hough ellipse fit inaccurate?

[Arno Dietz]

Hello,

I'm working on a project where I need to test various 
<http://www.dict.cc/englisch-deutsch/various.html> methods to fit ellipses 
<http://www.dict.cc/englisch-deutsch/ellipses.html> as accurate as 
possible. The hough ellipse fit from scikit-image for my images with 
perfect Ellipses is quite inaccurate as you can see in the examples.
The white ellipse is my edge image. The red are the fitted ones.
 

<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg> 
<https://lh4.googleusercontent.com/-_e1EN1j0e1A/VPeI_gVKenI/AAAAAAAAAHU/_6yJ5gHw8_c/s1600/ellipse4.jpg> 
<https://lh4.googleusercontent.com/-76eRCwkROTk/VPeI9td-ZbI/AAAAAAAAAHM/XOY0d2iIOSk/s1600/ellipse3.jpg>Why 
is there always a offset  
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg>although 
my source image has perfect ellipses? 
<http://www.dict.cc/englisch-deutsch/although.html> I tried to vary the 
parameters but without success. 
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg>
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg> 
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg>Thank 
you so far. 
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg> 
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg>best 
regards 
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg>
Arno 
<https://lh5.googleusercontent.com/-9LZJ3vPmnAU/VPeIuqOv-pI/AAAAAAAAAG8/oDeC5ySkTHE/s1600/ellipse1.jpg>
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