===== Laboratory Coordinate System =====

There is no standard coordinate system used in the motion analysis community, despite the efforts of the International Society of Biomechanics. Nonetheless, we assume that each individual user has a standard coordinate system within their own laboratory. Visual3D allows the user to express their own coordinate system which is saved saved to the user's registry and will be in effect until changed explicitly by this program or if the registry entries are deleted.

==== Visual3D's Global Coordinate System ====

The axes of Visual3D's default Global Coordinate System (GCS) are oriented such that the GCS Z-axis points vertically upward, the GCS Y-axis points along the direction of progression and the GCS X-axis is perpendicular to the other two axes as described by a right-handed Cartesian coordinate system.

==== Modifying the Laboratory Coordinate System ====

If the user employs a Laboratory Coordinate System (LCS) with an orientation that is different than Visual3D's default GCS, then they must modify the coordinate system defined for the laboratory by selecting the "Modify Lab Coordinate System" option in the "Properties" menu.

{{:LaboratoryCoordinateSystemDialog.png}}

The coordinate system's axial direction is assumed to represent the vertical direction and the gravity unit vector is modified accordingly. For example,

  - If the axial direction is +z then the gravity unit vector is {0, 0, -1};
  - If the axial direction is +y then the gravity unit vector is {0, -1, 0};
  - If the axial direction is +x then the gravity unit vector is {-1, 0, 0}.

==== Effects of the Laboratory Coordinate System ====

As you might expect, specifying a new LCS in Visual3D has many consequences:

  - The LCS defines the coordinate system in which the motion capture data was recorded
  - The LCS defines the direction of gravity
  - The LCS defines the default [[visual3d:documentation:modeling:coordinate_system|Segment Coordinate System]] when a segment is created in [[visual3d:documentation:definitions:model_builder_mode|Model Builder mode]]

{{:SegmentCoordinateSytems.gif}}


**IMPORTANT NOTE**: If the user changes the laboratory coordinate system in the Workspace after one or more segments have been created, the segments created after the change will have a different segment coordinate system for those created before the change!