env/ceres-solver/docs/source/nnls_solving.rst

.. default-domain:: cpp

.. highlight:: c++

.. cpp:namespace:: ceres

.. _chapter-nnls_solving:

================================
Solving Non-linear Least Squares
================================

Introduction
============

Effective use of Ceres requires some familiarity with the basic
components of a non-linear least squares solver, so before we describe
how to configure and use the solver, we will take a brief look at how
some of the core optimization algorithms in Ceres work.

Let :math:`x \in \mathbb{R}^n` be an :math:`n`-dimensional vector of
variables, and
:math:`F(x) = \left[f_1(x), ... ,  f_{m}(x) \right]^{\top}` be a
:math:`m`-dimensional function of :math:`x`.  We are interested in
solving the optimization problem [#f1]_

.. math:: \arg \min_x \frac{1}{2}\|F(x)\|^2\ . \\
          L \le x \le U
  :label: nonlinsq

Where, :math:`L` and :math:`U` are lower and upper bounds on the
parameter vector :math:`x`.

Since the efficient global minimization of :eq:`nonlinsq` for
general :math:`F(x)` is an intractable problem, we will have to settle
for finding a local minimum.

In the following, the Jacobian :math:`J(x)` of :math:`F(x)` is an
:math:`m\times n` matrix, where :math:`J_{ij}(x) = \partial_j f_i(x)`
and the gradient vector is :math:`g(x) = \nabla \frac{1}{2}\|F(x)\|^2
= J(x)^\top F(x)`.

The general strategy when solving non-linear optimization problems is
to solve a sequence of approximations to the original problem
[NocedalWright]_. At each iteration, the approximation is solved to
determine a correction :math:`\Delta x` to the vector :math:`x`. For
non-linear least squares, an approximation can be constructed by using
the linearization :math:`F(x+\Delta x) \approx F(x) + J(x)\Delta x`,
which leads to the following linear least squares problem:

.. math:: \min_{\Delta x} \frac{1}{2}\|J(x)\Delta x + F(x)\|^2
   :label: linearapprox

Unfortunately, naively solving a sequence of these problems and
updating :math:`x \leftarrow x+ \Delta x` leads to an algorithm that
may not converge.  To get a convergent algorithm, we need to control
the size of the step :math:`\Delta x`. Depending on how the size of
the step :math:`\Delta x` is controlled, non-linear optimization
algorithms can be divided into two major categories [NocedalWright]_.

1. **Trust Region** The trust region approach approximates the
   objective function using a model function (often a quadratic) over
   a subset of the search space known as the trust region. If the
   model function succeeds in minimizing the true objective function
   the trust region is expanded; conversely, otherwise it is
   contracted and the model optimization problem is solved again.

2. **Line Search** The line search approach first finds a descent
   direction along which the objective function will be reduced and
   then computes a step size that decides how far should move along
   that direction. The descent direction can be computed by various
   methods, such as gradient descent, Newton's method and Quasi-Newton
   method. The step size can be determined either exactly or
   inexactly.

Trust region methods are in some sense dual to line search methods:
trust region methods first choose a step size (the size of the trust
region) and then a step direction while line search methods first
choose a step direction and then a step size. Ceres implements
multiple algorithms in both categories.

.. _section-trust-region-methods:

Trust Region Methods
====================

The basic trust region algorithm looks something like this.

   1. Given an initial point :math:`x` and a trust region radius :math:`\mu`.
   2. Solve

      .. math::
         \arg \min_{\Delta x}& \frac{1}{2}\|J(x)\Delta x + F(x)\|^2 \\
         \text{such that} &\|D(x)\Delta x\|^2 \le \mu\\
         &L \le x + \Delta x \le U.

   3. :math:`\rho = \frac{\displaystyle \|F(x + \Delta x)\|^2 -
      \|F(x)\|^2}{\displaystyle \|J(x)\Delta x + F(x)\|^2 -
      \|F(x)\|^2}`
   4. if :math:`\rho > \epsilon` then  :math:`x = x + \Delta x`.
   5. if :math:`\rho > \eta_1` then :math:`\mu = 2  \mu`
   6. else if :math:`\rho < \eta_2` then :math:`\mu = 0.5 * \mu`
   7. Go to 2.

Here, :math:`\mu` is the trust region radius, :math:`D(x)` is some
matrix used to define a metric on the domain of :math:`F(x)` and
:math:`\rho` measures the quality of the step :math:`\Delta x`, i.e.,
how well did the linear model predict the decrease in the value of the
non-linear objective. The idea is to increase or decrease the radius
of the trust region depending on how well the linearization predicts
the behavior of the non-linear objective, which in turn is reflected
in the value of :math:`\rho`.

The key computational step in a trust-region algorithm is the solution
of the constrained optimization problem

.. math::
   \arg \min_{\Delta x}&\quad \frac{1}{2}\|J(x)\Delta x + F(x)\|^2 \\
   \text{such that} &\quad \|D(x)\Delta x\|^2 \le \mu\\
    &\quad L \le x + \Delta x \le U.
   :label: trp

There are a number of different ways of solving this problem, each
giving rise to a different concrete trust-region algorithm. Currently,
Ceres implements two trust-region algorithms - Levenberg-Marquardt
and Dogleg, each of which is augmented with a line search if bounds
constraints are present [Kanzow]_. The user can choose between them by
setting :member:`Solver::Options::trust_region_strategy_type`.

.. rubric:: Footnotes

.. [#f1] At the level of the non-linear solver, the block structure is
         not relevant, therefore our discussion here is in terms of an
         optimization problem defined over a state vector of size
         :math:`n`. Similarly the presence of loss functions is also
         ignored as the problem is internally converted into a pure
         non-linear least squares problem.


.. _section-levenberg-marquardt:

Levenberg-Marquardt
-------------------

The Levenberg-Marquardt algorithm [Levenberg]_  [Marquardt]_ is the
most popular algorithm for solving non-linear least squares problems.
It was also the first trust region algorithm to be developed
[Levenberg]_ [Marquardt]_. Ceres implements an exact step [Madsen]_
and an inexact step variant of the Levenberg-Marquardt algorithm
[WrightHolt]_ [NashSofer]_.

It can be shown, that the solution to :eq:`trp` can be obtained by
solving an unconstrained optimization of the form

.. math:: \arg\min_{\Delta x} \frac{1}{2}\|J(x)\Delta x + F(x)\|^2 +\lambda  \|D(x)\Delta x\|^2

Where, :math:`\lambda` is a Lagrange multiplier that is inverse
related to :math:`\mu`. In Ceres, we solve for

.. math:: \arg\min_{\Delta x} \frac{1}{2}\|J(x)\Delta x + F(x)\|^2 + \frac{1}{\mu} \|D(x)\Delta x\|^2
   :label: lsqr

The matrix :math:`D(x)` is a non-negative diagonal matrix, typically
the square root of the diagonal of the matrix :math:`J(x)^\top J(x)`.

Before going further, let us make some notational simplifications. We
will assume that the matrix :math:`\frac{1}{\sqrt{\mu}} D` has been concatenated
at the bottom of the matrix :math:`J` and similarly a vector of zeros
has been added to the bottom of the vector :math:`f` and the rest of
our discussion will be in terms of :math:`J` and :math:`F`, i.e, the
linear least squares problem.

.. math:: \min_{\Delta x} \frac{1}{2} \|J(x)\Delta x + F(x)\|^2 .
   :label: simple

For all but the smallest problems the solution of :eq:`simple` in
each iteration of the Levenberg-Marquardt algorithm is the dominant
computational cost in Ceres. Ceres provides a number of different
options for solving :eq:`simple`. There are two major classes of
methods - factorization and iterative.

The factorization methods are based on computing an exact solution of
:eq:`lsqr` using a Cholesky or a QR factorization and lead to an exact
step Levenberg-Marquardt algorithm. But it is not clear if an exact
solution of :eq:`lsqr` is necessary at each step of the LM algorithm
to solve :eq:`nonlinsq`. In fact, we have already seen evidence
that this may not be the case, as :eq:`lsqr` is itself a regularized
version of :eq:`linearapprox`. Indeed, it is possible to
construct non-linear optimization algorithms in which the linearized
problem is solved approximately. These algorithms are known as inexact
Newton or truncated Newton methods [NocedalWright]_.

An inexact Newton method requires two ingredients. First, a cheap
method for approximately solving systems of linear
equations. Typically an iterative linear solver like the Conjugate
Gradients method is used for this
purpose [NocedalWright]_. Second, a termination rule for
the iterative solver. A typical termination rule is of the form

.. math:: \|H(x) \Delta x + g(x)\| \leq \eta_k \|g(x)\|.
   :label: inexact

Here, :math:`k` indicates the Levenberg-Marquardt iteration number and
:math:`0 < \eta_k <1` is known as the forcing sequence.  [WrightHolt]_
prove that a truncated Levenberg-Marquardt algorithm that uses an
inexact Newton step based on :eq:`inexact` converges for any
sequence :math:`\eta_k \leq \eta_0 < 1` and the rate of convergence
depends on the choice of the forcing sequence :math:`\eta_k`.

Ceres supports both exact and inexact step solution strategies. When
the user chooses a factorization based linear solver, the exact step
Levenberg-Marquardt algorithm is used. When the user chooses an
iterative linear solver, the inexact step Levenberg-Marquardt
algorithm is used.

.. _section-dogleg:

Dogleg
------

Another strategy for solving the trust region problem :eq:`trp` was
introduced by M. J. D. Powell. The key idea there is to compute two
vectors

.. math::

        \Delta x^{\text{Gauss-Newton}} &= \arg \min_{\Delta x}\frac{1}{2} \|J(x)\Delta x + f(x)\|^2.\\
        \Delta x^{\text{Cauchy}} &= -\frac{\|g(x)\|^2}{\|J(x)g(x)\|^2}g(x).

Note that the vector :math:`\Delta x^{\text{Gauss-Newton}}` is the
solution to :eq:`linearapprox` and :math:`\Delta
x^{\text{Cauchy}}` is the vector that minimizes the linear
approximation if we restrict ourselves to moving along the direction
of the gradient. Dogleg methods finds a vector :math:`\Delta x`
defined by :math:`\Delta x^{\text{Gauss-Newton}}` and :math:`\Delta
x^{\text{Cauchy}}` that solves the trust region problem. Ceres
supports two variants that can be chose by setting
:member:`Solver::Options::dogleg_type`.

``TRADITIONAL_DOGLEG`` as described by Powell, constructs two line
segments using the Gauss-Newton and Cauchy vectors and finds the point
farthest along this line shaped like a dogleg (hence the name) that is
contained in the trust-region. For more details on the exact reasoning
and computations, please see Madsen et al [Madsen]_.

``SUBSPACE_DOGLEG`` is a more sophisticated method that considers the
entire two dimensional subspace spanned by these two vectors and finds
the point that minimizes the trust region problem in this subspace
[ByrdSchnabel]_.

The key advantage of the Dogleg over Levenberg-Marquardt is that if
the step computation for a particular choice of :math:`\mu` does not
result in sufficient decrease in the value of the objective function,
Levenberg-Marquardt solves the linear approximation from scratch with
a smaller value of :math:`\mu`. Dogleg on the other hand, only needs
to compute the interpolation between the Gauss-Newton and the Cauchy
vectors, as neither of them depend on the value of :math:`\mu`.

