scipy curve_fit 提高 "OptimizeWarning: Covariance of the parameters could not be estimated"
scipy curve_fit raises "OptimizeWarning: Covariance of the parameters could not be estimated"
我正在尝试使这个函数适合一些数据:
但是当我使用我的代码时
import numpy as np
from scipy.optimize import curve_fit
import matplotlib.pyplot as plt
def f(x, start, end):
res = np.empty_like(x)
res[x < start] =-1
res[x > end] = 1
linear = np.all([[start <= x], [x <= end]], axis=0)[0]
res[linear] = np.linspace(-1., 1., num=np.sum(linear))
return res
if __name__ == '__main__':
xdata = np.linspace(0., 1000., 1000)
ydata = -np.ones(1000)
ydata[500:1000] = 1.
ydata = ydata + np.random.normal(0., 0.25, len(ydata))
popt, pcov = curve_fit(f, xdata, ydata, p0=[495., 505.])
print(popt, pcov)
plt.figure()
plt.plot(xdata, f(xdata, *popt), 'r-', label='fit')
plt.plot(xdata, ydata, 'b-', label='data')
plt.show()
我收到错误
OptimizeWarning: Covariance of the parameters could not be estimated
输出:
在此示例中,开始和结束应该接近 500,但它们与我最初的猜测完全没有变化。
的警告(非错误)
OptimizeWarning: Covariance of the parameters could not be estimated
表示拟合无法确定拟合参数的不确定性(方差)。
主要问题是您的模型函数 f
将参数 start
和 end
视为离散值——它们被用作函数形式变化的整数位置。 scipy 的 curve_fit
(以及 scipy.optimize
中的所有其他优化例程)假设参数是 连续的 变量,而不是离散的。
拟合过程将尝试在参数中采取小步长(通常围绕机器精度)以获得残差相对于变量(雅可比行列式)的数值导数。将值用作离散变量时,这些导数将为零,并且拟合过程将不知道如何更改值以改善拟合。
您似乎在尝试将阶跃函数拟合到某些数据。请允许我推荐尝试 lmfit
(https://lmfit.github.io/lmfit-py),它提供了更高级别的曲线拟合接口,并具有许多内置模型。例如,它包含一个 StepModel
应该能够为您的数据建模。
为了对您的数据进行轻微修改(使其具有有限步长),以下带有 lmfit
的脚本可以适合此类数据:
#!/usr/bin/python
import numpy as np
from lmfit.models import StepModel, LinearModel
import matplotlib.pyplot as plt
np.random.seed(0)
xdata = np.linspace(0., 1000., 1000)
ydata = -np.ones(1000)
ydata[500:1000] = 1.
# note that a linear step is added here:
ydata[490:510] = -1 + np.arange(20)/10.0
ydata = ydata + np.random.normal(size=len(xdata), scale=0.1)
# model data as Step + Line
step_mod = StepModel(form='linear', prefix='step_')
line_mod = LinearModel(prefix='line_')
model = step_mod + line_mod
# make named parameters, giving initial values:
pars = model.make_params(line_intercept=ydata.min(),
line_slope=0,
step_center=xdata.mean(),
step_amplitude=ydata.std(),
step_sigma=2.0)
# fit data to this model with these parameters
out = model.fit(ydata, pars, x=xdata)
# print results
print(out.fit_report())
# plot data and best-fit
plt.plot(xdata, ydata, 'b')
plt.plot(xdata, out.best_fit, 'r-')
plt.show()
打印出
的报告
[[Model]]
(Model(step, prefix='step_', form='linear') + Model(linear, prefix='line_'))
[[Fit Statistics]]
# fitting method = leastsq
# function evals = 49
# data points = 1000
# variables = 5
chi-square = 9.72660131
reduced chi-square = 0.00977548
Akaike info crit = -4622.89074
Bayesian info crit = -4598.35197
[[Variables]]
step_sigma: 20.6227793 +/- 0.77214167 (3.74%) (init = 2)
step_center: 490.167878 +/- 0.44804412 (0.09%) (init = 500)
step_amplitude: 1.98946656 +/- 0.01304854 (0.66%) (init = 0.996283)
