fuse_core
- The fuse_core package provides the base class interfaces for the various fuse components. Concrete implementations of these
interfaces are provided in other packages.
README
fuse_core
This package provides the base class interfaces for the various fuse components. Concrete implementations of these interfaces are provided in other packages.
Variable
Within the fuse system, a “variable” is a convenient group of one or more related scalar values. These scalars are treated as a block within the optimizer. If the variable is modeling a time-varying quatity, the an instance of the variable represents value of the state at one specific point in time. The fuse system maintains a history of variable instances, allowing measurements to involve previous states as well as the current state.
When defining a new variable type, there is a balance that must be struct between reusability and convenience. If you define a complex composite state, it is unlikely that any other available components will use that same state definition. If you make the state too granular, then more book-keeping and value lookups will be required to piece together a useful concept from many smallar scalar components.
As an example, let’s consider a 3D pose consisting of a 3D position (x, y, z) and a quaternion orientation (qx, qy, qx, qw). We could define a single state for the 3D pose consisting of all 7 scalar components. Alternatively, we could define two variables types, a 3 dimension position vector and a 4 dimension quaternion. Or we could even define seven variable types, one for each dimension.
Within the fuse_core package, no concrete variables are actually created. We only define the common interface to which
all types must adhere. A set of common variable types are provided in the fuse_variables
package. And new variable types can be created outside the fuse stack completely. However, similar to how using common
ROS messages across nodes allow for code reuse, using common variable types will allow sensor models and motion models
to be shared across the community.
Constraint
A Constraint defines a cost function that is connected to one or more variables. This base class defines the required interface of all Constraint objects, and holds the ordered list of involved variable UUIDs. All other functionality is left to the derived classes to implement.
Most importantly, the implementation of the cost function is left to the derived classes, allowing arbitrarily complex sensor models to be implemented outside of the core fuse packages. The cost function must be a valid ceres::CostFunction object. Ceres provides many nice features to make implementing the cost function easier, including an automatic differentiation system. Please see the Ceres documentation for details on creating valid ceres::CostFunction objects (http://ceres-solver.org/nnls_modeling.html). In addition to the cost function itself, an optional loss function may be provided. Loss functions provide a mechanism for reducing the impact of outlier measurements on the final optimization results. Again, see the Ceres documentation for details (http://ceres-solver.org/nnls_modeling.html#lossfunction).
Transaction
A transaction is a group of variable and constraint additions and subtractions that should all be processed at the same time. This arises most often with graph edits, when you want to remove an existing constraint and replace it with one or more new constraints. You don’t want the removal to happen independently of the additions. All graph operations are contained within a Transaction object so that all operations are treated equally.