ControlPoints are used in surface construction. Every subject has an associated control.dat file in their tmp directory; this is the list of ControlPoints, in TalairachSpace, that is read in to TkMedit automatically upon start.
Display Options
ControlPoints appear as green cross hairs in the Display Window. They can be turned off by unchecking Views->Control Points.
Making and Deleting Control Points
ControlPoints can be edited with the Edit Control Points tool, which can be activated by pushing the attachment:Icons/TkMedit/icon_edit_ctrlpts.gif button on the Main Toolbar, by choosing Tools->Edit Control Points, or by pressing the 'c' key when the mouse is in the Display Window.
To make a new ControlPoint, click anywhere in the Display Window with mouse button 2. To remove a control point, click near it with mouse button 3. Button 3 will remove the closest control point to the voxel clicked, within 3 voxels.
Saving
Whenever a new ControlPoint is made, it is automatically written to the control.dat file. However, removed ControlPoints are not removed from the file until you choose File->Save Control Points or quit normally with the File->Quit command.