The Dogleg method can only be used with the exact factorization based
linear solvers.

.. _section-inner-iterations:

Inner Iterations
----------------

Some non-linear least squares problems have additional structure in
the way the parameter blocks interact that it is beneficial to modify
the way the trust region step is computed. For example, consider the
following regression problem

.. math::   y = a_1 e^{b_1 x} + a_2 e^{b_3 x^2 + c_1}


Given a set of pairs :math:`\{(x_i, y_i)\}`, the user wishes to estimate
:math:`a_1, a_2, b_1, b_2`, and :math:`c_1`.

Notice that the expression on the left is linear in :math:`a_1` and
:math:`a_2`, and given any value for :math:`b_1, b_2` and :math:`c_1`,
it is possible to use linear regression to estimate the optimal values
of :math:`a_1` and :math:`a_2`. It's possible to analytically
eliminate the variables :math:`a_1` and :math:`a_2` from the problem
entirely. Problems like these are known as separable least squares
problem and the most famous algorithm for solving them is the Variable
Projection algorithm invented by Golub & Pereyra [GolubPereyra]_.

Similar structure can be found in the matrix factorization with
missing data problem. There the corresponding algorithm is known as
Wiberg's algorithm [Wiberg]_.

Ruhe & Wedin present an analysis of various algorithms for solving
separable non-linear least squares problems and refer to *Variable
Projection* as Algorithm I in their paper [RuheWedin]_.

Implementing Variable Projection is tedious and expensive. Ruhe &
Wedin present a simpler algorithm with comparable convergence
properties, which they call Algorithm II.  Algorithm II performs an
additional optimization step to estimate :math:`a_1` and :math:`a_2`
exactly after computing a successful Newton step.


This idea can be generalized to cases where the residual is not
linear in :math:`a_1` and :math:`a_2`, i.e.,

.. math:: y = f_1(a_1, e^{b_1 x}) + f_2(a_2, e^{b_3 x^2 + c_1})

In this case, we solve for the trust region step for the full problem,
and then use it as the starting point to further optimize just `a_1`
and `a_2`. For the linear case, this amounts to doing a single linear
least squares solve. For non-linear problems, any method for solving
the :math:`a_1` and :math:`a_2` optimization problems will do. The
only constraint on :math:`a_1` and :math:`a_2` (if they are two
different parameter block) is that they do not co-occur in a residual
block.

This idea can be further generalized, by not just optimizing
:math:`(a_1, a_2)`, but decomposing the graph corresponding to the
Hessian matrix's sparsity structure into a collection of
non-overlapping independent sets and optimizing each of them.

Setting :member:`Solver::Options::use_inner_iterations` to ``true``
enables the use of this non-linear generalization of Ruhe & Wedin's
Algorithm II.  This version of Ceres has a higher iteration
complexity, but also displays better convergence behavior per
iteration.

Setting :member:`Solver::Options::num_threads` to the maximum number
possible is highly recommended.

.. _section-non-monotonic-steps:

Non-monotonic Steps
-------------------

Note that the basic trust-region algorithm described in
:ref:`section-trust-region-methods` is a descent algorithm in that it
only accepts a point if it strictly reduces the value of the objective
function.

Relaxing this requirement allows the algorithm to be more efficient in
the long term at the cost of some local increase in the value of the
objective function.

This is because allowing for non-decreasing objective function values
in a principled manner allows the algorithm to *jump over boulders* as
the method is not restricted to move into narrow valleys while
preserving its convergence properties.

Setting :member:`Solver::Options::use_nonmonotonic_steps` to ``true``
enables the non-monotonic trust region algorithm as described by Conn,
Gould & Toint in [Conn]_.

Even though the value of the objective function may be larger
than the minimum value encountered over the course of the
optimization, the final parameters returned to the user are the
ones corresponding to the minimum cost over all iterations.

The option to take non-monotonic steps is available for all trust
region strategies.


.. _section-line-search-methods:

Line Search Methods
===================

The line search method in Ceres Solver cannot handle bounds
constraints right now, so it can only be used for solving
unconstrained problems.

Line search algorithms

   1. Given an initial point :math:`x`
   2. :math:`\Delta x = -H^{-1}(x) g(x)`
   3. :math:`\arg \min_\mu \frac{1}{2} \| F(x + \mu \Delta x) \|^2`
   4. :math:`x = x + \mu \Delta x`
   5. Goto 2.

Here :math:`H(x)` is some approximation to the Hessian of the
objective function, and :math:`g(x)` is the gradient at
:math:`x`. Depending on the choice of :math:`H(x)` we get a variety of
different search directions :math:`\Delta x`.

Step 4, which is a one dimensional optimization or `Line Search` along
:math:`\Delta x` is what gives this class of methods its name.

Different line search algorithms differ in their choice of the search
direction :math:`\Delta x` and the method used for one dimensional
optimization along :math:`\Delta x`. The choice of :math:`H(x)` is the
primary source of computational complexity in these
methods. Currently, Ceres Solver supports three choices of search
directions, all aimed at large scale problems.

1. ``STEEPEST_DESCENT`` This corresponds to choosing :math:`H(x)` to
   be the identity matrix. This is not a good search direction for
   anything but the simplest of the problems. It is only included here
   for completeness.

2. ``NONLINEAR_CONJUGATE_GRADIENT`` A generalization of the Conjugate
   Gradient method to non-linear functions. The generalization can be
   performed in a number of different ways, resulting in a variety of
   search directions. Ceres Solver currently supports
   ``FLETCHER_REEVES``, ``POLAK_RIBIERE`` and ``HESTENES_STIEFEL``
   directions.

3. ``BFGS`` A generalization of the Secant method to multiple
   dimensions in which a full, dense approximation to the inverse
   Hessian is maintained and used to compute a quasi-Newton step
   [NocedalWright]_.  BFGS is currently the best known general
   quasi-Newton algorithm.

4. ``LBFGS`` A limited memory approximation to the full ``BFGS``
   method in which the last `M` iterations are used to approximate the
   inverse Hessian used to compute a quasi-Newton step [Nocedal]_,
   [ByrdNocedal]_.

Currently Ceres Solver supports both a backtracking and interpolation
based Armijo line search algorithm, and a sectioning / zoom
interpolation (strong) Wolfe condition line search algorithm.
However, note that in order for the assumptions underlying the
``BFGS`` and ``LBFGS`` methods to be guaranteed to be satisfied the
Wolfe line search algorithm should be used.

.. _section-linear-solver:

LinearSolver
============

Recall that in both of the trust-region methods described above, the
key computational cost is the solution of a linear least squares
problem of the form

.. math:: \min_{\Delta x} \frac{1}{2} \|J(x)\Delta x + f(x)\|^2 .
   :label: simple2

Let :math:`H(x)= J(x)^\top J(x)` and :math:`g(x) = -J(x)^\top
f(x)`. For notational convenience let us also drop the dependence on
:math:`x`. Then it is easy to see that solving :eq:`simple2` is
equivalent to solving the *normal equations*.

.. math:: H \Delta x = g
   :label: normal

Ceres provides a number of different options for solving :eq:`normal`.

.. _section-qr:

``DENSE_QR``
------------

For small problems (a couple of hundred parameters and a few thousand
residuals) with relatively dense Jacobians, ``DENSE_QR`` is the method
of choice [Bjorck]_. Let :math:`J = QR` be the QR-decomposition of
:math:`J`, where :math:`Q` is an orthonormal matrix and :math:`R` is
an upper triangular matrix [TrefethenBau]_. Then it can be shown that
the solution to :eq:`normal` is given by

.. math:: \Delta x^* = -R^{-1}Q^\top f


Ceres uses ``Eigen`` 's dense QR factorization routines.

.. _section-cholesky:

``DENSE_NORMAL_CHOLESKY`` & ``SPARSE_NORMAL_CHOLESKY``
------------------------------------------------------

Large non-linear least square problems are usually sparse. In such
cases, using a dense QR factorization is inefficient. Let :math:`H =
R^\top R` be the Cholesky factorization of the normal equations, where
:math:`R` is an upper triangular matrix, then the solution to
:eq:`normal` is given by

.. math::

    \Delta x^* = R^{-1} R^{-\top} g.


The observant reader will note that the :math:`R` in the Cholesky
factorization of :math:`H` is the same upper triangular matrix
:math:`R` in the QR factorization of :math:`J`. Since :math:`Q` is an
orthonormal matrix, :math:`J=QR` implies that :math:`J^\top J = R^\top
Q^\top Q R = R^\top R`. There are two variants of Cholesky
factorization -- sparse and dense.

``DENSE_NORMAL_CHOLESKY``  as the name implies performs a dense
Cholesky factorization of the normal equations. Ceres uses
``Eigen`` 's dense LDLT factorization routines.

``SPARSE_NORMAL_CHOLESKY``, as the name implies performs a sparse
Cholesky factorization of the normal equations. This leads to
substantial savings in time and memory for large sparse problems. The
use of this linear solver requires that Ceres is compiled with support
for at least one of:

 1. SuiteSparse
 2. Apple's Accelerate framework.
 3. Eigen's sparse linear solvers.

.. _section-cgnr:

``CGNR``
--------

For general sparse problems, if the problem is too large for
``CHOLMOD`` or a sparse linear algebra library is not linked into
Ceres, another option is the ``CGNR`` solver. This solver uses the
Conjugate Gradients solver on the *normal equations*, but without
forming the normal equations explicitly. It exploits the relation

.. math::
    H x = J^\top J x = J^\top(J x)

The convergence of Conjugate Gradients depends on the conditioner
number :math:`\kappa(H)`. Usually :math:`H` is poorly conditioned and
a :ref:`section-preconditioner` must be used to get reasonable
performance. Currently only the ``JACOBI`` preconditioner is available
for use with ``CGNR``. It uses the block diagonal of :math:`H` to
precondition the normal equations.

When the user chooses ``CGNR`` as the linear solver, Ceres
automatically switches from the exact step algorithm to an inexact
step algorithm.

.. _section-schur:

``DENSE_SCHUR`` & ``SPARSE_SCHUR``
----------------------------------

While it is possible to use ``SPARSE_NORMAL_CHOLESKY`` to solve bundle
adjustment problems, bundle adjustment problem have a special
structure, and a more efficient scheme for solving :eq:`normal`
can be constructed.

Suppose that the SfM problem consists of :math:`p` cameras and
:math:`q` points and the variable vector :math:`x` has the block
structure :math:`x = [y_{1}, ... ,y_{p},z_{1}, ... ,z_{q}]`. Where,
:math:`y` and :math:`z` correspond to camera and point parameters,
respectively.  Further, let the camera blocks be of size :math:`c` and
the point blocks be of size :math:`s` (for most problems :math:`c` =
:math:`6`--`9` and :math:`s = 3`). Ceres does not impose any constancy
requirement on these block sizes, but choosing them to be constant
simplifies the exposition.

