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Fig. 4.
Free clipping in VR (The original data set was provided by Dr. A. Koning,
SARA, the Netherlands).
Using a wand the user may navigate through a virtual world, explore the
patient's body and even walk through an artery. However, to navigate and ma-
nipulate successfully in a 3D virtual world a user should possess special motor
skills, which is not an easy task for all people [6].
3.2
A Personal Desktop Assistant
It is known that the desktop projection modality suits the individual work the
best [6]. That is why we called the interaction style provided by the desktop VRE
'a Personal Desktop Assistant'. In principle a user does not need additional motor
skills to interact with the desktop VRE. The biggest problem arises from the
fact that within a desktop application we cannot manipulate 3D objects directly,
we always deal with 2D projected representations of these objects [7]. Even
though the 3D representation of data is provided by many desktop applications,
it does not play an important role with respect to the manipulation or navigation
capabilities. It is used mostly as a passive viewer, which helps a user to orient
better.
Thus, to add a bypass or a stent within the desktop VRE a user has to
deal with several projected representations of an artery and auxiliary dialogue
menus. The same concerns interactive measurements in a desktop. The procedure
of adding a marker is similar to the procedure of grid editing. If in VR a user
can add a marker via a direct manipulation using the position and orientation
of a wand, switching to the desktop projection modality leads to the necessity
to deploy extra menus and sliders to help the user to orient him or herself in a
projected 3D world.
The GUI of the clipping engine of the VRE deployed for the desktop pro-
jection modality is shown in Fig. 5. In comparison to VR versions, additional
interface capabilities have been applied. Thus, a user may select a slice of interest
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