hough ellipse fit inaccurate?

[Arno Dietz]

Ok sorry, maybe I need to explain my project better.
I have a Autodesk Maya Model of a simplified eyeball with the pupil (see 
image). I render the images of the eyeball from different angles and try to 
detect the pupil center as accurate as possible. Since I know the geometry 
<http://www.dict.cc/englisch-deutsch/geometry.html> and rotation of the 
eyeball, I can calculate the mapping of the real center of the pupil on my 
virtual maya camera sensor. I proved the validity 
<http://www.dict.cc/englisch-deutsch/validity.html> of my calculation with 
several other methods of ellipse center detection (center of mass, distance 
transform, opencv ellipse fit, starbust) where I get errors between my 
calculation and measurement with sometimes less than 0.02 pixels.
But nevertheless <http://www.dict.cc/englisch-deutsch/nevertheless.html> I 
want to try the hough ellipse approach, because it may be more robust 
against noise or other errors I want to simulate later. And it seems so far 
only the hough ellipse approach is quite inaccurate so I was wondering why. 
I think the reason could be the ellipse center detection has only a half 
pixel accuracy while my other approaches have sub-pixel accuracy.

I think posting the calculation code would be too much, but I am quite sure 
the calculation is right.

Kind regards
Arno
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