An orb web with hanging threads.
For resampled curve segments on a spider web, a is the user defined hanging amount, u the relative and r the absolute length. u * (1.0 - u) is a function to make the curves hang in their center.
float a = chf('amount');
float u = vertexcurveparam(0, i@vtxnum);
float r = prim(0, 'restlength', i@primnum);
v@P.y -= u * (1.0 - u) * a * r;
