Caculate the tangle vec of trace

[HandsomeDragon3]

Is it a mistake that s0 is used rather than s? Can you explain it simply, thank you ~ @fangedward ~ // normal spline vector double x0 = csp->calc_x(s0); double y0 = csp->calc_y(s0); double x1 = csp->calc_x(s0 + 2); double y1 = csp->calc_y(s0 + 2);