Hi Arno,

Using the image you provided here, I fit the black pupil center using a ellipse estimation algorithm.
The result is 295.3148 along the horizontal direction from left to right 
106.7683 along the vertical direction from the top to down. Since I don't know the exact pupil center for this image, I‘m not sure the result is good or not.

Best wishes!
Xu Cheng

在 2015年3月5日星期四 UTC+8下午7:09:34,Arno Dietz写道:
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 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 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 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