A key characteristic of the bundle adjustment problem is that there is
no term :math:`f_{i}` that includes two or more point blocks.  This in
turn implies that the matrix :math:`H` is of the form

.. math:: H = \left[ \begin{matrix} B & E\\ E^\top & C \end{matrix} \right]\ ,
   :label: hblock

where :math:`B \in \mathbb{R}^{pc\times pc}` is a block sparse matrix
with :math:`p` blocks of size :math:`c\times c` and :math:`C \in
\mathbb{R}^{qs\times qs}` is a block diagonal matrix with :math:`q` blocks
of size :math:`s\times s`. :math:`E \in \mathbb{R}^{pc\times qs}` is a
general block sparse matrix, with a block of size :math:`c\times s`
for each observation. Let us now block partition :math:`\Delta x =
[\Delta y,\Delta z]` and :math:`g=[v,w]` to restate :eq:`normal`
as the block structured linear system

.. math:: \left[ \begin{matrix} B & E\\ E^\top & C \end{matrix}
                \right]\left[ \begin{matrix} \Delta y \\ \Delta z
                    \end{matrix} \right] = \left[ \begin{matrix} v\\ w
                    \end{matrix} \right]\ ,
   :label: linear2

and apply Gaussian elimination to it. As we noted above, :math:`C` is
a block diagonal matrix, with small diagonal blocks of size
:math:`s\times s`.  Thus, calculating the inverse of :math:`C` by
inverting each of these blocks is cheap. This allows us to eliminate
:math:`\Delta z` by observing that :math:`\Delta z = C^{-1}(w - E^\top
\Delta y)`, giving us

.. math:: \left[B - EC^{-1}E^\top\right] \Delta y = v - EC^{-1}w\ .
   :label: schur

The matrix

.. math:: S = B - EC^{-1}E^\top

is the Schur complement of :math:`C` in :math:`H`. It is also known as
the *reduced camera matrix*, because the only variables
participating in :eq:`schur` are the ones corresponding to the
cameras. :math:`S \in \mathbb{R}^{pc\times pc}` is a block structured
symmetric positive definite matrix, with blocks of size :math:`c\times
c`. The block :math:`S_{ij}` corresponding to the pair of images
:math:`i` and :math:`j` is non-zero if and only if the two images
observe at least one common point.


Now, :eq:`linear2` can be solved by first forming :math:`S`, solving for
:math:`\Delta y`, and then back-substituting :math:`\Delta y` to
obtain the value of :math:`\Delta z`.  Thus, the solution of what was
an :math:`n\times n`, :math:`n=pc+qs` linear system is reduced to the
inversion of the block diagonal matrix :math:`C`, a few matrix-matrix
and matrix-vector multiplies, and the solution of block sparse
:math:`pc\times pc` linear system :eq:`schur`.  For almost all
problems, the number of cameras is much smaller than the number of
points, :math:`p \ll q`, thus solving :eq:`schur` is
significantly cheaper than solving :eq:`linear2`. This is the
*Schur complement trick* [Brown]_.

This still leaves open the question of solving :eq:`schur`. The
method of choice for solving symmetric positive definite systems
exactly is via the Cholesky factorization [TrefethenBau]_ and
depending upon the structure of the matrix, there are, in general, two
options. The first is direct factorization, where we store and factor
:math:`S` as a dense matrix [TrefethenBau]_. This method has
:math:`O(p^2)` space complexity and :math:`O(p^3)` time complexity and
is only practical for problems with up to a few hundred cameras. Ceres
implements this strategy as the ``DENSE_SCHUR`` solver.


But, :math:`S` is typically a fairly sparse matrix, as most images
only see a small fraction of the scene. This leads us to the second
option: Sparse Direct Methods. These methods store :math:`S` as a
sparse matrix, use row and column re-ordering algorithms to maximize
the sparsity of the Cholesky decomposition, and focus their compute
effort on the non-zero part of the factorization [Chen]_. Sparse
direct methods, depending on the exact sparsity structure of the Schur
complement, allow bundle adjustment algorithms to significantly scale
up over those based on dense factorization. Ceres implements this
strategy as the ``SPARSE_SCHUR`` solver.

.. _section-iterative_schur:

``ITERATIVE_SCHUR``
-------------------

Another option for bundle adjustment problems is to apply
Preconditioned Conjugate Gradients to the reduced camera matrix
:math:`S` instead of :math:`H`. One reason to do this is that
:math:`S` is a much smaller matrix than :math:`H`, but more
importantly, it can be shown that :math:`\kappa(S)\leq \kappa(H)`.
Ceres implements Conjugate Gradients on :math:`S` as the
``ITERATIVE_SCHUR`` solver. When the user chooses ``ITERATIVE_SCHUR``
as the linear solver, Ceres automatically switches from the exact step
algorithm to an inexact step algorithm.

The key computational operation when using Conjuagate Gradients is the
evaluation of the matrix vector product :math:`Sx` for an arbitrary
vector :math:`x`. There are two ways in which this product can be
evaluated, and this can be controlled using
``Solver::Options::use_explicit_schur_complement``. Depending on the
problem at hand, the performance difference between these two methods
can be quite substantial.

  1. **Implicit** This is default. Implicit evaluation is suitable for
     large problems where the cost of computing and storing the Schur
     Complement :math:`S` is prohibitive. Because PCG only needs
     access to :math:`S` via its product with a vector, one way to
     evaluate :math:`Sx` is to observe that

     .. math::  x_1 &= E^\top x\\
                x_2 &= C^{-1} x_1\\
                x_3 &= Ex_2\\
                x_4 &= Bx\\
                Sx &= x_4 - x_3
        :label: schurtrick1

     Thus, we can run PCG on :math:`S` with the same computational
     effort per iteration as PCG on :math:`H`, while reaping the
     benefits of a more powerful preconditioner. In fact, we do not
     even need to compute :math:`H`, :eq:`schurtrick1` can be
     implemented using just the columns of :math:`J`.

     Equation :eq:`schurtrick1` is closely related to *Domain
     Decomposition methods* for solving large linear systems that
     arise in structural engineering and partial differential
     equations. In the language of Domain Decomposition, each point in
     a bundle adjustment problem is a domain, and the cameras form the
     interface between these domains. The iterative solution of the
     Schur complement then falls within the sub-category of techniques
     known as Iterative Sub-structuring [Saad]_ [Mathew]_.

  2. **Explicit** The complexity of implicit matrix-vector product
     evaluation scales with the number of non-zeros in the
     Jacobian. For small to medium sized problems, the cost of
     constructing the Schur Complement is small enough that it is
     better to construct it explicitly in memory and use it to
     evaluate the product :math:`Sx`.

When the user chooses ``ITERATIVE_SCHUR`` as the linear solver, Ceres
automatically switches from the exact step algorithm to an inexact
step algorithm.

  .. NOTE::

     In exact arithmetic, the choice of implicit versus explicit Schur
     complement would have no impact on solution quality. However, in
     practice if the Jacobian is poorly conditioned, one may observe
     (usually small) differences in solution quality. This is a
     natural consequence of performing computations in finite arithmetic.


.. _section-preconditioner:

Preconditioner
==============

The convergence rate of Conjugate Gradients for
solving :eq:`normal` depends on the distribution of eigenvalues
of :math:`H` [Saad]_. A useful upper bound is
:math:`\sqrt{\kappa(H)}`, where, :math:`\kappa(H)` is the condition
number of the matrix :math:`H`. For most bundle adjustment problems,
:math:`\kappa(H)` is high and a direct application of Conjugate
Gradients to :eq:`normal` results in extremely poor performance.

The solution to this problem is to replace :eq:`normal` with a
*preconditioned* system.  Given a linear system, :math:`Ax =b` and a
preconditioner :math:`M` the preconditioned system is given by
:math:`M^{-1}Ax = M^{-1}b`. The resulting algorithm is known as
Preconditioned Conjugate Gradients algorithm (PCG) and its worst case
complexity now depends on the condition number of the *preconditioned*
matrix :math:`\kappa(M^{-1}A)`.

The computational cost of using a preconditioner :math:`M` is the cost
of computing :math:`M` and evaluating the product :math:`M^{-1}y` for
arbitrary vectors :math:`y`. Thus, there are two competing factors to
consider: How much of :math:`H`'s structure is captured by :math:`M`
so that the condition number :math:`\kappa(HM^{-1})` is low, and the
computational cost of constructing and using :math:`M`.  The ideal
preconditioner would be one for which :math:`\kappa(M^{-1}A)
=1`. :math:`M=A` achieves this, but it is not a practical choice, as
applying this preconditioner would require solving a linear system
equivalent to the unpreconditioned problem.  It is usually the case
that the more information :math:`M` has about :math:`H`, the more
expensive it is use. For example, Incomplete Cholesky factorization
based preconditioners have much better convergence behavior than the
Jacobi preconditioner, but are also much more expensive.

For a survey of the state of the art in preconditioning linear least
squares problems with general sparsity structure see [GouldScott]_.

Ceres Solver comes with an number of preconditioners suited for
problems with general sparsity as well as the special sparsity
structure encountered in bundle adjustment problems.

``JACOBI``
----------

The simplest of all preconditioners is the diagonal or Jacobi
preconditioner, i.e., :math:`M=\operatorname{diag}(A)`, which for
block structured matrices like :math:`H` can be generalized to the
block Jacobi preconditioner. The ``JACOBI`` preconditioner in Ceres
when used with :ref:`section-cgnr` refers to the block diagonal of
:math:`H` and when used with :ref:`section-iterative_schur` refers to
the block diagonal of :math:`B` [Mandel]_. For detailed performance
data about the performance of ``JACOBI`` on bundle adjustment problems
see [Agarwal]_.


``SCHUR_JACOBI``
----------------

Another obvious choice for :ref:`section-iterative_schur` is the block
diagonal of the Schur complement matrix :math:`S`, i.e, the block
Jacobi preconditioner for :math:`S`. In Ceres we refer to it as the
``SCHUR_JACOBI`` preconditioner.  For detailed performance data about
the performance of ``SCHUR_JACOBI`` on bundle adjustment problems see
[Agarwal]_.


``CLUSTER_JACOBI`` and ``CLUSTER_TRIDIAGONAL``
----------------------------------------------

For bundle adjustment problems arising in reconstruction from
community photo collections, more effective preconditioners can be
constructed by analyzing and exploiting the camera-point visibility
structure of the scene.

The key idea is to cluster the cameras based on the visibility
structure of the scene. The similarity between a pair of cameras
:math:`i` and :math:`j` is given by:

  .. math:: S_{ij} = \frac{|V_i \cap V_j|}{|V_i| |V_j|}

Here :math:`V_i` is the set of scene points visible in camera
:math:`i`. This idea was first exploited by [KushalAgarwal]_ to create
the ``CLUSTER_JACOBI`` and the ``CLUSTER_TRIDIAGONAL`` preconditioners
which Ceres implements.

The performance of these two preconditioners depends on the speed and
clustering quality of the clustering algorithm used when building the
preconditioner. In the original paper, [KushalAgarwal]_ used the
Canonical Views algorithm [Simon]_, which while producing high quality
clusterings can be quite expensive for large graphs. So, Ceres
supports two visibility clustering algorithms - ``CANONICAL_VIEWS``
and ``SINGLE_LINKAGE``. The former is as the name implies Canonical
Views algorithm of [Simon]_. The latter is the the classic `Single
Linkage Clustering
<https://en.wikipedia.org/wiki/Single-linkage_clustering>`_
algorithm. The choice of clustering algorithm is controlled by
:member:`Solver::Options::visibility_clustering_type`.

