include/ceres/evaluation_callback.h

// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2018 Google Inc. All rights reserved.
// http://ceres-solver.org/
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// Author: mierle@gmail.com (Keir Mierle)

#ifndef CERES_PUBLIC_EVALUATION_CALLBACK_H_
#define CERES_PUBLIC_EVALUATION_CALLBACK_H_

namespace ceres {

// Using this callback interface, Ceres can notify you when it is about to
// evaluate the residuals or jacobians. With the callback, you can share
// computation between residual blocks by doing the shared computation in
// PrepareForEvaluation() before Ceres calls CostFunction::Evaluate() on all
// the residuals. It also enables caching results between a pure residual
// evaluation and a residual & jacobian evaluation, via the
// new_evaluation_point argument.
//
// One use case for this callback is if the cost function compute is moved to
// the GPU. In that case, the prepare call does the actual cost function
// evaluation, and subsequent calls from Ceres to the actual cost functions
// merely copy the results from the GPU onto the corresponding blocks for Ceres
// to plug into the solver.
//
// NOTE: Ceres provides no mechanism to share data other than the notification
// from the callback. Users must provide access to pre-computed shared data to
// their cost functions behind the scenes; this all happens without Ceres
// knowing. One approach is to put a pointer to the shared data in each cost
// function (recommended) or to use a global shared variable (discouraged;
// bug-prone).  As far as Ceres is concerned, it is evaluating cost functions
// like any other; it just so happens that behind the scenes the cost functions
// reuse pre-computed data to execute faster.
class CERES_EXPORT EvaluationCallback {
 public:
  virtual ~EvaluationCallback() {}

  // Called before Ceres requests residuals or jacobians for a given setting of
  // the parameters. User parameters (the double* values provided to the cost
  // functions) are fixed until the next call to PrepareForEvaluation(). If
  // new_evaluation_point == true, then this is a new point that is different
  // from the last evaluated point. Otherwise, it is the same point that was
  // evaluated previously (either jacobian or residual) and the user can use
  // cached results from previous evaluations.
  virtual void PrepareForEvaluation(bool evaluate_jacobians,
                                    bool new_evaluation_point) = 0;
};

}  // namespace ceres

#endif  // CERES_PUBLIC_EVALUATION_CALLBACK_H_