Question about osqp solver

[ImCabbage]

Hi,
I would like to know where do the -cos(theta) and -sin(theta) come from in function setConstraintMatrix (tension_smoother.cpp). Is it related to the frenet coordinates transformation?
If it is possible, is there any material about transforming the reference line smoothing problem to qp form? I want to know how to build the hessian, gradient and constraint matrix from scratch. Thank you.