When a dragger is constructed, it checks whether a resource file for overriding the defaults has been created. When reading from this file (if found), if the dragger encounters a second definition of a particular geometry, the new geometry replaces any previously defined geometry of the same name.

Inventor will look for files only if the environment variable SO_DRAGGER_DIR has been set. If it has, Inventor will look in that directory.

In all cases, a given dragger class will read only files of its same name: the SoTranslate1Dragger SoTranslate1Dragger SoTranslate1Dragger class reads only resource files named translate1Dragger.iv, the SoTrackballDragger SoTrackballDragger SoTrackballDragger class reads only files named trackballDragger.iv.

Changing the default geometry of a dragger part is a simple matter of creating a new file that contains a scene graph defining the new resource. When encountered, this new definition overrides the default definitions of that resource. The next two sections show how to do this using the SoTrackballDragger SoTrackballDragger SoTrackballDragger as a foundation.

To change the default geometry of a part, you merely redefine the scene graph for the geometry used by the part. When creating new resource files, it is necessary to define only the geometry that you wish to change. Other geometry will use the default values.

Keep in mind that you should never edit the resource files in Inventor/resources. If you want your application to use alternate resources, put your files in a convenient place and set the SO_DRAGGER_DIR environment variable to point there.

As an example, let's replace the trackballRotator resource of the trackball with a cube. (For more information on the Inventor file format, see Chapter 12, Importing data.) Looking at the default geometry file for the trackball, we see that the trackballRotator resource is defined by this scene graph:

# default geometry for SoTrackballDragger's "rotator" part (inactive)
DEF trackballRotator Separator {
   DrawStyle { style INVISIBLE }
   Sphere {}
}
# default geometry for SoTrackballDragger's "rotatorActive" part
DEF trackballRotatorActive Separator {
   DrawStyle { style INVISIBLE }
   Sphere {}
}

Note that, in the case of the trackball, the resources specify that the rotator and rotatorActive parts have the same geometry, an invisible sphere. Although this is common, some draggers may have completely different geometry for when they are inactive and active (and most manipulators have more complicated scene graphs than just a sphere).

To change the trackballRotator and trackballRotatorActive resources from an invisible sphere to a visible cube, you simply replace the sphere with a cube in both scene graphs:

# default geometry for the SoTrackballDragger's "rotator" part
DEF trackballRotator Separator {
   BaseColor {
      rgb 1. 1. 1.        #white
   }
   Cube {}
}
# default geometry for the SoTrackballDragger's "rotatorActive" part
DEF trackballRotatorActive Separator {
   BaseColor {
      rgb .5 .5  0.        #yellow
   }
   Cube {}
}

Using this mechanism, you can not only change the geometry of a given part, but also remove the functionality of that part entirely. For example, to disable the trackball's "rotator" part but still leave the cube visible, you can make the cube unpickable:

# default geometry for the SoTrackballDragger's "rotator" part
DEF trackballRotator Separator {
   BaseColor {
      rgb 1. 1. 1.        #white
   }
   PickStyle {
      style UNPICKABLE
   }
   Cube {}
}
# default geometry for the SoTrackballDragger's "rotatorActive" part
DEF trackballRotatorActive Separator {
   BaseColor {
      rgb .5 .5  0.        #yellow
   }
   PickStyle {
      style UNPICKABLE
   }
   Cube {}
}

To remove the trackball's rotator part altogether, leaving a trackball that can only rotate about its x, y, and z axes, you could redefine its geometry to be an empty scene graph:

# default geometry for SoTrackballDragger's "rotator" part
DEF trackballRotator Separator {
}
# default geometry for SoTrackballDragger's "rotatorActive" part
DEF trackballRotatorActive Separator {
}

You can also read the geometry from a file instead of defining it inline:

DEF trackballRotator Separator {
   File { name "myCustomRotator.iv" }
}
DEF trackballRotatorActive Separator {
   File { name "myCustomRotatorActive.iv" }
}

	Note: Never set a dragger part to NULL. Internal methods require a node to be present, even if it's simply an empty separator as shown in the previous example. (Often, the dragger parts are the children of a switch node. Changing a node to NULL could result in an incorrect ordering of the switch node's children.)