Station Defined Frames

This tutorial focuses on the StationDefinedFrame component, merged in opensim-core/pull/3694 , available in OpenSim >= v4.3.1. Specifically, this tutorial focuses on adding StationDefinedFrames via OpenSim Creator. If you’re more interested in the underlying implementation, or its associated OpenSim API, then StationDefinedFrame.h provides a technical (C++) description.

Overview

Frames compose a right-handed set of three orthogonal axes (i.e. a 3D rotation) and an origin point (i.e. a 3D translation). OpenSim has a Frame abstraction that it uses when defining almost all spatial relationships. Joints join Frames bodies are a Frame ; muscle path points are defined within Frames (as are markers); and meshes, contact geometry, and wrap surfaces are attached to Frames - the list goes on.

A Frame ‘s transform can be constant (e.g. Ground), based on constraints (e.g. Body), or user-defined (e.g. PhysicalOffsetFrame). User-defined Frame s are the primary way that modellers define spatial relationships in a model.

An Example

An OpenSim model file might encode “PhysicalOffsetFrame A is (0.5, 0, 0) meters, and a 90 degree rotation along the Y axis, in the Frame of body B”). You can see this for yourself by searching for PhysicalOffsetFrame in the source code of an existing .osim file.

StationDefinedFrame s are user-defined Frame s that calculate their rotation and translation from relationships between stations (rigid points) in the model. This mirrors how anatomical joint/body frames are typically defined (e.g. ISB CITATION), making them easier to work with in that context. Another advantage is that the StationDefinedFrame definition is compatible with algorithms that scale/warp/move points (e.g. TPS, as described in The Mesh Warper).