pybaselines.Baseline.psalsa

Baseline.psalsa(data, lam=100000.0, p=0.5, k=None, diff_order=2, max_iter=50, tol=0.001, weights=None)

Peaked Signal's Asymmetric Least Squares Algorithm (psalsa).

Similar to the asymmetric least squares (AsLS) algorithm, but applies an exponential decay weighting to values greater than the baseline to allow using a higher p value to better fit noisy data.

Parameters:

dataarray_like, shape (N,)

The y-values of the measured data, with N data points. Must not contain missing data (NaN) or Inf.

lamfloat, optional

The smoothing parameter. Larger values will create smoother baselines. Default is 1e5.

pfloat, optional

The penalizing weighting factor. Must be between 0 and 1. Values greater than the baseline will be given p weight, and values less than the baseline will be given 1 - p weight. Default is 0.5.

kfloat, optional

A factor that controls the exponential decay of the weights for baseline values greater than the data. Should be approximately the height at which a value could be considered a peak. Default is None, which sets k to one-tenth of the standard deviation of the input data. A large k value will produce similar results to asls().

diff_orderint, optional

The order of the differential matrix. Must be greater than 0. Default is 2 (second order differential matrix). Typical values are 2 or 1.

max_iterint, optional

The max number of fit iterations. Default is 50.

tolfloat, optional

The exit criteria. Default is 1e-3.

weightsarray_like, shape (N,), optional

The weighting array. If None (default), then the initial weights will be an array with size equal to N and all values set to 1.

Returns:

baselinenumpy.ndarray, shape (N,)

The calculated baseline.

paramsdict

A dictionary with the following items:

• 'weights': numpy.ndarray, shape (N,)

  The weight array used for fitting the data.

• 'tol_history': numpy.ndarray

  An array containing the calculated tolerance values for each iteration. The length of the array is the number of iterations completed. If the last value in the array is greater than the input tol value, then the function did not converge.

Raises:

ValueError

Raised if p is not between 0 and 1. Also raised if k is not greater than 0.

Notes

The exit criteria for the original algorithm was to check whether the signs of the residuals do not change between two iterations, but the comparison of the l2 norms of the weight arrays between iterations is used instead to be more comparable to other Whittaker-smoothing-based algorithms.

References

Oller-Moreno, S., et al. Adaptive Asymmetric Least Squares baseline estimation for analytical instruments. 2014 IEEE 11th International Multi-Conference on Systems, Signals, and Devices, 2014, 1-5.