viewmodel.html

JAVA多媒体开发类库说明

position and orientation to that of a six-degrees-of-freedom tracking
device. By slaving the frustum to the tracker, Java 3D can
automatically modify the view frustum so that the generated images
match the end-user's viewpoint exactly.
<p>Java&nbsp;3D must handle two rather different head-tracking
situations.
In one case, we rigidly attach a tracker's <em>base</em>,
and thus its coordinate frame, to the display environment. This
corresponds to placing a tracker base in a fixed position and
orientation relative to a projection screen within a room, to a
computer display on a desk, or to the walls of a multiple-wall
projection display. In the second head-tracking situation, we rigidly
attach a tracker's <em>sensor</em>, not its base, to the display
device. This corresponds to rigidly attaching one of that tracker's
sensors to a head-mounted display and placing the tracker base
somewhere within the physical environment.
</p>
<p>
</p>
<h2>Physical Environments and
Their Effects</h2>
Imagine an application where the end user sits on a magic carpet. The
application flies the user through the virtual environment by
controlling the carpet's location and orientation within the virtual
world. At first glance, it might seem that the application also
controls what the end user will see-and it does, but only
superficially.
<p>The following two examples show how end-user environments can
significantly affect how an application must construct viewing
transformations.
</p>
<p>
</p>
<h3>A Head-Mounted Example</h3>
Imagine that the end user sees the magic carpet and the virtual world
with a head-mounted display and head tracker. As the application flies
the carpet through the virtual world, the user may turn to look to the
left, to the right, or even toward the rear of the carpet. Because the
head tracker keeps the renderer informed of the user's gaze direction,
it might not need to draw the scene directly in front of the magic
carpet. The view that the renderer draws on the head-mount's display
must match what the end user would see if the experience had occurred
in the real world.
<h3>A Room-Mounted Example</h3>
Imagine a slightly different scenario where the end user sits in a
darkened room in front of a large projection screen. The application
still controls the carpet's flight path; however, the position and
orientation of the user's head barely influences the image drawn on the
projection screen. If a user looks left or right, then he or she sees
only the darkened room. The screen does not move. It's as if the screen
represents the magic carpet's "front window" and the darkened room
represents the "dark interior" of the carpet.
<p>By adding a left and right screen, we give the magic carpet rider a
more complete view of the virtual world surrounding the carpet. Now our
end user sees the view to the left or right of the magic carpet by
turning left or right.
</p>
<p>
</p>
<h3>Impact of Head Position and
Orientation on the Camera</h3>
In the head-mounted example, the user's head position and orientation
significantly affects a camera model's camera position and orientation
but hardly has any effect on the projection matrix. In the room-mounted
example, the user's head position and orientation contributes little to
a camera model's camera position and orientation; however, it does
affect the projection matrix.
<p>From a camera-based perspective, the application developer must
construct the camera's position and orientation by combining the
virtual-world component (the position and orientation of the magic
carpet) and the physical-world component (the user's instantaneous head
position and orientation).
</p>
<p>Java&nbsp;3D's view model incorporates the appropriate abstractions
to
compensate automatically for such variability in end-user hardware
environments.
</p>
<p>
</p>
<h2>The Coordinate Systems</h2>
The basic view model consists of eight or nine coordinate systems,
depending on whether the end-user environment consists of a
room-mounted display or a head-mounted display. First, we define the
coordinate systems used in a room-mounted display environment. Next, we
define the added coordinate system introduced when using a head-mounted
display system.
<h3>Room-Mounted Coordinate
Systems</h3>
The room-mounted coordinate system is divided into the virtual
coordinate system and the physical coordinate system. <a
 href="#Figure_5">Figure
5</a>
shows these coordinate systems graphically. The coordinate systems
within the grayed area exist in the virtual world; those outside exist
in the physical world. Note that the coexistence coordinate system
exists in both worlds.
<h4>The Virtual Coordinate
Systems</h4>
<h5> The Virtual World Coordinate System</h5>
The virtual world coordinate system encapsulates
the unified coordinate system for all scene graph objects in the
virtual environment. For a given View, the virtual world coordinate
system is defined by the Locale object that contains the ViewPlatform
object attached to the View. It is a right-handed coordinate system
with +<em>x</em> to the right, +<em>y</em> up, and +<em>z</em> toward
the viewer.
<h5> The ViewPlatform Coordinate System</h5>
The ViewPlatform coordinate system is the local coordinate system of
the ViewPlatform leaf node to which the View is attached.
<p><a name="Figure_5"></a><img style="width: 500px; height: 181px;"
 alt="Display Rigidly Attached to Tracker Base"
 title="Display Rigidly Attached to Tracker Base" src="ViewModel5.gif"></p>
<p>
</p>
<ul>
  <font size="-1"><b><i>Figure 5</i> &#8211; Display Rigidly Attached to the
Tracker Base</b></font>
</ul>
<p>
</p>
<h5> The Coexistence Coordinate System</h5>
A primary implicit goal of any view model is to map a specified local
portion of the physical world onto a specified portion of the virtual
world. Once established, one can legitimately ask where the user's head
or hand is located within the virtual world or where a virtual object
is located in the local physical world. In this way the physical user
can interact with objects inhabiting the virtual world, and vice versa.
To establish this mapping, Java&nbsp;3D defines a special coordinate
system,
called coexistence coordinates, that is defined to exist in both the
physical world and the virtual world.