``SUBSET``
----------

This is a  preconditioner for problems with general  sparsity. Given a
subset  of residual  blocks of  a problem,  it uses  the corresponding
subset  of the  rows of  the  Jacobian to  construct a  preconditioner
[Dellaert]_.

Suppose the Jacobian :math:`J` has been horizontally partitioned as

  .. math:: J = \begin{bmatrix} P \\ Q \end{bmatrix}

Where, :math:`Q` is the set of rows corresponding to the residual
blocks in
:member:`Solver::Options::residual_blocks_for_subset_preconditioner`. The
preconditioner is the matrix :math:`(Q^\top Q)^{-1}`.

The efficacy of the preconditioner depends on how well the matrix
:math:`Q` approximates :math:`J^\top J`, or how well the chosen
residual blocks approximate the full problem.


.. _section-ordering:

Ordering
========

The order in which variables are eliminated in a linear solver can
have a significant of impact on the efficiency and accuracy of the
method. For example when doing sparse Cholesky factorization, there
are matrices for which a good ordering will give a Cholesky factor
with :math:`O(n)` storage, whereas a bad ordering will result in an
completely dense factor.

Ceres allows the user to provide varying amounts of hints to the
solver about the variable elimination ordering to use. This can range
from no hints, where the solver is free to decide the best possible
ordering based on the user's choices like the linear solver being
used, to an exact order in which the variables should be eliminated,
and a variety of possibilities in between.

Instances of the :class:`ParameterBlockOrdering` class are used to
communicate this information to Ceres.

Formally an ordering is an ordered partitioning of the
parameter blocks, i.e, each parameter block belongs to exactly
one group, and each group has a unique non-negative integer
associated with it, that determines its order in the set of
groups.


e.g. Consider the linear system

.. math::
   x + y &= 3 \\
   2x + 3y &= 7

There are two ways in which it can be solved. First eliminating
:math:`x` from the two equations, solving for :math:`y` and then back
substituting for :math:`x`, or first eliminating :math:`y`, solving
for :math:`x` and back substituting for :math:`y`. The user can
construct three orderings here.

1. :math:`\{0: x\}, \{1: y\}` - eliminate :math:`x` first.
2. :math:`\{0: y\}, \{1: x\}` - eliminate :math:`y` first.
3. :math:`\{0: x, y\}` - Solver gets to decide the elimination order.

Thus, to have Ceres determine the ordering automatically, put all the
variables in group 0 and to control the ordering for every variable,
create groups :math:`0 \dots N-1`, one per variable, in the desired
order.

``linear_solver_ordering == nullptr`` and an ordering where all the
parameter blocks are in one elimination group mean the same thing -
the solver is free to choose what it thinks is the best elimination
ordering. Therefore in the following we will only consider the case
where ``linear_solver_ordering != nullptr``.

The exact interpretation of the ``linear_solver_ordering depeneds`` on
the values of ``Solver::Options::linear_solver_ordering_type``,
``Solver::Options::linear_solver_type``,
``Solver::Options::preconditioner_type`` and
``Solver::Options::sparse_linear_algebra_type`` as we will explain
below.

Bundle Adjustment
-----------------

If the user is using one of the Schur solvers (DENSE_SCHUR,
SPARSE_SCHUR, ITERATIVE_SCHUR) and chooses to specify an ordering, it
must have one important property. The lowest numbered elimination
group must form an independent set in the graph corresponding to the
Hessian, or in other words, no two parameter blocks in the first
elimination group should co-occur in the same residual block. For the
best performance, this elimination group should be as large as
possible. For standard bundle adjustment problems, this corresponds to
the first elimination group containing all the 3d points, and the
second containing the parameter blocks for all the cameras.

If the user leaves the choice to Ceres, then the solver uses an
approximate maximum independent set algorithm to identify the first
elimination group [LiSaad]_.

sparse_linear_algebra_library_type = SUITE_SPARSE
-------------------------------------------------

**linear_solver_ordering_type = AMD**

A constrained Approximate Minimum Degree (CAMD) ordering used where
the parameter blocks in the lowest numbered group are eliminated
first, and then the parameter blocks in the next lowest numbered group
and so on. Within each group, CAMD is free to order the parameter blocks
as it chooses.

**linear_solver_ordering_type = NESDIS**

a. ``linear_solver_type = SPARSE_NORMAL_CHOLESKY`` or
   ``linear_solver_type = CGNR`` and ``preconditioner_type = SUBSET``

   The value of linear_solver_ordering is ignored and a Nested
   Dissection algorithm is used to compute a fill reducing ordering.

b. ``linear_solver_type = SPARSE_SCHUR/DENSE_SCHUR/ITERATIVE_SCHUR``

   ONLY the lowest group are used to compute the Schur complement, and
   Nested Dissection is used to compute a fill reducing ordering for
   the Schur Complement (or its preconditioner).

sparse_linear_algebra_library_type = EIGEN_SPARSE or ACCELERATE_SPARSE
----------------------------------------------------------------------

a. ``linear_solver_type = SPARSE_NORMAL_CHOLESKY`` or
   ``linear_solver_type = CGNR`` and ``preconditioner_type = SUBSET``

   The value of linear_solver_ordering is ignored and ``AMD`` or
   ``NESDIS`` is used to compute a fill reducing ordering as requested
   by the user.

b. ``linear_solver_type = SPARSE_SCHUR/DENSE_SCHUR/ITERATIVE_SCHUR``

   ONLY the lowest group are used to compute the Schur complement, and
   ``AMD`` or ``NESID`` is used to compute a fill reducing ordering
   for the Schur Complement (or its preconditioner).

.. _section-solver-options:

:class:`Solver::Options`
========================

.. class:: Solver::Options

   :class:`Solver::Options` controls the overall behavior of the
   solver. We list the various settings and their default values below.

.. function:: bool Solver::Options::IsValid(string* error) const

   Validate the values in the options struct and returns true on
   success. If there is a problem, the method returns false with
   ``error`` containing a textual description of the cause.

.. member:: MinimizerType Solver::Options::minimizer_type

   Default: ``TRUST_REGION``

   Choose between ``LINE_SEARCH`` and ``TRUST_REGION`` algorithms. See
   :ref:`section-trust-region-methods` and
   :ref:`section-line-search-methods` for more details.

.. member:: LineSearchDirectionType Solver::Options::line_search_direction_type

   Default: ``LBFGS``

   Choices are ``STEEPEST_DESCENT``, ``NONLINEAR_CONJUGATE_GRADIENT``,
   ``BFGS`` and ``LBFGS``.

.. member:: LineSearchType Solver::Options::line_search_type

   Default: ``WOLFE``

   Choices are ``ARMIJO`` and ``WOLFE`` (strong Wolfe conditions).
   Note that in order for the assumptions underlying the ``BFGS`` and
   ``LBFGS`` line search direction algorithms to be guaranteed to be
   satisfied, the ``WOLFE`` line search should be used.

.. member:: NonlinearConjugateGradientType Solver::Options::nonlinear_conjugate_gradient_type

   Default: ``FLETCHER_REEVES``

   Choices are ``FLETCHER_REEVES``, ``POLAK_RIBIERE`` and
   ``HESTENES_STIEFEL``.

.. member:: int Solver::Options::max_lbfgs_rank

   Default: 20

   The L-BFGS hessian approximation is a low rank approximation to the
   inverse of the Hessian matrix. The rank of the approximation
   determines (linearly) the space and time complexity of using the
   approximation. Higher the rank, the better is the quality of the
   approximation. The increase in quality is however is bounded for a
   number of reasons.

     1. The method only uses secant information and not actual
        derivatives.

     2. The Hessian approximation is constrained to be positive
        definite.

   So increasing this rank to a large number will cost time and space
   complexity without the corresponding increase in solution
   quality. There are no hard and fast rules for choosing the maximum
   rank. The best choice usually requires some problem specific
   experimentation.

.. member:: bool Solver::Options::use_approximate_eigenvalue_bfgs_scaling

   Default: ``false``

   As part of the ``BFGS`` update step / ``LBFGS`` right-multiply
   step, the initial inverse Hessian approximation is taken to be the
   Identity.  However, [Oren]_ showed that using instead :math:`I *
   \gamma`, where :math:`\gamma` is a scalar chosen to approximate an
   eigenvalue of the true inverse Hessian can result in improved
   convergence in a wide variety of cases.  Setting
   ``use_approximate_eigenvalue_bfgs_scaling`` to true enables this
   scaling in ``BFGS`` (before first iteration) and ``LBFGS`` (at each
   iteration).

   Precisely, approximate eigenvalue scaling equates to

   .. math:: \gamma = \frac{y_k' s_k}{y_k' y_k}

   With:

  .. math:: y_k = \nabla f_{k+1} - \nabla f_k
  .. math:: s_k = x_{k+1} - x_k

  Where :math:`f()` is the line search objective and :math:`x` the
  vector of parameter values [NocedalWright]_.

  It is important to note that approximate eigenvalue scaling does
  **not** *always* improve convergence, and that it can in fact
  *significantly* degrade performance for certain classes of problem,
  which is why it is disabled by default.  In particular it can
  degrade performance when the sensitivity of the problem to different
  parameters varies significantly, as in this case a single scalar
  factor fails to capture this variation and detrimentally downscales
  parts of the Jacobian approximation which correspond to
  low-sensitivity parameters. It can also reduce the robustness of the
  solution to errors in the Jacobians.

.. member:: LineSearchIterpolationType Solver::Options::line_search_interpolation_type

   Default: ``CUBIC``

   Degree of the polynomial used to approximate the objective
   function. Valid values are ``BISECTION``, ``QUADRATIC`` and
   ``CUBIC``.

.. member:: double Solver::Options::min_line_search_step_size

   The line search terminates if:

   .. math:: \|\Delta x_k\|_\infty < \text{min_line_search_step_size}

   where :math:`\|\cdot\|_\infty` refers to the max norm, and
   :math:`\Delta x_k` is the step change in the parameter values at
   the :math:`k`-th iteration.

.. member:: double Solver::Options::line_search_sufficient_function_decrease

   Default: ``1e-4``

   Solving the line search problem exactly is computationally
   prohibitive. Fortunately, line search based optimization algorithms
   can still guarantee convergence if instead of an exact solution,
   the line search algorithm returns a solution which decreases the
   value of the objective function sufficiently. More precisely, we
   are looking for a step size s.t.

   .. math:: f(\text{step_size}) \le f(0) + \text{sufficient_decrease} * [f'(0) * \text{step_size}]

   This condition is known as the Armijo condition.

.. member:: double Solver::Options::max_line_search_step_contraction

   Default: ``1e-3``

   In each iteration of the line search,

   .. math:: \text{new_step_size} >= \text{max_line_search_step_contraction} * \text{step_size}

   Note that by definition, for contraction:

   .. math:: 0 < \text{max_step_contraction} < \text{min_step_contraction} < 1

.. member:: double Solver::Options::min_line_search_step_contraction

   Default: ``0.6``

   In each iteration of the line search,

   .. math:: \text{new_step_size} <= \text{min_line_search_step_contraction} * \text{step_size}

   Note that by definition, for contraction:

   .. math:: 0 < \text{max_step_contraction} < \text{min_step_contraction} < 1

.. member:: int Solver::Options::max_num_line_search_step_size_iterations

   Default: ``20``

   Maximum number of trial step size iterations during each line
   search, if a step size satisfying the search conditions cannot be
   found within this number of trials, the line search will stop.

