A model class can be created to fit any function, or interface with external code.
.. todo:: There should be some description of what needs to be done, as well as examples. Does the 1D example need to be cleaned up to separate out unnescessary code, perhaps just hiding the setup code (and it would be nice if this could be shared with the setup). Should the 2D example add commentary to point out the following note I added at the time: "Hmmm, this looks similar to the Sherpa results. In particular the 0,0 value is -80 not 1. Aha, is it a normalization at (0,0) vs (1,1) sort of thing?"
An example is the AstroPy trapezoidal model, which has four parameters: the amplitude of the central region, the center and width of this region, and the slope. The following model class, which was not written for efficiancy or robustness, implements this interface:
.. literalinclude:: ../code/trap.py
This can be used in the same manner as the :py:class:`~sherpa.models.basic.Gauss1D` model in the :ref:`quick guide to Sherpa<quick-gauss1d>`.
First, create the data to fit:
>>> import numpy as np >>> import matplotlib.pyplot as plt >>> np.random.seed(0) >>> x = np.linspace(-5., 5., 200) >>> ampl_true = 3 >>> pos_true = 1.3 >>> sigma_true = 0.8 >>> err_true = 0.2 >>> y = ampl_true * np.exp(-0.5 * (x - pos_true)**2 / sigma_true**2) >>> y += np.random.normal(0., err_true, x.shape)
Now create a Sherpa data object:
>>> from sherpa.data import Data1D >>> d = Data1D('example', x, y)
Set up the user model:
>>> from trap import Trap1D >>> t = Trap1D() >>> print(t) trap1d Param Type Value Min Max Units ----- ---- ----- --- --- ----- trap1d.ampl thawed 1 0 3.40282e+38 trap1d.center thawed 1 -3.40282e+38 3.40282e+38 trap1d.width thawed 1 0 3.40282e+38 trap1d.slope thawed 1 0 3.40282e+38
Finally, perform the fit:
>>> from sherpa.fit import Fit >>> from sherpa.stats import LeastSq >>> from sherpa.optmethods import LevMar >>> tfit = Fit(d, t, stat=LeastSq(), method=LevMar()) >>> tres = tfit.fit() >>> if not tres.succeeded: print(tres.message)
Rather than use a :py:class:`~sherpa.plot.ModelPlot` object,
the overplot
argument can be set to allow multiple values
in the same plot:
>>> from sherpa import plot >>> dplot = plot.DataPlot() >>> dplot.prepare(d) >>> dplot.plot() >>> mplot = plot.ModelPlot() >>> mplot.prepare(d, t) >>> mplot.plot(overplot=True)
The two-dimensional case is similar to the one-dimensional case,
with the major difference being the number of independent axes to
deal with. In the following example the model is assumed to only be
applied to non-integrated data sets, as it simplifies the implementation
of the calc
method.
It also shows one way of embedding models from a different system, in this case the two-dimemensional polynomial model from the AstroPy package.
.. literalinclude:: ../code/poly.py
Repeating the 2D fit by first setting up the data to fit:
>>> np.random.seed(0) >>> y2, x2 = np.mgrid[:128, :128] >>> z = 2. * x2 ** 2 - 0.5 * y2 ** 2 + 1.5 * x2 * y2 - 1. >>> z += np.random.normal(0., 0.1, z.shape) * 50000.
Put this data into a Sherpa data object:
>>> from sherpa.data import Data2D >>> x0axis = x2.ravel() >>> x1axis = y2.ravel() >>> d2 = Data2D('img', x0axis, x1axis, z.ravel(), shape=(128,128))
Create an instance of the user model:
>>> from poly import WrapPoly2D >>> wp2 = WrapPoly2D('wp2') >>> wp2.c1_0.frozen = True >>> wp2.c0_1.frozen = True
Finally, perform the fit:
>>> f2 = Fit(d2, wp2, stat=LeastSq(), method=LevMar()) >>> res2 = f2.fit() >>> if not res2.succeeded: print(res2.message) >>> print(res2) datasets = None itermethodname = none methodname = levmar statname = leastsq succeeded = True parnames = ('wp2.c0_0', 'wp2.c2_0', 'wp2.c0_2', 'wp2.c1_1') parvals = (-80.289475553599914, 1.9894112623565667, -0.4817452191363118, 1.5022711710873158) statval = 400658883390.6685 istatval = 6571934382318.328 dstatval = 6.17127549893e+12 numpoints = 16384 dof = 16380 qval = None rstat = None message = successful termination nfev = 80 >>> print(wp2) wp2 Param Type Value Min Max Units ----- ---- ----- --- --- ----- wp2.c0_0 thawed -80.2895 -3.40282e+38 3.40282e+38 wp2.c1_0 frozen 0 -3.40282e+38 3.40282e+38 wp2.c2_0 thawed 1.98941 -3.40282e+38 3.40282e+38 wp2.c0_1 frozen 0 -3.40282e+38 3.40282e+38 wp2.c0_2 thawed -0.481745 -3.40282e+38 3.40282e+38 wp2.c1_1 thawed 1.50227 -3.40282e+38 3.40282e+38