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Welcome to the OpenSim Developer Week Wiki. The information in this Wiki page is intended to help you formulate a project that may both benefit from the OpenSim 4.0 API and is tractable for a one-week workshop. This wiki will be updated regularly so please check in as we get closer to the development week.  

Important Dates 

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OpenSim 4.0 will allow for greater access and flexibility to software builders, making possible what may have been impossible in previous releases. Below is a list of 4.0 feature descriptions and some enabled projects to give you a sense of the types of development projects that may now be achievable.  Though the workshop will predominantly use the new 4.0 API interface, people who are developing new tools for the OpenSim 3 releases are also encouraged to apply. 

Features in Version 4.0

1. Nested model components. Compose a complex model from simpler more robust/testable subcomponents; for example, a muscle model can be composed of standalone activation dynamics and fiber dynamics subcomponents. As a result, components have a “path name”; e.g., “soleus_r/fiber,” We also improved the mechanism for accessing the components within a model by either path name or type.

2. Connectors between model components. Components depend on each other: joints connect two bodies, muscles need wrapping surfaces, etc. We have formalized the method for specifying the connections between components to make it easier to build modular models and to make these connections far less bug-prone.

3. Reference Frames. Performing and reporting kinematic transforms is one of the most useful functions of a multibody model and defining reference frames is essential for transforming data. Frames are new abstraction for defining and attaching reference frames in OpenSim and they have the ability to re-express quantities (positions, velocities, etc.) defined in one Frame to any other Frame.

4. Simulink-style wiring of signals between components. Each component can have Inputs and Outputs. Any model-computed value based on the state is a candidate as an Output, and can be wired to another component’s Input. A Reporter is no longer an inflexible Analysis but a modular Component that reads from designated Outputs and writes to in-memory data tables of results. That means no more reading storage files just to access a computed quantity.

5. Use any data file format you want. A new DataTable class replaces Storage as OpenSim’s primary container for simulation inputs and outputs. You can create a DataTable from any file format (CSV, C3D, MAT) using the new Adapter classes. Adapters convert files of various formats to DataTables consumable by OpenSim Solvers and Tools. DataTables generated by Solvers can also be written out to various files via an adapter.

6. Save exact simulation results to a file. The new StatesTrajectory class allows perfect replay of simulations from ForwardDynamics and CMC without loss of precision (as with states storage files) or of the values for non-continuous state variables (e.g. modeling options and discrete state variables).

7. Compose your own “Study” by walking through States, realizing Model dynamics and accelerations, and piping Outputs to Reporters and writing results to a file format of your choice.

8. Extend OpenSim classes through Python (e.g., write your Analysis class in Python instead of C++).

Projects Enabled by New Features in 4.0

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1. Pure Kinematics Study. Using coordinate information only, differentiate them to estimate velocity and acceleration and report the position, velocity and/or acceleration of Frames of interest.

2. Custom muscle analysis. From simulation results (StatesTrajectory), extract muscle internal variables (tendon strain, pennation angle, active force, force-velocity multiplier, etc.) and write them to file without other information (moments, moment-arms, etc.) coming for the ride.

3. More detailed muscle models. Modeling fatigue or different fiber type dynamics does not require authoring yet another complete muscle model. Instead, existing muscle models with constituent components (for activation dynamics, fiber kinematics, and tendon dynamics) can be overridden to include additional activation and fiber states with associated differential equations.

4. Parametric model scaling using Frames. Since joint locations and muscle origins and insertions can all be specified with respect to Frames, the parameters that define a single Frame can transform muscle attachments and joint axes, for example to represent tibial torsion or otherwise elongate/shorten and bend bones.

5. Implement new file adapter to support .mat, .anc, etc…

6. Live data streaming to DataTables to be processed by OpenSim Solvers in real-time (e.g., IK/ID)

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1. Implicit model formulation for improved Optimization

2. Task-space controller design

3. Patient-specific joint modeling from high-fidelity data

4. Script based analysis pipeline 

5. Net moment/force on Bodies calculator

6. Point of force application calculator for Contact Model visualization

7. Modeling of human-device interaction (exoskeleton, robotic-assistive device)

Detailed Agenda (Draft)

This is a five-day workshop. On the first day of the workshop, we will provide an overview of OpenSim 4.0, with discussions, guided exercises and interactive lectures to introduce the new features. The second day will consist of guided exercises on contributing using GitHub and how you can incorporate collaboration and contribution to your project. Days three and four will be devoted to working on projects that you and other participants bring to the workshop. Day five will include group discussions and presentations about your workshop achievements and plans for wrapping up your development.

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