   The minimum allowed value is 0 for trust region minimizer and 1
   otherwise. If 0 is specified for the trust region minimizer, then
   line search will not be used when solving constrained optimization
   problems.

   As this is an 'artificial' constraint (one imposed by the user, not
   the underlying math), if ``WOLFE`` line search is being used, *and*
   points satisfying the Armijo sufficient (function) decrease
   condition have been found during the current search (in :math:`<=`
   ``max_num_line_search_step_size_iterations``).  Then, the step size
   with the lowest function value which satisfies the Armijo condition
   will be returned as the new valid step, even though it does *not*
   satisfy the strong Wolfe conditions.  This behaviour protects
   against early termination of the optimizer at a sub-optimal point.

.. member:: int Solver::Options::max_num_line_search_direction_restarts

   Default: ``5``

   Maximum number of restarts of the line search direction algorithm
   before terminating the optimization. Restarts of the line search
   direction algorithm occur when the current algorithm fails to
   produce a new descent direction. This typically indicates a
   numerical failure, or a breakdown in the validity of the
   approximations used.

.. member:: double Solver::Options::line_search_sufficient_curvature_decrease

   Default: ``0.9``

   The strong Wolfe conditions consist of the Armijo sufficient
   decrease condition, and an additional requirement that the
   step size be chosen s.t. the *magnitude* ('strong' Wolfe
   conditions) of the gradient along the search direction
   decreases sufficiently. Precisely, this second condition
   is that we seek a step size s.t.

   .. math:: \|f'(\text{step_size})\| <= \text{sufficient_curvature_decrease} * \|f'(0)\|

   Where :math:`f()` is the line search objective and :math:`f'()` is the derivative
   of :math:`f` with respect to the step size: :math:`\frac{d f}{d~\text{step size}}`.

.. member:: double Solver::Options::max_line_search_step_expansion

   Default: ``10.0``

   During the bracketing phase of a Wolfe line search, the step size
   is increased until either a point satisfying the Wolfe conditions
   is found, or an upper bound for a bracket containing a point
   satisfying the conditions is found.  Precisely, at each iteration
   of the expansion:

   .. math:: \text{new_step_size} <= \text{max_step_expansion} * \text{step_size}

   By definition for expansion

   .. math:: \text{max_step_expansion} > 1.0

.. member:: TrustRegionStrategyType Solver::Options::trust_region_strategy_type

   Default: ``LEVENBERG_MARQUARDT``

   The trust region step computation algorithm used by
   Ceres. Currently ``LEVENBERG_MARQUARDT`` and ``DOGLEG`` are the two
   valid choices. See :ref:`section-levenberg-marquardt` and
   :ref:`section-dogleg` for more details.

.. member:: DoglegType Solver::Options::dogleg_type

   Default: ``TRADITIONAL_DOGLEG``

   Ceres supports two different dogleg strategies.
   ``TRADITIONAL_DOGLEG`` method by Powell and the ``SUBSPACE_DOGLEG``
   method described by [ByrdSchnabel]_ .  See :ref:`section-dogleg`
   for more details.

.. member:: bool Solver::Options::use_nonmonotonic_steps

   Default: ``false``

   Relax the requirement that the trust-region algorithm take strictly
   decreasing steps. See :ref:`section-non-monotonic-steps` for more
   details.

.. member:: int Solver::Options::max_consecutive_nonmonotonic_steps

   Default: ``5``

   The window size used by the step selection algorithm to accept
   non-monotonic steps.

.. member:: int Solver::Options::max_num_iterations

   Default: ``50``

   Maximum number of iterations for which the solver should run.

.. member:: double Solver::Options::max_solver_time_in_seconds

   Default: ``1e6``
   Maximum amount of time for which the solver should run.

.. member:: int Solver::Options::num_threads

   Default: ``1``

   Number of threads used by Ceres to evaluate the Jacobian.

.. member::  double Solver::Options::initial_trust_region_radius

   Default: ``1e4``

   The size of the initial trust region. When the
   ``LEVENBERG_MARQUARDT`` strategy is used, the reciprocal of this
   number is the initial regularization parameter.

.. member:: double Solver::Options::max_trust_region_radius

   Default: ``1e16``

   The trust region radius is not allowed to grow beyond this value.

.. member:: double Solver::Options::min_trust_region_radius

   Default: ``1e-32``

   The solver terminates, when the trust region becomes smaller than
   this value.

.. member:: double Solver::Options::min_relative_decrease

   Default: ``1e-3``

   Lower threshold for relative decrease before a trust-region step is
   accepted.

.. member:: double Solver::Options::min_lm_diagonal

   Default: ``1e-6``

   The ``LEVENBERG_MARQUARDT`` strategy, uses a diagonal matrix to
   regularize the trust region step. This is the lower bound on
   the values of this diagonal matrix.

.. member:: double Solver::Options::max_lm_diagonal

   Default:  ``1e32``

   The ``LEVENBERG_MARQUARDT`` strategy, uses a diagonal matrix to
   regularize the trust region step. This is the upper bound on
   the values of this diagonal matrix.

.. member:: int Solver::Options::max_num_consecutive_invalid_steps

   Default: ``5``

   The step returned by a trust region strategy can sometimes be
   numerically invalid, usually because of conditioning
   issues. Instead of crashing or stopping the optimization, the
   optimizer can go ahead and try solving with a smaller trust
   region/better conditioned problem. This parameter sets the number
   of consecutive retries before the minimizer gives up.

.. member:: double Solver::Options::function_tolerance

   Default: ``1e-6``

   Solver terminates if

   .. math:: \frac{|\Delta \text{cost}|}{\text{cost}} <= \text{function_tolerance}

   where, :math:`\Delta \text{cost}` is the change in objective
   function value (up or down) in the current iteration of
   Levenberg-Marquardt.

.. member:: double Solver::Options::gradient_tolerance

   Default: ``1e-10``

   Solver terminates if

   .. math:: \|x - \Pi \boxplus(x, -g(x))\|_\infty <= \text{gradient_tolerance}

   where :math:`\|\cdot\|_\infty` refers to the max norm, :math:`\Pi`
   is projection onto the bounds constraints and :math:`\boxplus` is
   Plus operation for the overall manifold associated with the
   parameter vector.

.. member:: double Solver::Options::parameter_tolerance

   Default: ``1e-8``

   Solver terminates if

   .. math:: \|\Delta x\| <= (\|x\| + \text{parameter_tolerance}) * \text{parameter_tolerance}

   where :math:`\Delta x` is the step computed by the linear solver in
   the current iteration.

.. member:: LinearSolverType Solver::Options::linear_solver_type

   Default: ``SPARSE_NORMAL_CHOLESKY`` / ``DENSE_QR``

   Type of linear solver used to compute the solution to the linear
   least squares problem in each iteration of the Levenberg-Marquardt
   algorithm. If Ceres is built with support for ``SuiteSparse`` or
   ``Accelerate`` or ``Eigen``'s sparse Cholesky factorization, the
   default is ``SPARSE_NORMAL_CHOLESKY``, it is ``DENSE_QR``
   otherwise.

.. member:: PreconditionerType Solver::Options::preconditioner_type

   Default: ``JACOBI``

   The preconditioner used by the iterative linear solver. The default
   is the block Jacobi preconditioner. Valid values are (in increasing
   order of complexity) ``IDENTITY``, ``JACOBI``, ``SCHUR_JACOBI``,
   ``CLUSTER_JACOBI`` and ``CLUSTER_TRIDIAGONAL``. See
   :ref:`section-preconditioner` for more details.

.. member:: VisibilityClusteringType Solver::Options::visibility_clustering_type

   Default: ``CANONICAL_VIEWS``

   Type of clustering algorithm to use when constructing a visibility
   based preconditioner. The original visibility based preconditioning
   paper and implementation only used the canonical views algorithm.

   This algorithm gives high quality results but for large dense
   graphs can be particularly expensive. As its worst case complexity
   is cubic in size of the graph.

   Another option is to use ``SINGLE_LINKAGE`` which is a simple
   thresholded single linkage clustering algorithm that only pays
   attention to tightly coupled blocks in the Schur complement. This
   is a fast algorithm that works well.

   The optimal choice of the clustering algorithm depends on the
   sparsity structure of the problem, but generally speaking we
   recommend that you try ``CANONICAL_VIEWS`` first and if it is too
   expensive try ``SINGLE_LINKAGE``.

.. member:: std::unordered_set<ResidualBlockId> residual_blocks_for_subset_preconditioner

   ``SUBSET`` preconditioner is a preconditioner for problems with
   general sparsity. Given a subset of residual blocks of a problem,
   it uses the corresponding subset of the rows of the Jacobian to
   construct a preconditioner.

   Suppose the Jacobian :math:`J` has been horizontally partitioned as

       .. math:: J = \begin{bmatrix} P \\ Q \end{bmatrix}

   Where, :math:`Q` is the set of rows corresponding to the residual
   blocks in
   :member:`Solver::Options::residual_blocks_for_subset_preconditioner`. The
   preconditioner is the matrix :math:`(Q^\top Q)^{-1}`.

   The efficacy of the preconditioner depends on how well the matrix
   :math:`Q` approximates :math:`J^\top J`, or how well the chosen
   residual blocks approximate the full problem.

   If ``Solver::Options::preconditioner_type == SUBSET``, then
   ``residual_blocks_for_subset_preconditioner`` must be non-empty.

.. member:: DenseLinearAlgebraLibrary Solver::Options::dense_linear_algebra_library_type

   Default:``EIGEN``

   Ceres supports using multiple dense linear algebra libraries for
   dense matrix factorizations. Currently ``EIGEN``, ``LAPACK`` and
   ``CUDA`` are the valid choices. ``EIGEN`` is always available,
   ``LAPACK`` refers to the system ``BLAS + LAPACK`` library which may
   or may not be available. ``CUDA`` refers to Nvidia's GPU based
   dense linear algebra library which may or may not be available.

   This setting affects the ``DENSE_QR``, ``DENSE_NORMAL_CHOLESKY``
   and ``DENSE_SCHUR`` solvers. For small to moderate sized problem
   ``EIGEN`` is a fine choice but for large problems, an optimized
   ``LAPACK + BLAS`` or ``CUDA`` implementation can make a substantial
   difference in performance.

.. member:: SparseLinearAlgebraLibrary Solver::Options::sparse_linear_algebra_library_type

   Default: The highest available according to: ``SUITE_SPARSE`` >
   ``ACCELERATE_SPARSE`` > ``EIGEN_SPARSE`` > ``NO_SPARSE``

   Ceres supports the use of three sparse linear algebra libraries,
   ``SuiteSparse``, which is enabled by setting this parameter to
   ``SUITE_SPARSE``, ``Acclerate``, which can be selected by setting
   this parameter to ``ACCELERATE_SPARSE`` and ``Eigen`` which is
   enabled by setting this parameter to ``EIGEN_SPARSE``.  Lastly,
   ``NO_SPARSE`` means that no sparse linear solver should be used;
   note that this is irrespective of whether Ceres was compiled with
   support for one.

