Creating a LieGroup

This guide explains how to create a LieGroup class, which builds upon the Manifold concept. Please read Creating a Manifold Class first, as it explains the foundational traits-based design and concept checking used in GTSAM.

A Lie group is a manifold that is also a group, with smooth group operations. In GTSAM, this means it has all the properties of a Manifold plus notions of composition, identity, and inversion. GTSAM uses the Curiously Recurring Template Pattern (CRTP) to automatically provide many of the more complex manifold and group methods.

1. From Manifold to Lie Group¶

A Lie group has a special element: the identity element. This allows us to define global Expmap and Logmap operations that are centered at this identity. The retract and localCoordinates methods required by the Manifold concept are then implemented in terms of these identity-centered operations.

To implement a Lie group, you inherit from gtsam::LieGroup<MyGroup, D>.

2. Minimal Implementation Requirements for MyGroup¶

MyGroup.h - Header File Requirements¶

1. Class Definition and Inheritance:

   • Inherit publicly from gtsam::LieGroup<MyGroup, D>, where D is the dimension of the group’s tangent space.

2. Essential Typedefs and Constants:

   • static const size_t dimension = D;
   • using ChartJacobian = gtsam::OptionalJacobian<D, D>;

3. Constructors:

   • A default constructor that initializes to the identity element.

4. Group Primitives:

   • static MyGroup Identity();
   • MyGroup inverse() const;
   • MyGroup operator*(const MyGroup& other) const;

5. Lie Group Primitives:

   • static MyGroup Expmap(const gtsam::VectorD& v, ChartJacobian H = {});: The Exponential map at identity. Must support an optional derivative.
   • static gtsam::VectorD Logmap(const MyGroup& p, ChartJacobian H = {});: The Logarithm map at identity. Must support an optional derivative.
   • gtsam::MatrixD AdjointMap() const;: Computes the Adjoint map.
   • ChartAtOrigin struct: This nested struct implements a first-order retract and local. You must define:
     • static MyGroup Retract(const gtsam::VectorD& v, ChartJacobian H = {});
     • static gtsam::VectorD Local(const MyGroup& p, ChartJacobian H = {});

6. using Declaration for inverse:

   • using LieGroup<MyGroup, D>::inverse;: This makes the base class inverse(ChartJacobian H) method visible.

7. Utilities (Testable concept):

   • void print(const std::string& s) const;
   • bool equals(const MyGroup& other, double tol) const;

Handling Dynamically-Sized Lie Groups¶

If your Lie group can change size at runtime, you must make two corresponding changes:

1. Set the static dimension to Eigen::Dynamic in the LieGroup template parameter:
   • class MyGroup : public gtsam::LieGroup<MyGroup, Eigen::Dynamic> { ... };
2. You must also provide an instance method that returns the object’s runtime dimension:
   • int dim() const;

The LieGroup framework relies on this method to correctly size Jacobians and other matrices at runtime.

3. What You Get for Free (via CRTP)¶

By inheriting from gtsam::LieGroup and defining the primitives above, your class automatically receives correct implementations for all of the following methods, including their Jacobian versions. Do NOT implement these yourself, unless you are sure you are able to more efficiently (and correctly) implement the Jacobians.

• compose(const MyGroup& other): Composes *this with other.
• between(const MyGroup& other): Calculates the relative transformation from *this to other.
• retract(const VectorD& v): Implemented as compose(ChartAtOrigin::Retract(v)).
• localCoordinates(const MyGroup& other): Implemented as ChartAtOrigin::Local(between(other)).
• expmap(const VectorD& v): Implemented as compose(Expmap(v)).
• logmap(const MyGroup& other): Implemented as Logmap(between(other)).

4. Traits and Concept Checking¶

The LieGroup concept builds on Manifold. You do not need to specialize the traits struct yourself, as the LieGroup base class handles it.

To verify your implementation, use the GTSAM_CONCEPT_LIE_INST macro in your unit test file.

#include <gtsam/base/TestableAssertions.h>
#include <gtsam/base/Lie.h>

// Your class include
#include <gtsam/geometry/MyGroup.h>

// This macro will fail to compile if MyGroup is not a valid LieGroup.
GTSAM_CONCEPT_LIE_INST(MyGroup)

// ... your unit tests ...