The title of a paper about how to calculate the curvature is "Silicon Photonics Design Rule Checking: Application of a Programmable Modeling Engine for Non-Manhattan Geometry Verification"
I also cannot image the process is so easy, but the authout of this paper is also the engineer of SIEMENS EDA Mentor Graphic Calibre. So I think the way in this paper is authentic, but the problem is whether it is easy to be achieved in Klayout by using ruby.
I cannot find other public publication or reference.
I think equation-based DRC is really potential.
Thanks.