   ``SuiteSparse`` is a sophisticated sparse linear algebra library
   and should be used in general. On MacOS you may want to use the
   ``Accelerate`` framework.

   If your needs/platforms prevent you from using ``SuiteSparse``,
   consider using the sparse linear algebra routines in ``Eigen``. The
   sparse Cholesky algorithms currently included with ``Eigen`` are
   not as sophisticated as the ones in ``SuiteSparse`` and
   ``Accelerate`` and as a result its performance is considerably
   worse.

.. member:: LinearSolverOrderingType Solver::Options::linear_solver_ordering_type

   Default: ``AMD``

    The order in which variables are eliminated in a linear solver can
    have a significant impact on the efficiency and accuracy of the
    method. e.g., when doing sparse Cholesky factorization, there are
    matrices for which a good ordering will give a Cholesky factor
    with :math:`O(n)` storage, where as a bad ordering will result in
    an completely dense factor.

    Sparse direct solvers like ``SPARSE_NORMAL_CHOLESKY`` and
    ``SPARSE_SCHUR`` use a fill reducing ordering of the columns and
    rows of the matrix being factorized before computing the numeric
    factorization.

    This enum controls the type of algorithm used to compute this fill
    reducing ordering. There is no single algorithm that works on all
    matrices, so determining which algorithm works better is a matter
    of empirical experimentation.

.. member:: shared_ptr<ParameterBlockOrdering> Solver::Options::linear_solver_ordering

   Default: ``nullptr``

   An instance of the ordering object informs the solver about the
   desired order in which parameter blocks should be eliminated by the
   linear solvers.

   If ``nullptr``, the solver is free to choose an ordering that it
   thinks is best.

   See :ref:`section-ordering` for more details.

.. member:: bool Solver::Options::use_explicit_schur_complement

   Default: ``false``

   Use an explicitly computed Schur complement matrix with
   ``ITERATIVE_SCHUR``.

   By default this option is disabled and ``ITERATIVE_SCHUR``
   evaluates evaluates matrix-vector products between the Schur
   complement and a vector implicitly by exploiting the algebraic
   expression for the Schur complement.

   The cost of this evaluation scales with the number of non-zeros in
   the Jacobian.

   For small to medium sized problems there is a sweet spot where
   computing the Schur complement is cheap enough that it is much more
   efficient to explicitly compute it and use it for evaluating the
   matrix-vector products.

   Enabling this option tells ``ITERATIVE_SCHUR`` to use an explicitly
   computed Schur complement. This can improve the performance of the
   ``ITERATIVE_SCHUR`` solver significantly.

   .. NOTE::

     This option can only be used with the ``SCHUR_JACOBI``
     preconditioner.

.. member:: bool Solver::Options::dynamic_sparsity

   Some non-linear least squares problems are symbolically dense but
   numerically sparse. i.e. at any given state only a small number of
   Jacobian entries are non-zero, but the position and number of
   non-zeros is different depending on the state. For these problems
   it can be useful to factorize the sparse jacobian at each solver
   iteration instead of including all of the zero entries in a single
   general factorization.

   If your problem does not have this property (or you do not know),
   then it is probably best to keep this false, otherwise it will
   likely lead to worse performance.

   This setting only affects the `SPARSE_NORMAL_CHOLESKY` solver.

.. member:: bool Solver::Options::use_mixed_precision_solves

   Default: ``false``

   .. NOTE::

     This feature is EXPERIMENTAL and under development, use at your
     own risk!

   If true, the Gauss-Newton matrix is computed in *double* precision, but
   its factorization is computed in **single** precision. This can result in
   significant time and memory savings at the cost of some accuracy in the
   Gauss-Newton step. Iterative refinement is used to recover some
   of this accuracy back.

   If ``use_mixed_precision_solves`` is true, we recommend setting
   ``max_num_refinement_iterations`` to 2-3.

   This option is currently only available if
   ``sparse_linear_algebra_library_type`` is ``EIGEN_SPARSE`` or
   ``ACCELERATE_SPARSE``, and ``linear_solver_type`` is
   ``SPARSE_NORMAL_CHOLESKY`` or ``SPARSE_SCHUR``.

.. member:: int Solver::Options::max_num_refinement_iterations

   Default: ``0``

   Number steps of the iterative refinement process to run when
   computing the Gauss-Newton step, see ``use_mixed_precision_solves``.

.. member:: int Solver::Options::min_linear_solver_iterations

   Default: ``0``

   Minimum number of iterations used by the linear solver. This only
   makes sense when the linear solver is an iterative solver, e.g.,
   ``ITERATIVE_SCHUR`` or ``CGNR``.

.. member:: int Solver::Options::max_linear_solver_iterations

   Default: ``500``

   Minimum number of iterations used by the linear solver. This only
   makes sense when the linear solver is an iterative solver, e.g.,
   ``ITERATIVE_SCHUR`` or ``CGNR``.

.. member:: double Solver::Options::eta

   Default: ``1e-1``

   Forcing sequence parameter. The truncated Newton solver uses this
   number to control the relative accuracy with which the Newton step
   is computed. This constant is passed to
   ``ConjugateGradientsSolver`` which uses it to terminate the
   iterations when

   .. math:: \frac{Q_i - Q_{i-1}}{Q_i} < \frac{\eta}{i}

.. member:: bool Solver::Options::jacobi_scaling

   Default: ``true``

   ``true`` means that the Jacobian is scaled by the norm of its
   columns before being passed to the linear solver. This improves the
   numerical conditioning of the normal equations.

.. member:: bool Solver::Options::use_inner_iterations

   Default: ``false``

   Use a non-linear version of a simplified variable projection
   algorithm. Essentially this amounts to doing a further optimization
   on each Newton/Trust region step using a coordinate descent
   algorithm.  For more details, see :ref:`section-inner-iterations`.

   **Note** Inner iterations cannot be used with :class:`Problem`
   objects that have an :class:`EvaluationCallback` associated with
   them.

.. member:: double Solver::Options::inner_iteration_tolerance

   Default: ``1e-3``

   Generally speaking, inner iterations make significant progress in
   the early stages of the solve and then their contribution drops
   down sharply, at which point the time spent doing inner iterations
   is not worth it.

   Once the relative decrease in the objective function due to inner
   iterations drops below ``inner_iteration_tolerance``, the use of
   inner iterations in subsequent trust region minimizer iterations is
   disabled.

.. member:: shared_ptr<ParameterBlockOrdering> Solver::Options::inner_iteration_ordering

   Default: ``nullptr``

   If :member:`Solver::Options::use_inner_iterations` true, then the
   user has two choices.

   1. Let the solver heuristically decide which parameter blocks to
      optimize in each inner iteration. To do this, set
      :member:`Solver::Options::inner_iteration_ordering` to ``nullptr``.

   2. Specify a collection of of ordered independent sets. The lower
      numbered groups are optimized before the higher number groups
      during the inner optimization phase. Each group must be an
      independent set. Not all parameter blocks need to be included in
      the ordering.

   See :ref:`section-ordering` for more details.

.. member:: LoggingType Solver::Options::logging_type

   Default: ``PER_MINIMIZER_ITERATION``

.. member:: bool Solver::Options::minimizer_progress_to_stdout

   Default: ``false``

   By default the :class:`Minimizer` progress is logged to ``STDERR``
   depending on the ``vlog`` level. If this flag is set to true, and
   :member:`Solver::Options::logging_type` is not ``SILENT``, the logging
   output is sent to ``STDOUT``.

   For ``TRUST_REGION_MINIMIZER`` the progress display looks like

   .. code-block:: bash

      iter      cost      cost_change  |gradient|   |step|    tr_ratio  tr_radius  ls_iter  iter_time  total_time
         0  4.185660e+06    0.00e+00    1.09e+08   0.00e+00   0.00e+00  1.00e+04       0    7.59e-02    3.37e-01
         1  1.062590e+05    4.08e+06    8.99e+06   5.36e+02   9.82e-01  3.00e+04       1    1.65e-01    5.03e-01
         2  4.992817e+04    5.63e+04    8.32e+06   3.19e+02   6.52e-01  3.09e+04       1    1.45e-01    6.48e-01

   Here

   #. ``cost`` is the value of the objective function.
   #. ``cost_change`` is the change in the value of the objective
      function if the step computed in this iteration is accepted.
   #. ``|gradient|`` is the max norm of the gradient.
   #. ``|step|`` is the change in the parameter vector.
   #. ``tr_ratio`` is the ratio of the actual change in the objective
      function value to the change in the value of the trust
      region model.
   #. ``tr_radius`` is the size of the trust region radius.
   #. ``ls_iter`` is the number of linear solver iterations used to
      compute the trust region step. For direct/factorization based
      solvers it is always 1, for iterative solvers like
      ``ITERATIVE_SCHUR`` it is the number of iterations of the
      Conjugate Gradients algorithm.
   #. ``iter_time`` is the time take by the current iteration.
   #. ``total_time`` is the total time taken by the minimizer.

   For ``LINE_SEARCH_MINIMIZER`` the progress display looks like

   .. code-block:: bash

      0: f: 2.317806e+05 d: 0.00e+00 g: 3.19e-01 h: 0.00e+00 s: 0.00e+00 e:  0 it: 2.98e-02 tt: 8.50e-02
      1: f: 2.312019e+05 d: 5.79e+02 g: 3.18e-01 h: 2.41e+01 s: 1.00e+00 e:  1 it: 4.54e-02 tt: 1.31e-01
      2: f: 2.300462e+05 d: 1.16e+03 g: 3.17e-01 h: 4.90e+01 s: 2.54e-03 e:  1 it: 4.96e-02 tt: 1.81e-01

   Here

   #. ``f`` is the value of the objective function.
   #. ``d`` is the change in the value of the objective function if
      the step computed in this iteration is accepted.
   #. ``g`` is the max norm of the gradient.
   #. ``h`` is the change in the parameter vector.
   #. ``s`` is the optimal step length computed by the line search.
   #. ``it`` is the time take by the current iteration.
   #. ``tt`` is the total time taken by the minimizer.

.. member:: vector<int> Solver::Options::trust_region_minimizer_iterations_to_dump

   Default: ``empty``

   List of iterations at which the trust region minimizer should dump
   the trust region problem. Useful for testing and benchmarking. If
   ``empty``, no problems are dumped.

.. member:: string Solver::Options::trust_region_problem_dump_directory

   Default: ``/tmp``

    Directory to which the problems should be written to. Should be
    non-empty if
    :member:`Solver::Options::trust_region_minimizer_iterations_to_dump` is
    non-empty and
    :member:`Solver::Options::trust_region_problem_dump_format_type` is not
    ``CONSOLE``.

.. member:: DumpFormatType Solver::Options::trust_region_problem_dump_format

   Default: ``TEXTFILE``

   The format in which trust region problems should be logged when
   :member:`Solver::Options::trust_region_minimizer_iterations_to_dump`
   is non-empty.  There are three options:

   * ``CONSOLE`` prints the linear least squares problem in a human
      readable format to ``stderr``. The Jacobian is printed as a
      dense matrix. The vectors :math:`D`, :math:`x` and :math:`f` are
      printed as dense vectors. This should only be used for small
      problems.

