# Framework Design 1¶

Stone Soup is initially targeted at two different groups of users:

• Academics conducting research into tracking and state estimation, looking to quickly develop algorithms, and compare against other algorithms assessed against metrics.

• User, owners and processors of real data, looking to identify the best approach for their application, without the need for deep expertise required to implement many algorithms.

The central theme of the Stone Soup design philosophy is interchangeability. The framework is designed with the idea that users can easily insert their own components into existing constructs, and that they could mix and match components in new and unexpected ways. In support of this goal, the Stone Soup code architecture has been built on the principles of modularity and uniformity of external interfaces.

Stone Soup is object oriented and makes use of encapsulation, abstraction and inheritance:

Abstraction

Stone Soup trackers are built as hierarchical objects. For example, a MultiTargetTracker object may contain track Initiator, a track Deleter, Detector, DataAssociator, and Updater objects. Each of these objects is defined by an abstract class that specifies the external interface for that class; that is, the parameters and functions an object of that class must make available to the outside world.

Inheritance

An example, the Updater abstract class specifies that an Updater object must have a measurement_model attribute, and that it must have methods predict_measurement() and update() that returns a MeasurementPrediction and State object respectively. Therefore, all implementations of Updaters in Stone Soup (KalmanUpdater, ExtendedKalmanUpdater, ParticleUpdater, etc.) must have the specified elements.

Encapsulation

With the Updater example, this approach ensures that different Updaters are interchangeable (within limits), and that the Tracker can utilize them without knowing the details of the Updater implementation.

## Components¶

Stone Soup has a number of components used to both build a algorithm, but also enable an environment for testing and assessment.

### Enabling Components¶

The enabling components in Stone Soup consist of components for reading/simulating data, feeding into the algorithm, and then writing and assessing the output.

Stone Soup Logical Data Flow

The figure above shows the overall flow of data from the various components, showing for example how Detection data can be read directly from a DetectionReader, or maybe via a DetectionSimulator which uses GroundTruthPath data. This could also include reading data direct from a sensor. Outputs can also be passed into MetricGenerator, or written to file or database for later analysis. Note that all these components are optional.

### Algorithm Components¶

The algorithm components are those used to create a tracking or state estimation algorithm, where the main component (e.g. MultiTargetTracker) will define the parts required. These are currently mainly focused on target tracking use case in Stone Soup, but intent is also for general state estimation as well.

Stone Soup Multi Target Tracker

The figure above shows an example of a MultiTargetTracker, but note that other types of algorithms may use different components, and different combination/sequence. In this examples, this is processing detections over time, which then is predicting, associating, updating, initiating and deleting tracks. By using in here an KalmanPredictor, a KalmanUpdater, and a GaussianInitiator, this becomes a Kalman tracker; but with a ParticlePredictor, a ParticleUpdater, and a ParticleInitiator, this becomes a Particle tracker.

### Data Types¶

A key part of Stone Soup is the data types, that allow data to be passed between components. A fundamental example of this in Stone Soup is the State data type. This has a state_vector, and optional timestamp, which describes the state of something at a particular time. One child class of this is the Detection class, which describes a measurement from a sensor at a particular time; or the GaussianState which not only has a state_vector (Gaussian mean), but also has a covar (Gaussian covariance) and such describes the state with uncertainty in the form of a multivariate Gaussian distribution.

Stone Soup also employs duck typing, a technique that means that data types are flexible. For example, a State (as mentioned above) and GroundTruthPath (describing how target’s true state changes over time), both have a similar interface in terms of have a state vector and timestamp (in case of GroundTruthPath the most recent GroundTruthState). They therefore can both be used with TransitionModel instances, allowing models to be used for both target prediction (in case with Predictor) and in simulating targets (in case with Platform).

Footnotes

1

David Last, Paul Thomas, Steven Hiscocks, Jordi Barr, David Kirkland, Mamoon Rashid, Sang Bin Li, and Lyudmil Vladimirov “Stone Soup: announcement of beta release of an open-source framework for tracking and state estimation”, Proc. SPIE 11018, Signal Processing, Sensor/Information Fusion, and Target Recognition XXVIII, 1101807 (7 May 2019); https://doi.org/10.1117/12.2518514 [PDF]