trvalues.py

# trvalues - track values
#
# SYNTAX
# [y,dydx,d2ydx2,alpha,R]=trvalues(p,x)
#
# INPUT
# p: the n+1 coefficients of a polynomial of degree n, given in descending
# order. (For instance the output from p=iptrack(filename).)
# x: ordinate value at which the polynomial is evaluated.
#
# OUTPUT
# [y,dydx,d2ydx2,alpha,R]=trvalues(p,x) returns the value y of the
# polynomial at x, the derivative dydx and the second derivative d2ydx2 in
# that point, as well as the slope alpha(x) and the radius of the
# osculating circle.
# The slope angle alpha is positive for a curve with a negative derivative.
# The sign of the radius of the osculating circle is the same as that of
# the second derivative.

import numpy as np


def trvalues(p,x):
    y=np.polyval(p,x)
    dp=np.polyder(p)
    dydx=np.polyval(dp,x)
    ddp=np.polyder(dp)
    d2ydx2=np.polyval(ddp,x)
    alpha=np.arctan(-dydx)
    R=(1.0+dydx**2)**1.5/d2ydx2
    return [y,dydx,d2ydx2,alpha,R]