To define the local coordinate system (LCS) of one of the objects in your subject file, the object should already have a surface file specified for it. This file, whose vertices are expressed in the object's CT reference frame, was probably created by Surface3D or a third-party application. Before tracking an object in X4D or Locate3D, you should define an LCS for it because the tracking output for each X-ray frame is the transform from the X-ray lab frame to the object's LCS.

Follow these steps for defining an LCS:

1. Load the subject file.
2. Choose the session and object.
3. Turn on the display of the global axes.
4. Turn on the display of the local axes.
5. You'll see that the global and local axes are coincident (unless an LCS is already defined in the subject file). There are three ways to define the LCS:
   1. Manually: Ctrl-left-click on the local axes in the 3D view to display a trackball. Use the trackball to rotate and translate the local axes to the appropriate location. When you are done, use the menu option LCS→Save to Subject Object to save the LCS transform to the subject that is loaded in memory (it does not save the subject file).
   2. From File: Select LCS→Load from File… and select the data file that contains the 4×4 transform from the global frame to the local frame. The LCS file must contain exactly 16 numbers, separated by commas or white space. The subject file loaded in memory will be automatically updated with the LCS.
   3. Algorithmically: Open the //Local Coordinate System// widget and click on the appropriate algorithm. Most of the available algorithms use manually placed landmarks to guide the algorithm, though in some cases these landmarks are optional. Consult the documentation on the chosen algorithm for details on the landmarks and how they are used. Once you are done specifying landmarks, press Apply to run the algorithm and calculate the LCS. The subject file loaded in memory will be automatically updated with the LCS.
6. Select File→Save Surface (local frame) to write the ROI Surface File for this object. If this file is already specified for this object in the subject file, it will be [over-]written without prompting you for a file name. Otherwise, select the name of a new OBJ file. Select “meters” as the units so the file can be used in Visual3D.
7. Select File→Save Subject to save the subject file with the LCS and the link to the ROI Surface File. If you later add regions of interest to the surface model and save them to an OBJ file, this ROI Surface File will be overwritten. This is by design; if you create a surface model in the LCS with ROIs, the DSX Suite does not also need a surface model in the LCS without the ROIs.