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#!/usr/bin/env python3
# -*- python -*-
# Copyright © 2014 Mattias Andrée (maandree@member.fsf.org)
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.
# Test of monotone cubic interpolation
# using the Fritsch–Carlson method.
# Does not overshoot, but regular
# cubic interpolation with linear
# replace for overshots is better.
# Load matplotlib.pyplot,
# it can take some time so
# print information about it.
print('Loading matplotlib.pyplot...')
import matplotlib.pyplot as plot
print('Done loading matplotlib.pyplot')
# Modules used for input data generation
from math import *
from random import *
def main():
# Create a page with graphs
fig = plot.figure()
# Add graphs
add_graph(fig, 221, [i / 15 for i in range(16)])
add_graph(fig, 222, [sin(6 * i / 15) for i in range(16)])
add_graph(fig, 223, [(i / 15) ** 0.5 for i in range(16)])
#add_graph(fig, 221, [random() for i in range(16)])
#add_graph(fig, 222, [random() for i in range(16)])
#add_graph(fig, 223, [random() for i in range(16)])
#add_graph(fig, 224, [random() for i in range(16)])
#add_graph(fig, 224, [(-1) ** (i // 2) for i in range(16)])
add_graph(fig, 224, [i / 15 * (-1) ** (i // 2) for i in range(16)])
# Show graphs
plot.show()
def add_graph(fig, graph_pos, input_values):
'''
Add a graph
@param fig:Figure The page to which to add the graph
@param graph_pos:int Where to place the graph
@param input_values:list<float> The input values for each point
'''
# Interpolate data
output_values = interpolate(input_values)
# Number of input points
n = len(input_values)
# Number of output points
m = len(output_values)
# Create graph
graph = fig.add_subplot(graph_pos)
# Plot interpolated data
graph.plot([i / (m - 1) for i in range(m)], output_values, 'b-')
# Plot input data
graph.plot([i / (n - 1) for i in range(n)], input_values, 'ro')
def interpolate(small, tension = 0):
'''
Interpolate data
@param small:list<float> The input values for each point
@param tension:float A [0, 1] value of the interpolation tension
@return :list<float> The values for each point in a scaled up version
'''
large = [None] * len(small) ** 2
small_, large_ = len(small) - 1, len(large) - 1
# Basis functions
h00 = lambda t : (1 + 2 * t) * (1 - t) ** 2
h10 = lambda t : t * (1 - t) ** 2
h01 = lambda t : t ** 2 * (3 - 2 * t)
h11 = lambda t : t ** 2 * (t - 1)
# Tension coefficent
c_ = 1 - tension
## Interpolant selection
# Compute the slopes of the secant
# lines between successive points
ds = [small[i + 1] - small[i] for i in range(small_)]
# Initialize the tangents at every
# data point as the average of the secants
ms = [ds[0]] + [(ds[i - 1] + ds[i]) / 2 for i in range(1, small_)] + [ds[small_ - 1]]
βlast = 0
for i in range(small_):
if ds[i] == 0:
# Two successive values are equal, ms[i],
# must be zero to preserve monotonicity,
# no idea to do further work on them.
ms[i], βlast = 0, -1
continue
# Look for local extremums
α, β = ms[i] / ds[i], ms[i + 1] / ds[i]
if (α < 0) or (βlast < 0):
# Local extremum found,
# ensure piecewise monotonicity
ms[i], β = 0, -1
elif α ** 2 + β ** 2 > 9:
# Otherwise, prevent overshoot and ensure
# monotonicity by restricting the (α, β)
# vector to a circle of radius 3.
τ = 3 / (α ** 2 + β ** 2) ** 0.5
ms[i], ms[i + 1] = τ * α * ds[i], τ * β * ds[i]
βlast = β
## Interpolation
for i in range(len(large)):
# Scaling
j = i * small_ / large_
# Floor, weight, ceiling
j, w, k = int(j), j % 1, min(int(j) + 1, small_)
# Points
pj, pk = small[j], small[k]
# Tangents
mj, mk = c_ * ms[j], c_ * ms[k]
# Interpolation
large[i] = h00(w) * pj + h10(w) * mj + h01(w) * pk + h11(w) * mk
return large
# Plot interpolation
main()
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