The saturation formula in chroma key

[xiangcong]

Thanks a lot for the project~! The chroma key algorithm works quite well!
However, I'm curious about the saturation formula in the chroma key function, which is
pixelSat = fmax - mix(secondaryComponents - fmin, secondaryComponents / 2.0, balance);
the function is not the same as the saturation formula I found in the website(like http://www.rapidtables.com/convert/color/rgb-to-hsl.htm for example)
I wonder why the saturation is like this in this project.
Thanks a lot!

[xiangcong]

secondaryComponents = dot(1.0 - pixelPrimary, sourcePixel.rgb);
secondaryComponents is the sum of two channels with are not the max of rgb.
while formulas I found that convert rgb to saturation are only related to max and min of rgb.