ba_demo_ceres

Bundle adjustment demo using Ceres Solver

Also read a blog post.

Dependencies

• Ceres Solver

Introduction

This project implements a simple bundle adjustment problem using Cere Solver to solve it.

The constraints are defined with customized cost functions and self-defined Jacobians.

Two versions of on-manifold local parameterization of SE3 poses (both of which using 6-dof Lie algebra for updating) are used, including

• quaternion + translation vector (7d), and
• rotation vector + translation vector (6d)

Please check parametersse3.hpp parametersse3.cpp for the local parameterization implementation.

Jacobians

For convenience in expression, we denote the Jacobian matrix of the error function w.r.t. the Lie group by J_err_grp, the Jacobian matrix of the Lie group w.r.t. the local Lie algebra increment by J_grp_alg, and the Jacobian matrix of the error function w.r.t. the local Lie algebra increment by J_err_alg.

In many on-manifold graph optimization problems, only J_err_alg are required. However, in Ceres Solver, one can only seperately assign J_err_grp and J_grp_alg, but cannot directly define J_err_alg. This may be redundant and cost extra computational resources:

• One has to derive the explicit Jacobians equations of both J_err_grp and J_grp_alg.
• The solver may spend extra time in computing J_err_grp and J_grp_alg, and multiplying J_err_grp and J_grp_alg to get J_err_alg.

Therefore, for convenience, we use a not-that-elegant but effective trick. Let's say that the error function term is of dimension m, the Lie group N, and the Lie algebra n. We define the leading m*n block in J_err_grp to be the actual J_err_alg, with other elements to be zero; and define the leading n*n block in J_grp_alg to be identity matrix, with other elements to be zero. Thus, we are free of deriving the two extra Jacobians, and the computational burden of the solver is reduced -- although Ceres still has to multiply the two Jacobians, the overall computational process gets simpler.

Other tips

One advice for self-defined Jacobians: do not access jacobians[0] and jacobians[1] in the meantime, which may cause seg-fault.

License

BSD New