hough ellipse fit inaccurate?

[xu cheng]

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 
> <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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