Optimization Over Lie Groups

Birds-Eye View

Optimization methods incorporating Lie Groups are, by definition, nonlinear. Thus, the focus here is on nonlinear "local search" methods and adapting them to accommodate state vectors $x$ containing at least one Lie group component:

$$\boldsymbol x\in \mathbb{R}^n\times \mathcal{M} \times \cdots.$$

Acknowledging that a state vector can contain both vector and group components, define the following composite addition and subtraction operators:

$$\boldsymbol{a}\boxplus\boldsymbol{b}=\begin{cases} (\boldsymbol{a}+\boldsymbol{b})\in \mathbb{R}^n & \boldsymbol{a},\boldsymbol{b}\in\mathbb{R}^{n}\ (\boldsymbol{a}\oplus\boldsymbol{b})\in \mathcal{M} & \boldsymbol{a}\in\mathcal{M},\boldsymbol{b}\in \mathbb{R}^m \cong \mathfrak{m} \end{cases}$$

$$\boldsymbol{a}\boxminus\boldsymbol{b}=\begin{cases} (\boldsymbol{a}-\boldsymbol{b})\in \mathbb{R}^n & \boldsymbol{a},\boldsymbol{b}\in\mathbb{R}^{n}\ (\boldsymbol{a}\ominus\boldsymbol{b})\in \mathbb{R}^m\cong \mathfrak{m} & \boldsymbol{a},\boldsymbol{b}\in\mathbb{\mathcal{M}} \end{cases}$$

Local search, as the name suggests, utilizes the local, right- versions of the $\oplus$ and $\ominus$ operators, as well as right-Jacobians.

Lie-ify Your Optimization Algorithm

Local, iterative search algorithms (as with nonlinear least squares) take the following form when $\boldsymbol x$ belongs to a vector space:

$$\boldsymbol x_{k+1}=\boldsymbol x_k+\boldsymbol{\Delta x},~\boldsymbol{\Delta x}=f(\boldsymbol{x}_k,\boldsymbol J(\boldsymbol x_k))\in \mathbb{R}^n,$$

where $\boldsymbol J(\boldsymbol x_k)$ is the Jacobian of the residual function, $r(\boldsymbol{x}_k)$.

Acknowledging that the state can contain Lie group components, the above must be modified to:

$$\boldsymbol x_{k+1}=\boldsymbol x_k\boxplus\boldsymbol{\Delta x},~\boldsymbol{\Delta x}=f(\boldsymbol{x}_k,\boldsymbol J(\boldsymbol x_k))\in \mathbb{R}^n\times \mathbb{R}^m\cong \mathfrak{m}\times \cdots.$$

In essence, to perform local search with a state that evolves on the manifold, the following three steps must be taken:

  • Ensure that $r(\boldsymbol x)$ returns a vector (doesn't necessarily have to be this way, but it makes things much more straightforward).
  • Be able to calculate $\boldsymbol J(\boldsymbol x_k)$, which may contain both Jacobian terms over vector spaces as well as Jacobian terms on the manifold. An example below.
  • Ensure that, for your formulation, $f(\boldsymbol{x}_k,\boldsymbol J(\boldsymbol x_k))$ also returns a vector, but with components that are isomorphic to a Lie algebra (i.e., $\mathbb{R}^m\cong \mathfrak{m}$) where appropriate so that the $\boxplus$ operation makes sense.

If you can do those three things, then your optimization will behave as any other local search over vector spaces.


Simple "Mixed" Jacobian Example

Say that your state vector looks like

$$\boldsymbol x=\begin{bmatrix}\boldsymbol p\ \boldsymbol{R}\end{bmatrix}\in \mathbb{R}^3 \times SO(3),$$

and your residual function is

$$r(\boldsymbol x)=\begin{bmatrix}\boldsymbol p_\text{measured}-\boldsymbol p\ \boldsymbol R_\text{measured}\boldsymbol R^{-1}\end{bmatrix}\triangleq \begin{bmatrix}r_1(\boldsymbol x)\r_2(\boldsymbol x)\end{bmatrix}.$$

The Jacobian matrix will then look like

$$\boldsymbol J=\begin{bmatrix}\frac{\partial r_1}{\partial \boldsymbol p} & \frac{\partial r_1}{\partial \boldsymbol R} \ \frac{\partial r_2}{\partial \boldsymbol p} & \frac{\partial r_2}{\partial \boldsymbol R}\end{bmatrix}\in \mathbb{R}^{6\times 6},$$

where the Jacobians $\partial r_1/\partial \boldsymbol R$ and $\partial r_2/\partial \boldsymbol R$ are the (nontrivial) right-Jacobians on the manifold, which must be calculated as explained in the Jacobians section and demonstrated in the next example.

Gauss-Newton for a Nonlinear Least-Squares Problem

Suppose we want to derive the Gauss-Newton recursive update step for the nonlinear least-squares problem

$$\min_{\boldsymbol T\in SE(3)}\frac{1}{n}\sum_{i=1}^n\text{Tr}((\boldsymbol T-\boldsymbol T_i)^T(\boldsymbol T-\boldsymbol T_i)),$$

where $\boldsymbol T_i$ are a series of $n$ pose measurements. Let's go through the three steps for formulating local search on the manifold:

1: Ensure that the residual function returns a vector.

