C++ Question

[lanz]

I don't know for shure, but I guess it is here to catch reader's attention, that a conversion is taking place.
AFAIK !! doesn't have any special meaning in C++.

EDIT: OK, I did additional research, it seems to be C idiom of clamping result to the range 0,1. So if it was not zero it becomes 1, if it was zero it stays zero, so that variable will be always either 1 or 0.
I think it have no meaning here, just a C leftover and a point of attention.