Fits

Functions

Details

Collection of fitting functions

fits.demo_ransac()[source]

Find the best-fit circle in an image, using the RANSAC algorithm

fits.fit_circle(x, y)[source]

Determine the best-fit circle to given datapoints.

Parameters:

x : array (N,)

x-values.

y : array (N,)

corresponding y-values.
Returns:

center : array (2,)

x/y coordinates of center of the circle

radius : float

Circle radius.

Examples

>>> r = 2
>>> center = np.r_[5,5]
>>> theta = np.r_[0:2*np.pi:10j]
>>> x = r*np.cos(theta)+center[0]
>>> y = r*np.sin(theta)+center[1]
>>> cFit,rFit = thLib.fits.fit_circle(x,y)
fits.fit_ellipse(x, y)[source]

Ellipse fit by Taubin’s Method

Parameters:

x : array

x-coordinates of the ellipse points

y : array

y-coordinates of the ellipse points
Returns:

A : array

Ellipse parameters A = [a b c d e f] is the vector of algebraic parameters of thefitting ellipse: ax^2 + bxy + cy^2 +dx + ey + f = 0 The vector A is normed, so that ||A||=1.

Notes

Among fast non-iterative ellipse fitting methods, this is perhaps the most accurate and robust.

This method fits a quadratic curve (conic) to a set of points; if points are better approximated by a hyperbola, this fit will return a hyperbola. To fit ellipses only, use “Direct Ellipse Fit”.

Published in G. Taubin, “Estimation Of Planar Curves, Surfaces And Nonplanar Space Curves Defined By Implicit Equations, With Applications To Edge And Range Image Segmentation”, IEEE Trans. PAMI, Vol. 13, pages 1115-1138, (1991)

fits.fit_exp(tFit, yFit, plotFlag=False)[source]

Calculates best-fit parameters for the exponential decay to an offset. This can serve as an example for a general non-linear fit.

Parameters:

tFit : array (N,)

Time values.

yFit : array (N,)

Function values
Returns:

offset : float

Function offset/bias.

amp : float

Amplitude of exponential function

tau : float

Decay time.

Examples

>>> t = np.arange(10)
>>> tau = 2.
>>> amp = 1.
>>> offset = 2.
>>> x = offset + amp*np.exp(-t/tau)    
>>> fitted =  thLib.fits.fit_exp(t,x)
fits.fit_line(x, y, alpha=0.05, newx=[], plotFlag=False)[source]

Linear regression fit.

Parameters:

x : ndarray

Input / Predictor.

y : ndarray

Input / Estimator.

alpha : float

Confidence limit [default=0.05]

newx : float or ndarray

Values for which the fit and the prediction limits are calculated (optional)

plotFlag: int, optional

1 = plot, 0 = no_plot [default]
Returns:

a : float

Intercept

b : float

Slope

ci : ndarray

Lower and upper confidence interval for the slope

info : dictionary

contains return information on - residuals - var_res - sd_res - alpha - tval - df

newy : list(ndarray)

Predictions for (newx, newx-ciPrediction, newx+ciPrediction)

Notes

Example data and formulas are taken from D. Altman, “Practical Statistics for Medicine”

Examples

>>> x = np.r_[0:10:11j]
>>> y = x**2
>>> (a,b,(ci_a, ci_b),_) = thLib.fits.fit_line(x,y)    
Summary: a=-15.0000+/-12.4590, b=10.0000+/-2.1060
Confidence intervals: ci_a=(-27.4590 - -2.5410), ci_b=(7.8940 - 12.1060)
Residuals: variance = 95.3333, standard deviation = 9.7639
alpha = 0.050, tval = 2.2622, df=9
fits.fit_sin(tList, yList, freq)[source]

Fit a sine wave with a known frequency to a given set of data.

y = amplitude * sin(2*pi*freq * tList + phase*pi/180) + bias

Parameters:

yList : array

datapoints

tList : float

time base, in sec

freq : float

in Hz
Returns:

phase : float

in degrees

amplitude : float

bias : float

Examples

>>> np.random.seed(1234)
>>> t = np.arange(0,10,0.1)
>>> x = 3 + 4*np.sin(2*np.pi*t + 5*np.pi/180) + np.random.randn(len(t))
>>> (phase, amp, offset) = thLib.fits.fit_sin(t, x, 1)
fits.regress(y, X, alpha=0.05, intercept=True)[source]

Multilinear regression and confidence intervals. Note that by default, an intercept is added to the design matrix!

Parameters:

X : ndarray (N,) or (N,p)

predictors at each of N observations

y : ndarray(N,)

observed responses

alpha : float, optional

Defines the 100*(1-alpha)% confidence level in ci. Default alpha=0.05

intercept : boolean

If ‘True’, an intercept is automatically added to the design matrix. If ‘False’, the behavior is like the Matlab “regress” function, and no intercept is added.
Returns:

fit : float

best fit intercept and regression parameters

ci : ndarray (2,)

confidence intervals for the coefficient estimates at the given alpha level (default: 95%-level)

See also

fit_line

Examples

>>> x = np.array([0, 0, 10, 10])
>>> y = np.array([1, 3, 11, 13])
>>> (fit,ci) = thLib.fits.regress(y,x)    

>>> X = np.random.randn(10,2)
>>> mat = np.hstack( (np.ones( (len(X),1) ), X) )
>>> pars = np.c_[[2, 4, 5]]
>>> y = mat.dot(pars)
>>> y += 0.1*np.random.randn(*y.shape)
>>> (fit,ci) = thLib.fits.regress(y, mat)