gtsam/gtsam/slam/ReadMe.md

SLAM Factors

GenericProjectionFactor (defined in ProjectionFactor.h)

Non-linear factor that minimizes the re-projection error with respect to a 2D measurement. The calibration is assumed known and passed in the constructor. The main building block for visual SLAM.

Templated on

• POSE, default Pose3
• LANDMARK, default Point3
• CALIBRATION, default Cal3_S2

SmartFactors

These are "structure-less" factors, i.e., rather than introducing a new variable for an observed 3D point or landmark, a single factor is created that provides a multi-view constraint on several poses and/or cameras. While one typically adds multiple GenericProjectionFactors (one for each observation of a landmark), a SmartFactor collects all measurements for a landmark, i.e., the factor graph contains 1 smart factor per landmark.

SmartFactorBase

This is the base class for smart factors, templated on a CAMERA type. It has no internal point, but it saves the measurements, keeps a noise model, and an optional sensor pose.

SmartProjectionFactor

Also templated on CAMERA. Triangulates a 3D point and keeps an estimate of it around. This factor operates with monocular cameras, and is used to optimize the camera pose and calibration for each camera, and requires variables of type CAMERA in values.

If the calibration is fixed use SmartProjectionPoseFactor instead!

SmartProjectionPoseFactor

Derives from SmartProjectionFactor but is templated on a CALIBRATION type, setting CAMERA = PinholePose<CALIBRATION>. This factor assumes that the camera calibration is fixed and the same for all cameras involved in this factor. The factor only constrains poses.

If the calibration should be optimized, as well, use SmartProjectionFactor instead!

SmartProjectionFactorP

Same as SmartProjectionPoseFactor, except:

• it is templated on CAMERA, i.e., it allows cameras beyond pinhole;
• it admits a different calibration for each measurement, i.e., it can model a multi-camera system;
• it allows multiple observations from the same pose/key, again, to model a multi-camera system.

TODO: DimPose and ZDim are hardcoded. Copy/paste from SmartProjectionPoseFactor. Unclear what the use case is.

Linearized Smart Factors

The factors below are less likely to be relevant to the user, but result from using the non-linear smart factors above.

RegularImplicitSchurFactor

A specialization of a GaussianFactor to structure-less SFM, which is very fast in a conjugate gradient (CG) solver. It is produced by calling createRegularImplicitSchurFactor in SmartFactorBase or SmartProjectionFactor.

JacobianFactorQ

A RegularJacobianFactor that uses some badly documented reduction on the Jacobians.

JacobianFactorQR

A RegularJacobianFactor that eliminates a point using sequential elimination.

JacobianFactorQR

A RegularJacobianFactor that uses the "Nullspace Trick" by Mourikis et al. See the documentation in the file, which is well documented.