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Defining our Global Coordinate System
Defining our Global Coordinate System

Outlining the Global Coordinate System used by Uplift Capture.

Written by Steven Hirsch
Updated over a week ago

The purpose of this article is to outline the Coordinate System used by Uplift Capture. Although we outline some technical aspects of the system, the purpose of this document was not to provide an encompassing technical overview of how Uplift Capture exports 3D biomechanical data from two iPhones.

Before outlining the kinematic outputs, it is essential to first outline the Global Coordinate System (GCS) of the biomechanical data. First, we’ll outline the “original” GCS used internally for the computer vision data. Then, we’ll outline the “External” GCS that is included in the csv exports (i.e., what you receive in Webhooks or by clicking “Download Kinematics Data” in the web visualizer).

Figure 1 outlines our “original” GCS:

The global origin is situated in the top left of the primary camera. The X-axis points to the right in the video, the Y-axis points downwards in the video, and the Z-axis points “into” the video.

However, since most biomechanical analyses assume a vertical axis pointing “up”, our data exports are rotated 180 degrees about the global X-axis. Then, we translate the origin to the average (with outliers removed) position of the left hip joint center of the capture. Thus, our “External” GCS (i.e., the GCS in the data exports) is shown in Figure 2 below:

The global origin is located in the average (outliers removed) position of the left hip joint center throughout the session. The X-axis points relatively to the right in the video captured by the primary camera, the Y-axis points vertically, and the Z-axis points “back” towards the primary camera.

In summary, our Y-axis points vertically in our data exports, but the X-axis and Z-axis are dependent on the positioning of the primary camera. Depending on your applications, you may want to rotate your GCS so a particular axis points (roughly) to a particular object (e.g., the Z-axis pointing towards the mound during a swing). If this is something you are interested in, please contact us for more information about how to perform this transformation.

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