Operator() on Vec3d?

I am reading the source code of opencv about the function initUndistortRectifyMapComputer.
I am confused about the below codes, what does vecTilt(2) represent , I think vec3d type has no () member functions.

Vec3d vecTilt = matTilt*cv::Vec3d(xd, yd, 1);
double invProj = vecTilt(2) ? 1./vecTilt(2) : 1;


3-element vector
last element, (2), is the projective coordinate. that is why it is used for a divisor.

see also:

Yes, it is. Thank you very much.