   * ``TEXTFILE`` Write out the linear least squares problem to the
     directory pointed to by
     :member:`Solver::Options::trust_region_problem_dump_directory` as
     text files which can be read into ``MATLAB/Octave``. The Jacobian
     is dumped as a text file containing :math:`(i,j,s)` triplets, the
     vectors :math:`D`, `x` and `f` are dumped as text files
     containing a list of their values.

     A ``MATLAB/Octave`` script called
     ``ceres_solver_iteration_???.m`` is also output, which can be
     used to parse and load the problem into memory.

.. member:: bool Solver::Options::check_gradients

   Default: ``false``

   Check all Jacobians computed by each residual block with finite
   differences. This is expensive since it involves computing the
   derivative by normal means (e.g. user specified, autodiff, etc),
   then also computing it using finite differences. The results are
   compared, and if they differ substantially, the optimization fails
   and the details are stored in the solver summary.

.. member:: double Solver::Options::gradient_check_relative_precision

   Default: ``1e-8``

   Precision to check for in the gradient checker. If the relative
   difference between an element in a Jacobian exceeds this number,
   then the Jacobian for that cost term is dumped.

.. member:: double Solver::Options::gradient_check_numeric_derivative_relative_step_size

   Default: ``1e-6``

   .. NOTE::

      This option only applies to the numeric differentiation used for
      checking the user provided derivatives when when
      `Solver::Options::check_gradients` is true. If you are using
      :class:`NumericDiffCostFunction` and are interested in changing
      the step size for numeric differentiation in your cost function,
      please have a look at :class:`NumericDiffOptions`.

   Relative shift used for taking numeric derivatives when
   `Solver::Options::check_gradients` is `true`.

   For finite differencing, each dimension is evaluated at slightly
   shifted values, e.g., for forward differences, the numerical
   derivative is

   .. math::

     \delta &= gradient\_check\_numeric\_derivative\_relative\_step\_size\\
     \Delta f &= \frac{f((1 + \delta)  x) - f(x)}{\delta x}

   The finite differencing is done along each dimension. The reason to
   use a relative (rather than absolute) step size is that this way,
   numeric differentiation works for functions where the arguments are
   typically large (e.g. :math:`10^9`) and when the values are small
   (e.g. :math:`10^{-5}`). It is possible to construct *torture cases*
   which break this finite difference heuristic, but they do not come
   up often in practice.

.. member:: bool Solver::Options::update_state_every_iteration

   Default: ``false``

   If ``update_state_every_iteration`` is ``true``, then Ceres Solver
   will guarantee that at the end of every iteration and before any
   user :class:`IterationCallback` is called, the parameter blocks are
   updated to the current best solution found by the solver. Thus the
   IterationCallback can inspect the values of the parameter blocks
   for purposes of computation, visualization or termination.

   If ``update_state_every_iteration`` is ``false`` then there is no
   such guarantee, and user provided :class:`IterationCallback` s
   should not expect to look at the parameter blocks and interpret
   their values.

.. member:: vector<IterationCallback> Solver::Options::callbacks

   Callbacks that are executed at the end of each iteration of the
   :class:`Minimizer`. They are executed in the order that they are
   specified in this vector.

   By default, parameter blocks are updated only at the end of the
   optimization, i.e., when the :class:`Minimizer` terminates. This
   means that by default, if an :class:`IterationCallback` inspects
   the parameter blocks, they will not see them changing in the course
   of the optimization.

   To tell Ceres to update the parameter blocks at the end of each
   iteration and before calling the user's callback, set
   :member:`Solver::Options::update_state_every_iteration` to
   ``true``.

   The solver does NOT take ownership of these pointers.

:class:`ParameterBlockOrdering`
===============================

.. class:: ParameterBlockOrdering

   ``ParameterBlockOrdering`` is a class for storing and manipulating
   an ordered collection of groups/sets with the following semantics:

   Group IDs are non-negative integer values. Elements are any type
   that can serve as a key in a map or an element of a set.

   An element can only belong to one group at a time. A group may
   contain an arbitrary number of elements.

   Groups are ordered by their group id.

.. function:: bool ParameterBlockOrdering::AddElementToGroup(const double* element, const int group)

   Add an element to a group. If a group with this id does not exist,
   one is created. This method can be called any number of times for
   the same element. Group ids should be non-negative numbers.  Return
   value indicates if adding the element was a success.

.. function:: void ParameterBlockOrdering::Clear()

   Clear the ordering.

.. function:: bool ParameterBlockOrdering::Remove(const double* element)

   Remove the element, no matter what group it is in. If the element
   is not a member of any group, calling this method will result in a
   crash.  Return value indicates if the element was actually removed.

.. function:: void ParameterBlockOrdering::Reverse()

   Reverse the order of the groups in place.

.. function:: int ParameterBlockOrdering::GroupId(const double* element) const

   Return the group id for the element. If the element is not a member
   of any group, return -1.

.. function:: bool ParameterBlockOrdering::IsMember(const double* element) const

   True if there is a group containing the parameter block.

.. function:: int ParameterBlockOrdering::GroupSize(const int group) const

   This function always succeeds, i.e., implicitly there exists a
   group for every integer.

.. function:: int ParameterBlockOrdering::NumElements() const

   Number of elements in the ordering.

.. function:: int ParameterBlockOrdering::NumGroups() const

   Number of groups with one or more elements.

:class:`IterationSummary`
==========================

.. class:: IterationSummary

   :class:`IterationSummary` describes the state of the minimizer at
   the end of each iteration.

.. member:: int IterationSummary::iteration

   Current iteration number.

.. member:: bool IterationSummary::step_is_valid

   Step was numerically valid, i.e., all values are finite and the
   step reduces the value of the linearized model.

    **Note**: :member:`IterationSummary::step_is_valid` is `false`
    when :member:`IterationSummary::iteration` = 0.

.. member::  bool IterationSummary::step_is_nonmonotonic

    Step did not reduce the value of the objective function
    sufficiently, but it was accepted because of the relaxed
    acceptance criterion used by the non-monotonic trust region
    algorithm.

    **Note**: :member:`IterationSummary::step_is_nonmonotonic` is
    `false` when when :member:`IterationSummary::iteration` = 0.

.. member:: bool IterationSummary::step_is_successful

   Whether or not the minimizer accepted this step or not.

   If the ordinary trust region algorithm is used, this means that the
   relative reduction in the objective function value was greater than
   :member:`Solver::Options::min_relative_decrease`. However, if the
   non-monotonic trust region algorithm is used
   (:member:`Solver::Options::use_nonmonotonic_steps` = `true`), then
   even if the relative decrease is not sufficient, the algorithm may
   accept the step and the step is declared successful.

   **Note**: :member:`IterationSummary::step_is_successful` is `false`
   when when :member:`IterationSummary::iteration` = 0.

.. member:: double IterationSummary::cost

   Value of the objective function.

.. member:: double IterationSummary::cost_change

   Change in the value of the objective function in this
   iteration. This can be positive or negative.

.. member:: double IterationSummary::gradient_max_norm

   Infinity norm of the gradient vector.

.. member:: double IterationSummary::gradient_norm

   2-norm of the gradient vector.

.. member:: double IterationSummary::step_norm

   2-norm of the size of the step computed in this iteration.

.. member:: double IterationSummary::relative_decrease

   For trust region algorithms, the ratio of the actual change in cost
   and the change in the cost of the linearized approximation.

   This field is not used when a linear search minimizer is used.

.. member:: double IterationSummary::trust_region_radius

   Size of the trust region at the end of the current iteration. For
   the Levenberg-Marquardt algorithm, the regularization parameter is
   1.0 / :member:`IterationSummary::trust_region_radius`.

.. member:: double IterationSummary::eta

   For the inexact step Levenberg-Marquardt algorithm, this is the
   relative accuracy with which the step is solved. This number is
   only applicable to the iterative solvers capable of solving linear
   systems inexactly. Factorization-based exact solvers always have an
   eta of 0.0.

.. member:: double IterationSummary::step_size

   Step sized computed by the line search algorithm.

   This field is not used when a trust region minimizer is used.

.. member:: int IterationSummary::line_search_function_evaluations

   Number of function evaluations used by the line search algorithm.

   This field is not used when a trust region minimizer is used.

.. member:: int IterationSummary::linear_solver_iterations

   Number of iterations taken by the linear solver to solve for the
   trust region step.

   Currently this field is not used when a line search minimizer is
   used.

.. member:: double IterationSummary::iteration_time_in_seconds

   Time (in seconds) spent inside the minimizer loop in the current
   iteration.

.. member:: double IterationSummary::step_solver_time_in_seconds

   Time (in seconds) spent inside the trust region step solver.

.. member:: double IterationSummary::cumulative_time_in_seconds

   Time (in seconds) since the user called Solve().

:class:`IterationCallback`
==========================

.. class:: IterationCallback

   Interface for specifying callbacks that are executed at the end of
   each iteration of the minimizer.

   .. code-block:: c++

      class IterationCallback {
       public:
        virtual ~IterationCallback() {}
        virtual CallbackReturnType operator()(const IterationSummary& summary) = 0;
      };


  The solver uses the return value of ``operator()`` to decide whether
  to continue solving or to terminate. The user can return three
  values.

  #. ``SOLVER_ABORT`` indicates that the callback detected an abnormal
     situation. The solver returns without updating the parameter
     blocks (unless ``Solver::Options::update_state_every_iteration`` is
     set true). Solver returns with ``Solver::Summary::termination_type``
     set to ``USER_FAILURE``.

  #. ``SOLVER_TERMINATE_SUCCESSFULLY`` indicates that there is no need
     to optimize anymore (some user specified termination criterion
     has been met). Solver returns with
     ``Solver::Summary::termination_type``` set to ``USER_SUCCESS``.

  #. ``SOLVER_CONTINUE`` indicates that the solver should continue
     optimizing.

  For example, the following :class:`IterationCallback` is used
  internally by Ceres to log the progress of the optimization.

  .. code-block:: c++

    class LoggingCallback : public IterationCallback {
     public:
      explicit LoggingCallback(bool log_to_stdout)
          : log_to_stdout_(log_to_stdout) {}

      ~LoggingCallback() {}

      CallbackReturnType operator()(const IterationSummary& summary) {
        const char* kReportRowFormat =
            "% 4d: f:% 8e d:% 3.2e g:% 3.2e h:% 3.2e "
            "rho:% 3.2e mu:% 3.2e eta:% 3.2e li:% 3d";
        string output = StringPrintf(kReportRowFormat,
                                     summary.iteration,
                                     summary.cost,
                                     summary.cost_change,
                                     summary.gradient_max_norm,
                                     summary.step_norm,
                                     summary.relative_decrease,
                                     summary.trust_region_radius,
                                     summary.eta,
                                     summary.linear_solver_iterations);
        if (log_to_stdout_) {
          cout << output << endl;
        } else {
          VLOG(1) << output;
        }
        return SOLVER_CONTINUE;
      }

     private:
      const bool log_to_stdout_;
    };



:class:`CRSMatrix`
==================

.. class:: CRSMatrix

   A compressed row sparse matrix used primarily for communicating the
   Jacobian matrix to the user.

.. member:: int CRSMatrix::num_rows

   Number of rows.

.. member:: int CRSMatrix::num_cols

   Number of columns.

