What does alpha mean in OpenCV fisheye camera model?

Question

I'm trying to implement the OpenCV fisheye camera model in OpenGL shader, so I checked the OpenCV document.

I don't understand what "α" mean in this function:

u=fx(x′+αy′)+cx

Answer 1

It is for skew.

The following can be seen in the explanation of the argument of fisheye::calibrate

fisheye::CALIB_FIX_SKEW Skew coefficient (alpha) is set to zero and stay zero.