Suppose there are 'n' lines in 3D. Since each line has 4 Degrees of Freedom (DOF), total DOF as I understand should be $4n$.
As per Camera Calibration Using Line Correspondences, Section 2.5: Degree of Freedom
up to projectivity n lines have $4n - 15$ DOF provided $n \ge 5.^2 $.
I don't quite understand this, can anyone please explain this to me.
Thanks in advance