What are the main references to the fish-eye camera model in OpenCV3.0.0dev?

Question

I am wrestling with the Fish-Eye Camera Model used in OpenCV 3.0.0.dev. I have read the documentation in this link several times, especial

Answer 1

After hours of reading, I found that the formula θ=atan(r) in OpenCV's fish-eye documentation, is the normalized inverse of r=f*tanθ pertaining to pinhole projection and hence, none of the fish-eye projection models mentioned in the above links are used in OpenCV.

In addition, regarding the distortion model, what I guess is that the Division Model of Fitzgibbon in his 2001 paper "Simultaneous linear estimation of multiple view geometry and lens distortion" is used. According to Hughes in his 2008 paper "Review of Geometric Distortion Compensation in Fish-Eye Cameras", among the other alternatives are "Odd Polynomial Model" and "Polynomial Fish-Eye Transform". In his paper, in page 2, he has written:

"(1) (which refers to Odd Polynomial Model) and (3) (which refers to Division Model, which I guess is the one used by OpenCV) can be used to describe distortion in standard,non-fisheye lenses. However, it is generally considered that these polynomial models are insufficient to describe the level of distortion introduced by fish-eye lenses. Shah and Aggarwal have shown in [9] (Intrinsic Parameter Calibration Procedure For A High-Distortion Fish-Eye Lens Camera With Distortion Model And Accuracy Estimation) that even when using a 7th order version of (1) to model fish-eye radial distortion, considerable distortion remains, to the extent that they had to use a model with greater degrees of freedom. Therefore, a polynomial that uses both odd and even coefficients (instead of simply one or the other) can be used to model the radial distortion introduced by a fisheye lens"

After all, I conclude that the fish-eye model in OpenCV has very limited applicability and could be much more strengthened in terms of distortion models and projection models. I'd like to re-emphasize that I still need to know what papers were used by the OpenCV developers to implement the fisheye namespace.

I'd deeply appreciate anyone's comments on this.