Bundle adjustment demo using Ceres Solver

Also read a blog post.

• Ceres Solver

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.

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.

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

BSD New