@@ -128,6 +128,10 @@ class) can be interpreted as an (implicitly defined) matrix.
It can be applied to :class:`Field` instances, resulting in other :class:`Field`
instances that potentially live on other domains.
Operator basics
---------------
There are four basic ways of applying an operator :math:`A` to a field :math:`f`:
- direct multiplication: :math:`A\cdot f`
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@@ -179,6 +183,10 @@ Further operator classes provided by NIFTy are
unstructured ones. This is typically needed when building instrument response
operators.
Syntatic sugar
--------------
Nifty4 allows simple and intuitive construction of altered and combined
operators.
As an example, if ``A``, ``B`` and ``C`` are of type :class:`LinearOperator`
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@@ -201,16 +209,68 @@ were the original operator's adjoint or inverse, respectively.
Minimization
============
Most problems in IFT are solved by (possibly nested) minimizations of high-dimensional functions, which are often nonlinear.
Most problems in IFT are solved by (possibly nested) minimizations of
high-dimensional functions, which are often nonlinear.
Energy functionals
------------------
In NIFTy4 such functions are represented by objects of type :class:`Energy`.
These hold the prescription how to calculate the function's value, gradient and (optionally) curvature at any given position.
Function values are floating-point scalars, gradients have the form of fields living on the energy's position domain, and curvatures are represented by linear operator objects.
These hold the prescription how to calculate the function's
:attr:`~Energy.value`, :attr:`~Energy.gradient` and
(optionally) :attr:`~Energy.curvature` at any given position.
Function values are floating-point scalars, gradients have the form of fields
living on the energy's position domain, and curvatures are represented by
linear operator objects.
Some examples of concrete energy classes delivered with NIFTy4 are :class:`QuadraticEnergy` (with position-independent curvature, mainly used with conjugate gradient minimization) and :class:`WienerFilterEnergy`.
Energies are classes that typically have to be provided by the user when tackling new IFT problems.
The minimization procedure can be carried out by one of several algorithms; NIFTy4 currently ships solvers based on
Convergence control
-------------------
Iterative minimization of an energy reqires some means of
controlling the quality of the current solution estimate and stopping once
it is sufficiently accurate. In case of numerical problems, the iteration needs
to be terminated as well, returning a suitable error description.
In NIFTy4, this functionality is encapsulated in the abstract
:class:`IterationController` class, which is provided with the initial energy
object before starting the minimization, and is updated with the improved
energy after every iteration. Based on this information, it can either continue
the minimization or return the current estimate indicating convergence or
failure.
Sensible stopping criteria can vary significantly with the problem being
solved; NIFTy provides one concrete sub-class of :class:`IterationController`
called :class:`GradientNormController`, which should be appropriate in many
circumstances, but users have complete freedom to implement custom sub-classes
for their specific applications.
Minimization algorithms
-----------------------
It is important to realize that the machinery presented here cannot only be
used for minimizing IFT Hamiltonians, but also for the numerical inversion of
linear operators, if the desired application mode is not directly available.
A classical example is the information propagator
:math:`D = \left(R^\dagger N^{-1} R + S^{-1}\right)^{-1}`,
which must be applied when calculating a Wiener filter. Only its inverse
application is straightforward; to use it in forward direction, we make use
of NIFTy's :class:`InversionEnabler` class, which internally performs a
minimization of a :class:`QuadraticEnergy` by means of a
:class:`ConjugateGradient` algorithm.
Some examples of concrete energy classes delivered with NIFTy4 are
:class:`QuadraticEnergy` (with position-independent curvature, mainly used with
conjugate gradient minimization) and :class:`WienerFilterEnergy`.
Energies are classes that typically have to be provided by the user when
tackling new IFT problems.
The minimization procedure can be carried out by one of several algorithms;
NIFTy4 currently ships solvers based on
- the conjugate gradient method (for quadratic energies)