1 // Ceres Solver - A fast non-linear least squares minimizer
2 // Copyright 2015 Google Inc. All rights reserved.
3 // http://ceres-solver.org/
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29 // Author: keir@google.com (Keir Mierle)
31 #ifndef CERES_INTERNAL_CGNR_LINEAR_OPERATOR_H_
32 #define CERES_INTERNAL_CGNR_LINEAR_OPERATOR_H_
35 #include "ceres/linear_operator.h"
36 #include "ceres/internal/scoped_ptr.h"
37 #include "ceres/internal/eigen.h"
44 // A linear operator which takes a matrix A and a diagonal vector D and
45 // performs products of the form
49 // This is used to implement iterative general sparse linear solving with
50 // conjugate gradients, where A is the Jacobian and D is a regularizing
51 // parameter. A brief proof that D^T D is the correct regularizer:
53 // Given a regularized least squares problem:
55 // min ||Ax - b||^2 + ||Dx||^2
58 // First expand into matrix notation:
60 // (Ax - b)^T (Ax - b) + xD^TDx
62 // Then multiply out to get:
64 // = xA^TAx - 2b^T Ax + b^Tb + xD^TDx
66 // Take the derivative:
68 // 0 = 2A^TAx - 2A^T b + 2 D^TDx
69 // 0 = A^TAx - A^T b + D^TDx
70 // 0 = (A^TA + D^TD)x - A^T b
72 // Thus, the symmetric system we need to solve for CGNR is
76 // with S = A^TA + D^TD
79 // Note: This class is not thread safe, since it uses some temporary storage.
80 class CgnrLinearOperator : public LinearOperator {
82 CgnrLinearOperator(const LinearOperator& A, const double *D)
83 : A_(A), D_(D), z_(new double[A.num_rows()]) {
85 virtual ~CgnrLinearOperator() {}
87 virtual void RightMultiply(const double* x, double* y) const {
88 std::fill(z_.get(), z_.get() + A_.num_rows(), 0.0);
91 A_.RightMultiply(x, z_.get());
94 A_.LeftMultiply(z_.get(), y);
98 int n = A_.num_cols();
99 VectorRef(y, n).array() += ConstVectorRef(D_, n).array().square() *
100 ConstVectorRef(x, n).array();
104 virtual void LeftMultiply(const double* x, double* y) const {
108 virtual int num_rows() const { return A_.num_cols(); }
109 virtual int num_cols() const { return A_.num_cols(); }
112 const LinearOperator& A_;
114 scoped_array<double> z_;
117 } // namespace internal
120 #endif // CERES_INTERNAL_CGNR_LINEAR_OPERATOR_H_