann@helmholtz-berlin.de</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Dear Richard,<br>
<br>
<br>
Many thanks for Your answers. Recommending the watershed filter for this<br>
was a great hint. I hadn't realized (since I was not looking for a<br>
segmentation) that the borderline of the watershed of the inverted image<br>
is exactly what I'm looking for.<br>
<br>
Still I'm wondering how I could find the saddle points (not the regional<br>
minima) of a 2D image (eg. point 0,0 of the plot of the function<br>
x*x-y*y). If I'm not mistaken those are the points where a watershed<br>
border starts when two basins meet. So kind of the origins of the<br>
watershed borderlines.<br>
<br>
Many thanks again.<br>
Roman<br>
<br>
Richard Beare wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hi,<br>
I'm not entirely sure what problem you are trying to solve, but there<br>
may be a simple approach if you outline it in more detail. The<br>
discussion of gradient descent makes me wonder whether you might be<br>
interested in watershed approaches, which are sometimes discussed in<br>
terms of gradient descent but are more usefully implemented as<br>
gradient ascent.. They are likely to be useful if you have a<br>
segmentation problem.<br>
<br>
Anyhow - regional minima can be found using a number of filters -<br>
search for RegionalMinima in documentation.<div><div></div><div class="h5"><br>
<br>
On 4/22/10, Roman Grothausmann <<a href="mailto:roman.grothausmann@helmholtz-berlin.de" target="_blank">roman.grothausmann@helmholtz-berlin.de</a>> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Dear mailing list members,<br>
<br>
<br>
How can I find saddle points and image border pixels with minimum<br>
gradient in a 2D image? I need those as starting points for tracing the<br>
gradient decent (as is done in the Random access iteration example in<br>
the user guide) because I want to detect valley traces in a 2D image<br>
that originate from saddle points (for no border pixels) or pixels with<br>
minimum gradient (for image border pixels).<br>
Is there a way to find these two types of special points with ITK?<br>
<br>
Any help is very much appreciated<br>
Roman<br>
<br>
--<br>
Roman Grothausmann<br>
<br>
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH<br>
Bereich Funktionale Materialien<br>
Institut für angewandte Materialforschung<br>
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<br>
-- <br>
Roman Grothausmann<br>
<br>
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH<br>
Bereich Funktionale Materialien<br>
Institut für angewandte Materialforschung<br>
Hahn-Meitner-Platz 1<br>
D-14109 Berlin <br>
<br>
Tel.: +49-(0)30-8062-2816<br>
Fax.: +49-(0)30-8062-3059<br>
<br>
Vorsitzender des Aufsichtsrats: Prof. Dr. Dr. h.c. mult. Joachim Treusch<br>
Stellvertretende Vorsitzende: Dr. Beatrix Vierkorn-Rudolph<br>
Geschäftsführer: Prof. Dr. Anke Rita Kaysser-Pyzalla, Prof. Dr. Dr. h.c.<br>
Wolfgang Eberhardt, Dr. Ulrich Breuer<br>
Sitz der Gesellschaft: Berlin<br>
Handelsregister: AG Charlottenburg, 89 HRB 5583<br>
<br>
<br>
_____________________________________<br>
Powered by <a href="http://www.kitware.com" target="_blank">www.kitware.com</a><br>
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