On-Manifold Optimization: Local Parameterization

Overview

Manifold Space vs Tangent Space

Jacobian w.r.t Error State

Jacobian w.r.t Error State vs True State

According ¹ 2.4,

The idea is that for a $\in N$ the function $g(\delta) := f (x \boxplus \delta)$ behaves locally in $0$ like $f$ does in $x$ . In particular $f(x)\|^2$ has a minimum in $x$ if and only if $\|g(\delta)\|^2$ has a minimum in $0$ . Therefore finding a local optimum of $g$ , $\delta = \arg \min_{\delta} \|g(\delta)\|^2$ implies $\boxplus \delta = \arg \min_{\xi} \|f(\xi)\|^2$ .

$\boxplus \delta)=f(x)+J_x \delta+\mathcal{O}\left(\|\delta\|^2\right)$

where

$\left. \frac{\partial f(x \boxplus \delta)}{\partial \delta} \right|_{\delta=0} \quad \longleftrightarrow \quad J = \left. \frac{\partial f(x)}{\partial x} \right|_{x}$

ESKF ² 6.1.1: Jacobian computation

$\left.\mathbf{H} \triangleq \frac{\partial h}{\partial \delta \mathbf{x}}\right|_{\mathbf{x}}=\left.\left.\frac{\partial h}{\partial \mathbf{x}_t}\right|_{\mathbf{x}} \frac{\partial \mathbf{x}_t}{\partial \delta \mathbf{x}}\right|_{\mathbf{x}}=\mathbf{H}_{\mathbf{x}} \mathbf{X}_{\delta \mathbf{x}}$

$x_t$ : true state
$x$ : normal state
$\delta x$ : error state

lifting and retraction:

$\left.\mathbf{X}_{\delta \mathbf{x}} \triangleq \frac{\partial \mathbf{x}_t}{\partial \delta \mathbf{x}}\right|_{\mathbf{x}}=\left[\begin{array}{ccc} \mathbf{I}_6 & 0 & 0 \\ 0 & \mathbf{Q}_{\delta \boldsymbol{\theta}} & 0 \\ 0 & 0 & \mathbf{I}_9 \end{array}\right]$

the quaternion term

$\begin{aligned} \left.\mathbf{Q}_{\delta \boldsymbol{\theta}} \triangleq \frac{\partial(\mathbf{q} \otimes \delta \mathbf{q})}{\partial \delta \boldsymbol{\theta}}\right|_{\mathbf{q}} &=\left.\left.\frac{\partial(\mathbf{q} \otimes \delta \mathbf{q})}{\partial \delta \mathbf{q}}\right|_{\mathbf{q}} \frac{\partial \delta \mathbf{q}}{\partial \delta \boldsymbol{\theta}}\right|_{\delta \hat{\boldsymbol{\theta}}=0} \\ &=\left.\left.\frac{\partial\left([\mathbf{q}]_L \delta \mathbf{q}\right)}{\partial \delta \mathbf{q}}\right|_{\mathbf{q}} \frac{\partial\left[\begin{array}{c} 1 \\ \frac{1}{2} \delta \boldsymbol{\theta} \end{array}\right]}{\partial \delta \boldsymbol{\theta}}\right|_{\hat{\delta}=0} \\ &=[\mathbf{q}]_L \frac{1}{2}\left[\begin{array}{lll} 0 & 0 & 0 \\ 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array}\right] \end{aligned}$

Least Squares on a Manifold ³

Local Parameterization in Ceres Solver ⁴ ⁵ ⁶ ⁷ ⁸

class LocalParameterization {public:virtual ~LocalParameterization() = default;virtual bool Plus(const double* x,const double* delta,double* x_plus_delta) const = 0;virtual bool ComputeJacobian(const double* x, double* jacobian) const = 0;virtual bool MultiplyByJacobian(const double* x,const int num_rows,const double* global_matrix,double* local_matrix) const;virtual int GlobalSize() const = 0;virtual int LocalSize() const = 0;
};

Plus

Retraction

$\boxplus(x, \Delta)=x \operatorname{Exp}(\Delta)$

ComputeJacobian

global w.r.t local

$J_{GL} = \frac{\partial x_G}{\partial x_L} = D_2 \boxplus(x, 0) = \left. \frac{\partial \boxplus(x, \Delta)}{\partial \Delta} \right|_{\Delta = 0}$

参考 ⁹

$r$ w.r.t $x_{L}$

在 ceres::CostFunction 处提供 residuals 对 Manifold 上变量的导数

$J_{rG} = \frac{\partial r}{\partial x_G}$

则对 Tangent Space 上变量的导数

$J_{rL} = \frac{\partial r}{\partial x_L} = \frac{\partial r}{\partial x_G} \cdot J_{GL}$

Sub Class

QuaternionParameterization
EigenQuaternionParameterization

自定义 QuaternionParameterization

参考 ⁷

Summary

QuaternionParameterization 的 Plus 与 ComputeJacobian 共同决定使用左扰动或使用右扰动形式

Quaternion in Eigen


Quaterniond q1(1, 2, 3, 4);           // wxyz
Quaterniond q2(Vector4d(1, 2, 3, 4)); // xyzw
Quaterniond q3(tmp_q);                // xyzw, double tmp_q[4];
q.coeffs();                           // xyzw

Quaternion in Ceres Solver

order: wxyz
Ceres Solver 中 Quaternion 是 Hamilton Quaternion，遵循 Hamilton 乘法法则
矩阵 raw memory 存储方式是 Row Major

A Framework for Sparse, Non-Linear Least Squares Problems on Manifolds ↩︎
Quaternion kinematics for the error-state Kalman filter, Joan Solà ↩︎
A Tutorial on Graph-Based SLAM ↩︎
http://ceres-solver.org/nnls_modeling.html#localparameterization ↩︎
On-Manifold Optimization Demo using Ceres Solver ↩︎
Matrix Manifold Local Parameterizations for Ceres Solver ↩︎
[ceres-solver] From QuaternionParameterization to LocalParameterization 😄 ↩︎ ↩︎
LocalParameterization子类说明：QuaternionParameterization类和EigenQuaternionParameterization类 ↩︎
优化库——ceres（二）深入探索ceres::Problem ↩︎