Trend computationΒΆ

In this example we are going to estimate a trend from a field.

We note (\underline{x}_0, \dots, \underline{x}_{N-1}) the values of the initial field associated to the mesh \mathcal{M} of \mathcal{D} \in \mathbb{R}^n, where \underline{x}_i \in \mathbb{R}^d and (\underline{x}^{stat}_0, \dots, \underline{x}^{stat}_{N-1}) the values of the resulting stationary field.

The object TrendFactory allows one to estimate a trend and is built from:

  • a regression strategy that generates a basis using the Least Angle Regression method thanks to the object LARS,

  • a fitting algorithm that estimates the empirical error on each sub-basis using the leave one out strategy, thanks to the object CorrectedLeaveOneOut or the k-fold algorithm thanks to the object KFold.

Then, the trend transformation is estimated from the initial field (\underline{x}_0, \dots, \underline{x}_{N-1}) and a function basis \mathcal{B} thanks to the method build of the object TrendFactory, which produces an object of type TrendTransform. This last object allows one to:

  • add the trend to a given field \underline{w}_0, \dots, \underline{w}_{N-1} defined on the same mesh \mathcal{M}: the resulting field shares the same mesh than the initial field. For example, it may be useful to add the trend to a realization of the stationary process X_{stat} in order to get a realization of the process X

  • get the function f_{trend} defined in that evaluates the trend thanks to the method getEvaluation();

  • create the inverse trend transformation in order to remove the trend the intiail field (\underline{x}_0, \dots, \underline{x}_{N-1}) and to create the resulting stationary field (\underline{x}^{stat}_0, \dots, \underline{x}^{stat}_{N-1}) such that:

\underline{x}^{stat}_i = \underline{x}_i - f_{trend}(\underline{t}_i)

where \underline{t}_i is the simplex associated to the value \underline{x}_i.

This creation of the inverse trend function -f_{trend} is done thanks to the method getInverse() which produces an object of type InverseTrendTransform that can be evaluated on a a field. For example, it may be useful in order to get the stationary field (\underline{x}^{stat}_0, \dots, \underline{x}^{stat}_{N-1}) and then analyze it with methods adapted to stationary processes (ARMA decomposition for example).

import openturns as ot


Define a bi dimensional mesh

myIndices = [40, 20]
myMesher = ot.IntervalMesher(myIndices)
lowerBound = [0.0, 0.0]
upperBound = [2.0, 1.0]
myInterval = ot.Interval(lowerBound, upperBound)
myMesh =

# Define a scalar temporal normal process on the mesh
# this process is stationary
amplitude = [1.0]
scale = [0.01] * 2
myCovModel = ot.ExponentialModel(scale, amplitude)
myXProcess = ot.GaussianProcess(myCovModel, myMesh)

# Create a trend function
# fTrend : R^2 --> R
#          (t,s) --> 1+2t+2s
fTrend = ot.SymbolicFunction(["t", "s"], ["1+2*t+2*s"])
fTemp = ot.TrendTransform(fTrend, myMesh)

# Add the trend to the initial process
myYProcess = ot.CompositeProcess(fTemp, myXProcess)

# Get a field from myYtProcess
myYField = myYProcess.getRealization()

CASE 1 : we estimate the trend from the field

# Define the regression stategy using the LAR method
myBasisSequenceFactory = ot.LARS()

# Define the fitting algorithm using the
# Corrected Leave One Out or KFold algorithms
myFittingAlgorithm = ot.CorrectedLeaveOneOut()
myFittingAlgorithm_2 = ot.KFold()

# Define the basis function
# For example composed of 5 functions
myFunctionBasis = list(
        lambda fst: ot.SymbolicFunction(["t", "s"], [fst]),
        ["1", "t", "s", "t^2", "s^2"],

# Define the trend function factory algorithm
myTrendFactory = ot.TrendFactory(myBasisSequenceFactory, myFittingAlgorithm)

# Create the trend transformation  of type TrendTransform
myTrendTransform =, ot.Basis(myFunctionBasis))

# Check the estimated trend function
print("Trend function = ", myTrendTransform)
Trend function =  1.67595 * ([t,s]->[1]) + 2.03881 * ([t,s]->[s]) + 0.939637 * ([t,s]->[t^2])

CASE 2 : we impose the trend (or its inverse)

# The function g computes the trend : R^2 -> R
# g :      R^2 --> R
#          (t,s) --> 1+2t+2s
g = ot.SymbolicFunction(["t", "s"], ["1+2*t+2*s"])
gTemp = ot.TrendTransform(g, myMesh)

# Get the inverse trend transformation
# from the trend transform already defined
myInverseTrendTransform = myTrendTransform.getInverse()
print("Inverse trend function = ", myInverseTrendTransform)

# Sometimes it is more useful to define
# the opposite trend h : R^2 -> R
# in fact h = -g
h = ot.SymbolicFunction(["t", "s"], ["-(1+2*t+2*s)"])
myInverseTrendTransform_2 = ot.InverseTrendTransform(h, myMesh)
Inverse trend function =  1.67595 * ([t,s]->[1]) + 2.03881 * ([t,s]->[s]) + 0.939637 * ([t,s]->[t^2])

Remove the trend from the field myYField myXField = myYField - f(t,s)

myXField2 = myTrendTransform.getInverse()(myYField)
# or from the class InverseTrendTransform
myXField3 = myInverseTrendTransform(myYField)

# Add the trend to the field myXField2
# myYField = f(t,s) + myXField2
myInitialYField = myTrendTransform(myXField2)

# Get the trend function f(t,s)
myEvaluation_f = myTrendTransform.getTrendFunction()

# Evaluate the trend function f at a particular vertex
# which is the lower corner of the mesh
myMesh = myYField.getMesh()
vertices = myMesh.getVertices()
vertex = vertices.getMin()
trend_t = myEvaluation_f(vertex)