Hello,
I have a question about the formula in the official documentation.
You can find the formula in ink below:
https://docs.opencv.org/4.5.4/db/d58/group__calib3d__fisheye.html

This is the formula about distorted point coordinates conversion into pixel coordinates:

\begin{align*}
x' &= (\theta_d / r) a ~~~ & ~~~ (1) \\
y' &= (\theta_d / r) b ~~~ & ~~~ (2) \\
u &= f_x (x' + \alpha y') + c_x ~~~ & ~~~ (3) \\
v &= f_y y' + c_y ~~~ & ~~~ (4) \\
\end{align*}

I found the source code in link below:

```
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//
//
// License Agreement
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```

This file has been truncated. show original

And this how the source code programed:

```
double scale = (r == 0) ? 1.0 : theta_d / r;
u = f[0]*x*scale + c[0];
v = f[1]*y*scale + c[1];
```

My question is:
1.what is Î± in formula(3)?
2.Why the calculation of formula(3) is different from source code?

Thank you!