数学图形(1.39)TN constant curve

这是个类似巴黎铁塔的曲线.

#http://www.mathcurve.com/courbes2d/tn/tn.shtml
vertices =
t = from  to (PI*0.999)
a = 

s = sin(t)
c = cos(t)

y = -a*(c*c + ln(s))
x = a*s*c

y = limit(y,  -, )

面的形式:

vertices = D1: D2:
u = from (0.01*PI) to (PI*0.99) D1
v = from  to  D2

s = sin(u)
c = cos(u)

y = -v*(c*c + ln(s))
x = v*s*c