Defining $\boldsymbol x\triangleq \boldsymbol T\in SE(3)$, we want the optimization above to look like:

$$\min_{\boldsymbol x}r(\boldsymbol x)^Tr(\boldsymbol x)$$

where $r(\boldsymbol x)$ returns a vector. We'll do that now:

$$\frac{1}{n}\sum_{i=1}^n\text{Tr}((\boldsymbol T-\boldsymbol T_i)^T(\boldsymbol T-\boldsymbol T_i))=\text{vec}\left(\frac{1}{n}\sum_{i=1}^n(\boldsymbol T-\boldsymbol T_i)\right)^T\text{vec}\left(\frac{1}{n}\sum_{i=1}^n(\boldsymbol T-\boldsymbol T_i)\right)$$

$$=\text{vec}\left(\boldsymbol T-\bar{\boldsymbol{T}} \right)^T\text{vec}\left( \boldsymbol T-\bar{\boldsymbol{T}}\right)=\text{vec}\left(\begin{bmatrix}\boldsymbol{R}-\bar{\boldsymbol{R}} & \boldsymbol{p}-\bar{\boldsymbol{p}}\ \boldsymbol{0} & \boldsymbol{1}\end{bmatrix} \right)^T\text{vec}\left(\begin{bmatrix}\boldsymbol{R}-\bar{\boldsymbol{R}} & \boldsymbol{p}-\bar{\boldsymbol{p}}\ \boldsymbol{0} & \boldsymbol{1}\end{bmatrix} \right),$$

where $\bar{\cdot}$ indicates the average value over $n$ samples. Throw out the constants (which don't affect the optimization) to save some space, and you have a concise definition for an equivalent vector-valued residual function:

$$r(\boldsymbol x)\triangleq \begin{bmatrix}\boldsymbol p-\bar{\boldsymbol{p}}\(\boldsymbol R-\bar{\boldsymbol{R}})\boldsymbol e_1\(\boldsymbol R-\bar{\boldsymbol{R}})\boldsymbol e_2 \ (\boldsymbol R-\bar{\boldsymbol{R}})\boldsymbol e_3\end{bmatrix}~:~SE(3)\rightarrow \mathbb{R}^{12}.$$

2: Calculate the Jacobian matrix of the residual function.

Most Jacobian elements can be trivially calculated as

$$\boldsymbol J=\begin{bmatrix} \partial r_1/\partial \boldsymbol p & \partial r_1/\partial \boldsymbol \theta \ \partial r_2/\partial \boldsymbol p & \partial r_2/\partial \boldsymbol \theta \ \partial r_3/\partial \boldsymbol p & \partial r_3/\partial \boldsymbol \theta \ \partial r_4/\partial \boldsymbol p & \partial r_4/\partial \boldsymbol \theta \end{bmatrix}=\begin{bmatrix}\boldsymbol I & \boldsymbol 0\ \boldsymbol 0 & \partial r_2/\partial \boldsymbol \theta\ \boldsymbol 0 & \partial r_3/\partial \boldsymbol \theta \ \boldsymbol 0 & \partial r_4/\partial \boldsymbol \theta\end{bmatrix}.$$

The remaining Jacobian blocks must be calculated on the manifold:

$$\frac{\partial r_2}{\partial \boldsymbol\theta}=\lim_{\boldsymbol \theta\rightarrow \boldsymbol{0}}\frac{r_2(\boldsymbol R \oplus \boldsymbol \theta)-r_2(\boldsymbol R)}{\boldsymbol \theta}$$

(...note the regular minus $-$ sign in the numerator, since $r$ returns a vector and thus $\ominus$ is not necessary...)

$$=\lim_{\boldsymbol \theta\rightarrow \boldsymbol{0}}\frac{(\boldsymbol R \text{Exp}\boldsymbol \theta-\bar{\boldsymbol{R}})\boldsymbol e_1-(\boldsymbol{R}-\bar{\boldsymbol{R}})\boldsymbol e_1}{\boldsymbol \theta}=\lim_{\boldsymbol \theta\rightarrow \boldsymbol{0}}\frac{\boldsymbol R(\text{Exp}\boldsymbol \theta-\boldsymbol I)\boldsymbol e_1}{\boldsymbol \theta}$$

$$\approx\lim_{\boldsymbol \theta\rightarrow \boldsymbol{0}}\frac{\boldsymbol R[\boldsymbol \theta]\times\boldsymbol e_1}{\boldsymbol \theta}=\lim{\boldsymbol \theta\rightarrow \boldsymbol{0}}\frac{-\boldsymbol R[\boldsymbol e_1]_\times\boldsymbol \theta}{\boldsymbol \theta}$$

$$=-\boldsymbol{R}[\boldsymbol e_1]_\times.$$


$$\boldsymbol J=\begin{bmatrix}\boldsymbol I & \boldsymbol 0\ \boldsymbol 0 & -\boldsymbol{R}[\boldsymbol e_1]\times\ \boldsymbol 0 & -\boldsymbol{R}[\boldsymbol e_2]\times \ \boldsymbol 0 & -\boldsymbol{R}[\boldsymbol e_3]_\times\end{bmatrix}\in \mathbb{R}^{12\times 6}.$$

3: Check that the $\boxplus$ operation makes sense.

For a Gauss-Newton update step, the recursive relation is

$$\boldsymbol x_{k+1}=\boldsymbol x_k \boxplus \alpha_k \boldsymbol J^\dagger r(\boldsymbol{x}_k),$$

so that means that we have the requirement

$$\boldsymbol J^\dagger r(\boldsymbol{x}_k)\in \mathbb{R}^6 \cong \mathfrak{se}(3).$$

Carrying out the pseudoinverse expansion of $\boldsymbol J^\dagger$ and multiplying with $r$ will indeed verify that the dimensions are all correct.