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