<p>The coexistence coordinate system exists half in the virtual world
and
half in the physical world. The two transforms that go from the
coexistence coordinate system to the virtual world coordinate system
and back again contain all the information needed to expand or shrink
the virtual world relative to the physical world. It also contains the
information needed to position and orient the virtual world relative to
the physical world.
</p>
<p>Modifying the transform that maps the coexistence coordinate system
into the virtual world coordinate system changes what the end user can
see. The Java&nbsp;3D application programmer moves the end user within
the
virtual world by modifying this transform.
</p>
<p>
</p>
<h4>The Physical Coordinate
Systems</h4>
<h5> The Head Coordinate System</h5>
The head coordinate system allows an application to import its user's
head geometry. The coordinate system provides a simple consistent
coordinate frame for specifying such factors as the location of the
eyes and ears.
<h5> The Image Plate Coordinate System</h5>
The image plate coordinate system corresponds with the physical
coordinate system of the image generator. The image plate is defined as
having its origin at the lower left-hand corner of the display area and
as lying in the display area's <em>XY</em>
plane. Note that image plate is a different coordinate system than
either left image plate or right image plate. These last two coordinate
systems are defined in head-mounted environments only.
<h5> The Head Tracker Coordinate System</h5>
The head tracker coordinate system corresponds to the
six-degrees-of-freedom tracker's sensor attached to the user's head.
The head tracker's coordinate system describes the user's instantaneous
head position.
<h5> The Tracker Base Coordinate System</h5>
The tracker base coordinate system corresponds to the emitter
associated with absolute position/orientation trackers. For those
trackers that generate relative position/orientation information, this
coordinate system is that tracker's initial position and orientation.
In general, this coordinate system is rigidly attached to the physical
world.
<h3>Head-Mounted Coordinate
Systems</h3>
Head-mounted coordinate systems divide the same virtual coordinate
systems and the physical coordinate systems. <a href="#Figure_6">Figure
6</a>
shows these coordinate systems graphically. As with the room-mounted
coordinate systems, the coordinate systems within the grayed area exist
in the virtual world; those outside exist in the physical world. Once
again, the coexistence coordinate system exists in both worlds. The
arrangement of the coordinate system differs from those for a
room-mounted display environment. The head-mounted version of
Java&nbsp;3D's
coordinate system differs in another way. It includes two image plate
coordinate systems, one for each of an end-user's eyes.
<h5> The Left Image Plate and Right Image Plate Coordinate Systems</h5>
The left image plate and right image plate
coordinate systems correspond with the physical coordinate system of
the image generator associated with the left and right eye,
respectively. The image plate is defined as having its origin at the
lower left-hand corner of the display area and lying in the display
area's <em>XY</em> plane. Note that the left image plate's <em>XY</em>
plane does not necessarily lie parallel to the right image plate's <em>XY</em>
plane. Note that the left image plate and the right image plate are
different coordinate systems than the room-mounted display
environment's image plate coordinate system.
<p><a name="Figure_6"></a><img style="width: 499px; height: 162px;"
 alt="Display Rigidly Attached to Head Tracker"
 title="Display Rigidly Attached to Head Tracker" src="ViewModel6.gif"></p>
<p>
</p>
<ul>
  <font size="-1"><b><i>Figure 6</i> &#8211; Display Rigidly Attached to the
Head Tracker (Sensor)</b></font>
</ul>
<p>
</p>
<h2>The Screen3D Object</h2>
A Screen3D object represents one independent display device. The most
common environment for a Java&nbsp;3D application is a desktop computer
with
or without a head tracker. <a href="#Figure_7">Figure
7</a> shows a scene graph fragment for a display environment designed
for such an end-user environment. <a href="#Figure_8">Figure
8</a> shows a display environment that matches the scene graph
fragment in <a href="#Figure_7">Figure
7</a>.
<p><a name="Figure_7"></a><img style="width: 499px; height: 185px;"
 alt="Environment with Single Screen3D Object"
 title="Environment with Single Screen3D Object" src="ViewModel7.gif"></p>
<p>
</p>
<ul>
  <font size="-1"><b><i>Figure 7</i> &#8211; A Portion of a Scene Graph
Containing a Single Screen3D
Object</b></font>
</ul>
<p>
<a name="Figure_8"></a><img style="width: 500px; height: 237px;"
 alt="Single-Screen Display Environment"
 title="Single-Screen Display Environment" src="ViewModel8.gif"></p>
<p>
</p>
<ul>
  <font size="-1"><b><i>Figure 8</i> &#8211; A Single-Screen Display
Environment</b></font>
</ul>
<p>
A multiple-projection wall display presents a more exotic environment.
Such environments have multiple screens, typically three or more. <a
 href="#Figure_9">Figure
9</a> shows a scene graph fragment representing such a system, and <a
 href="#Figure_10">Figure
10</a> shows the corresponding display environment.
</p>
<p><a name="Figure_9"></a><img style="width: 500px; height: 196px;"
 alt="Environment with Three Screen3D Object"
 title="Environment with Three Screen3D Object" src="ViewModel9.gif">
</p>
<p>
</p>
<ul>
  <font size="-1"><b><i>Figure 9</i> &#8211; A Portion of a Scene Graph
Containing Three Screen3D
Objects</b></font>
</ul>
<p>
<a name="Figure_10"></a><img style="width: 700px; height: 241px;"
 alt="Three-Screen Display Environment"
 title="Three-Screen Display Environment" src="ViewModel10.gif"></p>
<p>
</p>