.. member:: vector<int> CRSMatrix::rows

   :member:`CRSMatrix::rows` is a :member:`CRSMatrix::num_rows` + 1
   sized array that points into the :member:`CRSMatrix::cols` and
   :member:`CRSMatrix::values` array.

.. member:: vector<int> CRSMatrix::cols

   :member:`CRSMatrix::cols` contain as many entries as there are
   non-zeros in the matrix.

   For each row ``i``, ``cols[rows[i]]`` ... ``cols[rows[i + 1] - 1]``
   are the indices of the non-zero columns of row ``i``.

.. member:: vector<int> CRSMatrix::values

   :member:`CRSMatrix::values` contain as many entries as there are
   non-zeros in the matrix.

   For each row ``i``,
   ``values[rows[i]]`` ... ``values[rows[i + 1] - 1]`` are the values
   of the non-zero columns of row ``i``.

e.g., consider the 3x4 sparse matrix

.. code-block:: c++

   0 10  0  4
   0  2 -3  2
   1  2  0  0

The three arrays will be:

.. code-block:: c++

            -row0-  ---row1---  -row2-
   rows   = [ 0,      2,          5,     7]
   cols   = [ 1,  3,  1,  2,  3,  0,  1]
   values = [10,  4,  2, -3,  2,  1,  2]


:class:`Solver::Summary`
========================

.. class:: Solver::Summary

   Summary of the various stages of the solver after termination.

.. function:: string Solver::Summary::BriefReport() const

   A brief one line description of the state of the solver after
   termination.

.. function:: string Solver::Summary::FullReport() const

   A full multiline description of the state of the solver after
   termination.

.. function:: bool Solver::Summary::IsSolutionUsable() const

   Whether the solution returned by the optimization algorithm can be
   relied on to be numerically sane. This will be the case if
   `Solver::Summary:termination_type` is set to `CONVERGENCE`,
   `USER_SUCCESS` or `NO_CONVERGENCE`, i.e., either the solver
   converged by meeting one of the convergence tolerances or because
   the user indicated that it had converged or it ran to the maximum
   number of iterations or time.

.. member:: MinimizerType Solver::Summary::minimizer_type

   Type of minimization algorithm used.

.. member:: TerminationType Solver::Summary::termination_type

   The cause of the minimizer terminating.

.. member:: string Solver::Summary::message

   Reason why the solver terminated.

.. member:: double Solver::Summary::initial_cost

   Cost of the problem (value of the objective function) before the
   optimization.

.. member:: double Solver::Summary::final_cost

   Cost of the problem (value of the objective function) after the
   optimization.

.. member:: double Solver::Summary::fixed_cost

   The part of the total cost that comes from residual blocks that
   were held fixed by the preprocessor because all the parameter
   blocks that they depend on were fixed.

.. member:: vector<IterationSummary> Solver::Summary::iterations

   :class:`IterationSummary` for each minimizer iteration in order.

.. member:: int Solver::Summary::num_successful_steps

   Number of minimizer iterations in which the step was
   accepted. Unless :member:`Solver::Options::use_non_monotonic_steps`
   is `true` this is also the number of steps in which the objective
   function value/cost went down.

.. member:: int Solver::Summary::num_unsuccessful_steps

   Number of minimizer iterations in which the step was rejected
   either because it did not reduce the cost enough or the step was
   not numerically valid.

.. member:: int Solver::Summary::num_inner_iteration_steps

   Number of times inner iterations were performed.

 .. member:: int Solver::Summary::num_line_search_steps

    Total number of iterations inside the line search algorithm across
    all invocations. We call these iterations "steps" to distinguish
    them from the outer iterations of the line search and trust region
    minimizer algorithms which call the line search algorithm as a
    subroutine.

.. member:: double Solver::Summary::preprocessor_time_in_seconds

   Time (in seconds) spent in the preprocessor.

.. member:: double Solver::Summary::minimizer_time_in_seconds

   Time (in seconds) spent in the Minimizer.

.. member:: double Solver::Summary::postprocessor_time_in_seconds

   Time (in seconds) spent in the post processor.

.. member:: double Solver::Summary::total_time_in_seconds

   Time (in seconds) spent in the solver.

.. member:: double Solver::Summary::linear_solver_time_in_seconds

   Time (in seconds) spent in the linear solver computing the trust
   region step.

.. member:: int Solver::Summary::num_linear_solves

   Number of times the Newton step was computed by solving a linear
   system. This does not include linear solves used by inner
   iterations.

.. member:: double Solver::Summary::residual_evaluation_time_in_seconds

   Time (in seconds) spent evaluating the residual vector.

.. member:: int Solver::Summary::num_residual_evaluations

   Number of times only the residuals were evaluated.

.. member:: double Solver::Summary::jacobian_evaluation_time_in_seconds

   Time (in seconds) spent evaluating the Jacobian matrix.

.. member:: int Solver::Summary::num_jacobian_evaluations

   Number of times only the Jacobian and the residuals were evaluated.

.. member:: double Solver::Summary::inner_iteration_time_in_seconds

   Time (in seconds) spent doing inner iterations.

.. member:: int Solver::Summary::num_parameter_blocks

   Number of parameter blocks in the problem.

.. member:: int Solver::Summary::num_parameters

   Number of parameters in the problem.

.. member:: int Solver::Summary::num_effective_parameters

   Dimension of the tangent space of the problem (or the number of
   columns in the Jacobian for the problem). This is different from
   :member:`Solver::Summary::num_parameters` if a parameter block is
   associated with a :class:`Manifold`.

.. member:: int Solver::Summary::num_residual_blocks

   Number of residual blocks in the problem.

.. member:: int Solver::Summary::num_residuals

   Number of residuals in the problem.

.. member:: int Solver::Summary::num_parameter_blocks_reduced

   Number of parameter blocks in the problem after the inactive and
   constant parameter blocks have been removed. A parameter block is
   inactive if no residual block refers to it.

.. member:: int Solver::Summary::num_parameters_reduced

   Number of parameters in the reduced problem.

.. member:: int Solver::Summary::num_effective_parameters_reduced

   Dimension of the tangent space of the reduced problem (or the
   number of columns in the Jacobian for the reduced problem). This is
   different from :member:`Solver::Summary::num_parameters_reduced` if
   a parameter block in the reduced problem is associated with a
   :class:`Manifold`.

.. member:: int Solver::Summary::num_residual_blocks_reduced

   Number of residual blocks in the reduced problem.

.. member:: int Solver::Summary::num_residuals_reduced

   Number of residuals in the reduced problem.

.. member:: int Solver::Summary::num_threads_given

   Number of threads specified by the user for Jacobian and residual
   evaluation.

.. member:: int Solver::Summary::num_threads_used

   Number of threads actually used by the solver for Jacobian and
   residual evaluation.

.. member:: LinearSolverType Solver::Summary::linear_solver_type_given

   Type of the linear solver requested by the user.

.. member:: LinearSolverType Solver::Summary::linear_solver_type_used

   Type of the linear solver actually used. This may be different from
   :member:`Solver::Summary::linear_solver_type_given` if Ceres
   determines that the problem structure is not compatible with the
   linear solver requested or if the linear solver requested by the
   user is not available, e.g. The user requested
   `SPARSE_NORMAL_CHOLESKY` but no sparse linear algebra library was
   available.

.. member:: vector<int> Solver::Summary::linear_solver_ordering_given

   Size of the elimination groups given by the user as hints to the
   linear solver.

.. member:: vector<int> Solver::Summary::linear_solver_ordering_used

   Size of the parameter groups used by the solver when ordering the
   columns of the Jacobian.  This maybe different from
   :member:`Solver::Summary::linear_solver_ordering_given` if the user
   left :member:`Solver::Summary::linear_solver_ordering_given` blank
   and asked for an automatic ordering, or if the problem contains
   some constant or inactive parameter blocks.

.. member:: std::string Solver::Summary::schur_structure_given

    For Schur type linear solvers, this string describes the template
    specialization which was detected in the problem and should be
    used.

.. member:: std::string Solver::Summary::schur_structure_used

   For Schur type linear solvers, this string describes the template
   specialization that was actually instantiated and used. The reason
   this will be different from
   :member:`Solver::Summary::schur_structure_given` is because the
   corresponding template specialization does not exist.

   Template specializations can be added to ceres by editing
   ``internal/ceres/generate_template_specializations.py``

.. member:: bool Solver::Summary::inner_iterations_given

   `True` if the user asked for inner iterations to be used as part of
   the optimization.

.. member:: bool Solver::Summary::inner_iterations_used

   `True` if the user asked for inner iterations to be used as part of
   the optimization and the problem structure was such that they were
   actually performed. For example, in a problem with just one parameter
   block, inner iterations are not performed.

.. member:: vector<int> inner_iteration_ordering_given

   Size of the parameter groups given by the user for performing inner
   iterations.

.. member:: vector<int> inner_iteration_ordering_used

   Size of the parameter groups given used by the solver for
   performing inner iterations. This maybe different from
   :member:`Solver::Summary::inner_iteration_ordering_given` if the
   user left :member:`Solver::Summary::inner_iteration_ordering_given`
   blank and asked for an automatic ordering, or if the problem
   contains some constant or inactive parameter blocks.

.. member:: PreconditionerType Solver::Summary::preconditioner_type_given

   Type of the preconditioner requested by the user.

.. member:: PreconditionerType Solver::Summary::preconditioner_type_used

   Type of the preconditioner actually used. This may be different
   from :member:`Solver::Summary::linear_solver_type_given` if Ceres
   determines that the problem structure is not compatible with the
   linear solver requested or if the linear solver requested by the
   user is not available.

.. member:: VisibilityClusteringType Solver::Summary::visibility_clustering_type

   Type of clustering algorithm used for visibility based
   preconditioning. Only meaningful when the
   :member:`Solver::Summary::preconditioner_type` is
   ``CLUSTER_JACOBI`` or ``CLUSTER_TRIDIAGONAL``.

.. member:: TrustRegionStrategyType Solver::Summary::trust_region_strategy_type

   Type of trust region strategy.

.. member:: DoglegType Solver::Summary::dogleg_type

   Type of dogleg strategy used for solving the trust region problem.

.. member:: DenseLinearAlgebraLibraryType Solver::Summary::dense_linear_algebra_library_type

   Type of the dense linear algebra library used.

.. member:: SparseLinearAlgebraLibraryType Solver::Summary::sparse_linear_algebra_library_type

   Type of the sparse linear algebra library used.

.. member:: LineSearchDirectionType Solver::Summary::line_search_direction_type

   Type of line search direction used.

.. member:: LineSearchType Solver::Summary::line_search_type

   Type of the line search algorithm used.

.. member:: LineSearchInterpolationType Solver::Summary::line_search_interpolation_type

   When performing line search, the degree of the polynomial used to
   approximate the objective function.

.. member:: NonlinearConjugateGradientType Solver::Summary::nonlinear_conjugate_gradient_type

   If the line search direction is `NONLINEAR_CONJUGATE_GRADIENT`,
   then this indicates the particular variant of non-linear conjugate
   gradient used.

.. member:: int Solver::Summary::max_lbfgs_rank

   If the type of the line search direction is `LBFGS`, then this
   indicates the rank of the Hessian approximation.