line_intercept: -1.00628058 +/- 0.00706005 (0.70%) (init = -1.277259)
line_slope: 1.3947e-05 +/- 2.2340e-05 (160.18%) (init = 0)
[[Correlations]] (unreported correlations are < 0.100)
C(step_amplitude, line_slope) = -0.875
C(step_sigma, step_center) = -0.863
C(line_intercept, line_slope) = -0.774
C(step_amplitude, line_intercept) = 0.461
C(step_sigma, step_amplitude) = 0.170
C(step_sigma, line_slope) = -0.147
C(step_center, step_amplitude) = -0.146
C(step_center, line_slope) = 0.127
并生成一个图
Lmfit 有很多额外的功能。例如,如果你想对某些参数值设置边界或修复一些参数值的变化,你可以执行以下操作:
# make named parameters, giving initial values:
pars = model.make_params(line_intercept=ydata.min(),
line_slope=0,
step_center=xdata.mean(),
step_amplitude=ydata.std(),
step_sigma=2.0)
# now set max and min values for step amplitude"
pars['step_amplitude'].min = 0
pars['step_amplitude'].max = 100
# fix the offset of the line to be -1.0
pars['line_offset'].value = -1.0
pars['line_offset'].vary = False
# then run fit with these parameters
out = model.fit(ydata, pars, x=xdata)
如果您知道模型应该是 Step+Constant
并且常数应该是固定的,您也可以将模型修改为
from lmfit.models import ConstantModel
# model data as Step + Constant
step_mod = StepModel(form='linear', prefix='step_')
const_mod = ConstantModel(prefix='const_')
model = step_mod + const_mod
pars = model.make_params(const_c=-1,
step_center=xdata.mean(),
step_amplitude=ydata.std(),
step_sigma=2.0)
pars['const_c'].vary = False
我正在尝试使这个函数适合一些数据:
但是当我使用我的代码时
import numpy as np
from scipy.optimize import curve_fit
import matplotlib.pyplot as plt
def f(x, start, end):
res = np.empty_like(x)
res[x < start] =-1
res[x > end] = 1
linear = np.all([[start <= x], [x <= end]], axis=0)[0]
res[linear] = np.linspace(-1., 1., num=np.sum(linear))
return res
if __name__ == '__main__':
xdata = np.linspace(0., 1000., 1000)
ydata = -np.ones(1000)
ydata[500:1000] = 1.
ydata = ydata + np.random.normal(0., 0.25, len(ydata))
popt, pcov = curve_fit(f, xdata, ydata, p0=[495., 505.])
print(popt, pcov)
plt.figure()
plt.plot(xdata, f(xdata, *popt), 'r-', label='fit')
plt.plot(xdata, ydata, 'b-', label='data')
plt.show()
我收到错误
OptimizeWarning: Covariance of the parameters could not be estimated
输出:
在此示例中,开始和结束应该接近 500,但它们与我最初的猜测完全没有变化。
OptimizeWarning: Covariance of the parameters could not be estimated
表示拟合无法确定拟合参数的不确定性(方差)。
主要问题是您的模型函数 f
将参数 start
和 end
视为离散值——它们被用作函数形式变化的整数位置。 scipy 的 curve_fit
(以及 scipy.optimize
中的所有其他优化例程)假设参数是 连续的 变量,而不是离散的。
拟合过程将尝试在参数中采取小步长(通常围绕机器精度)以获得残差相对于变量(雅可比行列式)的数值导数。将值用作离散变量时,这些导数将为零,并且拟合过程将不知道如何更改值以改善拟合。
您似乎在尝试将阶跃函数拟合到某些数据。请允许我推荐尝试 lmfit
(https://lmfit.github.io/lmfit-py),它提供了更高级别的曲线拟合接口,并具有许多内置模型。例如,它包含一个 StepModel
应该能够为您的数据建模。
为了对您的数据进行轻微修改(使其具有有限步长),以下带有 lmfit
的脚本可以适合此类数据:
#!/usr/bin/python
import numpy as np
from lmfit.models import StepModel, LinearModel
import matplotlib.pyplot as plt
np.random.seed(0)
xdata = np.linspace(0., 1000., 1000)
ydata = -np.ones(1000)
ydata[500:1000] = 1.
