Approximating data with a multi segment cubic bezier curve and a distance as well as a curvature contraint

Question

I have some geo data (the image below shows the path of a river as red dots) which I want to approximate using a multi segment cubic bezier curve. Through other questions on stackoverflow here and here I found the algorithm by Philip J. Schneider from "Graphics Gems". I successfully implemented it and can report that even with thousands of points it is very fast. Unfortunately that speed comes with some disadvantages, namely that the fitting is done quite sloppily. Consider the following graphic:

The red dots are my original data and the blue line is the multi segment bezier created by the algorithm by Schneider. As you can see, the input to the algorithm was a tolerance which is at least as high as the green line indicates. Nevertheless, the algorithm creates a bezier curve which has too many sharp turns. You see too of these unnecessary sharp turns in the image. It is easy to imagine a bezier curve with less sharp turns for the shown data while still maintaining the maximum tolerance condition (just push the bezier curve a bit into the direction of the magenta arrows). The problem seems to be that the algorithm picks data points from my original data as end points of the individual bezier curves (the magenta arrows point indicate some suspects). With the endpoints of the bezier curves restricted like that, it is clear that the algorithm will sometimes produce rather sharp curvatures.

What I am looking for is an algorithm which approximates my data with a multi segment bezier curve with two constraints:

• the multi segment bezier curve must never be more than a certain distance away from the data points (this is provided by the algorithm by Schneider)
• the multi segment bezier curve must never create curvatures that are too sharp. One way to check for this criteria would be to roll a circle with the minimum curvature radius along the multisegment bezier curve and check whether it touches all parts of the curve along its path. Though it seems there is a better method involving the cross product of the first and second derivative

The solutions I found which create better fits sadly either work only for single bezier curves (and omit the question of how to find good start and end points for each bezier curve in the multi segment bezier curve) or do not allow a minimum curvature contraint. I feel that the minimum curvature contraint is the tricky condition here.

Here another example (this is hand drawn and not 100% precise):

Lets suppose that figure one shows both, the curvature constraint (the circle must fit along the whole curve) as well as the maximum distance of any data point from the curve (which happens to be the radius of the circle in green). A successful approximation of the red path in figure two is shown in blue. That approximation honors the curvature condition (the circle can roll inside the whole curve and touches it everywhere) as well as the distance condition (shown in green). Figure three shows a different approximation to the path. While it honors the distance condition it is clear that the circle does not fit into the curvature any more. Figure four shows a path which is impossible to be approximated with the given constraints because it is too pointy. This example is supposed to illustrate that to properly approximate some pointy turns in the path, it is necessary that the algorithm chooses control points which are not part of the path. Figure three shows that if control points along the path were chosen, the curvature constraint cannot be fulfilled anymore. This example also shows that the algorithm must quit on some inputs as it is not possible to approximate it with the given constraints.

Does there exist a solution to this problem? The solution does not have to be fast. If it takes a day to process 1000 points, then that's fine. The solution does also not have to be optimal in the sense that it must result in a least squares fit.

In the end I will implement this in C and Python but I can read most other languages too.

Answer 1

I found the solution that fulfills my criterea. The solution is to first find a B-Spline that approximates the points in the least square sense and then convert that spline into a multi segment bezier curve. B-Splines do have the advantage that in contrast to bezier curves they will not pass through the control points as well as providing a way to specify a desired "smoothness" of the approximation curve. The needed functionality to generate such a spline is implemented in the FITPACK library to which scipy offers a python binding. Lets suppose I read my data into the lists x and y, then I can do:

import matplotlib.pyplot as plt
import numpy as np
from scipy import interpolate
tck,u = interpolate.splprep([x,y],s=3)
unew = np.arange(0,1.01,0.01)
out = interpolate.splev(unew,tck)
plt.figure()
plt.plot(x,y,out[0],out[1])
plt.show()

The result then looks like this:

If I want the curve more smooth, then I can increase the s parameter to splprep. If I want the approximation closer to the data I can decrease the s parameter for less smoothness. By going through multiple s parameters programatically I can find a good parameter that fits the given requirements.

The question though is how to convert that result into a bezier curve. The answer in this email by Zachary Pincus. I will replicate his solution here to give a complete answer to my question:

def b_spline_to_bezier_series(tck, per = False):
  """Convert a parametric b-spline into a sequence of Bezier curves of the same degree.

  Inputs:
    tck : (t,c,k) tuple of b-spline knots, coefficients, and degree returned by splprep.
    per : if tck was created as a periodic spline, per *must* be true, else per *must* be false.

  Output:
    A list of Bezier curves of degree k that is equivalent to the input spline. 
    Each Bezier curve is an array of shape (k+1,d) where d is the dimension of the
    space; thus the curve includes the starting point, the k-1 internal control 
    points, and the endpoint, where each point is of d dimensions.
  """
  from fitpack import insert
  from numpy import asarray, unique, split, sum
  t,c,k = tck
  t = asarray(t)
  try:
    c[0][0]
  except:
    # I can't figure out a simple way to convert nonparametric splines to 
    # parametric splines. Oh well.
    raise TypeError("Only parametric b-splines are supported.")
  new_tck = tck
  if per:
    # ignore the leading and trailing k knots that exist to enforce periodicity 
    knots_to_consider = unique(t[k:-k])
  else:
    # the first and last k+1 knots are identical in the non-periodic case, so
    # no need to consider them when increasing the knot multiplicities below
    knots_to_consider = unique(t[k+1:-k-1])
  # For each unique knot, bring it's multiplicity up to the next multiple of k+1
  # This removes all continuity constraints between each of the original knots, 
  # creating a set of independent Bezier curves.
  desired_multiplicity = k+1
  for x in knots_to_consider:
    current_multiplicity = sum(t == x)
    remainder = current_multiplicity%desired_multiplicity
    if remainder != 0:
      # add enough knots to bring the current multiplicity up to the desired multiplicity
      number_to_insert = desired_multiplicity - remainder
      new_tck = insert(x, new_tck, number_to_insert, per)
  tt,cc,kk = new_tck
  # strip off the last k+1 knots, as they are redundant after knot insertion
  bezier_points = numpy.transpose(cc)[:-desired_multiplicity]
  if per:
    # again, ignore the leading and trailing k knots
    bezier_points = bezier_points[k:-k]
  # group the points into the desired bezier curves
  return split(bezier_points, len(bezier_points) / desired_multiplicity, axis = 0)

So B-Splines, FITPACK, numpy and scipy saved my day :)