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The topics covered in this section include:

Table of Contents

Laboratory Coordinates

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OpenSim uses the standard engineering coordinate system of X forward (Red), Y up (Green), Z right (Blue). This convention is confirmed using the right-hand rule.

When collecting data to input into OpenSim it is important to note the coordinate system of the collection system. Every set of (x, y, z) coordinates obtained from a motion capture system is given relative to some coordinate system. Typically, this coordinate system is called the laboratory coordinate system,

or simply laboratory coordinates. The laboratory coordinate system is generally an inertial frame fixed to the Earth. Before inputting any coordinates

laboratory coordinates, or Ground.

Typically, motion capture systems use laboratory coordinates such that X is forward, Y is left, and Z is up. Collecting any data in this coordinate system would require a 90 degree rotation about the X axis before using in OpenSim.


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Before inputting any data from motion capture into OpenSim, it is your responsibility to ensure that all (x, y, z)

coordinates

values have been transformed from the laboratory coordinate system to the model coordinate system used in OpenSim.

Although you can define an arbitrary model coordinate system, the standard convention used in OpenSim is as follows: Assume that the model is a full-body musculoskeletal model of the human body, standing in an upright position on the ground. The origin of the model coordinate system is halfway between its feet. The x-axis of the model coordinate system points forward from the model, the y-axis points upward, and the z-axis points to the right of the model.

If all positions and distances are converted to meters, then all (x, y, z) coordinates can be mapped from the laboratory coordinate system to the model coordinate system by an orthonormal transformation. An orthonormal transformation can be represented by a 3 X 3 rotation matrix whose rows (and columns) are a set of orthogonal vectors of length one,. This matrix represents the orientation of the laboratory coordinate frame in the model coordinate frame. So, to transform the coordinates of a point labP = (x, y, z) given in the laboratory coordinate frame to its coordinates modelP = (x', y', z') in the model coordinate frame, you would employ the following transformation, where modellabR is the matrix whose columns are the vectors of the laboratory coordinate frame specified in the model coordinate frame:
modelP = modellabR * labP

External forces and moments are usually given in the coordinate system of a particular force sensor, such as a force plate, which may be different than the laboratory coordinate system. In this case, the force and moment data must be transformed from the appropriate force sensor's coordinate system to the model coordinate system.

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In the figure to the left, marker data (blue spheres) and ground reaction forces (green arrows) have been transformed from the motion capture coordinate system to the OpenSim coordinate system. This required a 90-degree rotation and all distances being converted to meters.


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To perform the rotation, you could use conversion packages found here or use the rotation method in the OpenSim GUI.

To transform Data in the GUI, see section Previewing Motion Capture (Mocap) Data for further details on how to view and transform experimental data in the GUI.











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