# note that a linear step is added here:
ydata[490:510] = -1 + np.arange(20)/10.0
ydata = ydata + np.random.normal(size=len(xdata), scale=0.1)
# model data as Step + Line
step_mod = StepModel(form='linear', prefix='step_')
line_mod = LinearModel(prefix='line_')
model = step_mod + line_mod
# make named parameters, giving initial values:
pars = model.make_params(line_intercept=ydata.min(),
line_slope=0,
step_center=xdata.mean(),
step_amplitude=ydata.std(),
step_sigma=2.0)
# fit data to this model with these parameters
out = model.fit(ydata, pars, x=xdata)
# print results
print(out.fit_report())
# plot data and best-fit
plt.plot(xdata, ydata, 'b')
plt.plot(xdata, out.best_fit, 'r-')
plt.show()
打印出
的报告[[Model]]
(Model(step, prefix='step_', form='linear') + Model(linear, prefix='line_'))
[[Fit Statistics]]
# fitting method = leastsq
# function evals = 49
# data points = 1000
# variables = 5
chi-square = 9.72660131
reduced chi-square = 0.00977548
Akaike info crit = -4622.89074
Bayesian info crit = -4598.35197
[[Variables]]
step_sigma: 20.6227793 +/- 0.77214167 (3.74%) (init = 2)
step_center: 490.167878 +/- 0.44804412 (0.09%) (init = 500)
step_amplitude: 1.98946656 +/- 0.01304854 (0.66%) (init = 0.996283)
line_intercept: -1.00628058 +/- 0.00706005 (0.70%) (init = -1.277259)
line_slope: 1.3947e-05 +/- 2.2340e-05 (160.18%) (init = 0)
[[Correlations]] (unreported correlations are < 0.100)
C(step_amplitude, line_slope) = -0.875
C(step_sigma, step_center) = -0.863
C(line_intercept, line_slope) = -0.774
C(step_amplitude, line_intercept) = 0.461
C(step_sigma, step_amplitude) = 0.170
C(step_sigma, line_slope) = -0.147
C(step_center, step_amplitude) = -0.146
C(step_center, line_slope) = 0.127
并生成一个图
Lmfit 有很多额外的功能。例如,如果你想对某些参数值设置边界或修复一些参数值的变化,你可以执行以下操作:
# make named parameters, giving initial values:
pars = model.make_params(line_intercept=ydata.min(),
line_slope=0,
step_center=xdata.mean(),
step_amplitude=ydata.std(),
step_sigma=2.0)
# now set max and min values for step amplitude"
pars['step_amplitude'].min = 0
pars['step_amplitude'].max = 100
# fix the offset of the line to be -1.0
pars['line_offset'].value = -1.0
pars['line_offset'].vary = False
# then run fit with these parameters
out = model.fit(ydata, pars, x=xdata)
如果您知道模型应该是 Step+Constant
并且常数应该是固定的,您也可以将模型修改为
from lmfit.models import ConstantModel
# model data as Step + Constant
step_mod = StepModel(form='linear', prefix='step_')
const_mod = ConstantModel(prefix='const_')
model = step_mod + const_mod
pars = model.make_params(const_c=-1,
step_center=xdata.mean(),
step_amplitude=ydata.std(),
step_sigma=2.0)
pars['const_c'].vary = False