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Java 3D API - View Model
View Model
Java 3D introduces a new view model that takes Java's
vision of "write once, run anywhere" and generalizes it to include
display devices and six-degrees-of-freedom input peripherals such as
head trackers. This "write once, view everywhere" nature of the new
view model means that an application or applet written using the Java
3D view model can render images to a broad range of display devices,
including standard computer displays, multiple-projection display
rooms, and head-mounted displays, without modification of the scene
graph. It also means that the same application, once again without
modification, can render stereoscopic views and can take advantage of
the input from a head tracker to control the rendered view.
Java 3D's view model achieves this versatility by cleanly
separating
the virtual and the physical world. This model distinguishes between
how an application positions, orients, and scales a ViewPlatform object
(a viewpoint) within the virtual world and how the Java 3D
renderer
constructs the final view from that viewpoint's position and
orientation. The application controls the ViewPlatform's position and
orientation; the renderer computes what view to render using this
position and orientation, a description of the end-user's physical
environment, and the user's position and orientation within the
physical environment.
This document first explains why Java 3D chose a different view
model
and some of the philosophy behind that choice. It next describes how
that model operates in the simple case of a standard computer screen
without head tracking—the most common case. Finally, it presents
advanced material that was originally published in Appendix C of the
API specification guide.
Why a New Model?
Camera-based view models, as found in low-level APIs, give
developers
control over all rendering parameters. This makes sense when dealing
with custom applications, less sense when dealing with systems that
wish to have broader applicability: systems such as viewers or browsers
that load and display whole worlds as a single unit or systems where
the end users view, navigate, display, and even interact with the
virtual world.
Camera-based view models emulate a camera in the virtual world, not
a
human in a virtual world. Developers must continuously reposition a
camera to emulate "a human in the virtual world."
The Java 3D view model incorporates head tracking directly, if
present,
with no additional effort from the developer, thus providing end users
with the illusion that they actually exist inside a virtual world.
The Java 3D view model, when operating in a non-head-tracked
environment and rendering to a single, standard display, acts very much
like a traditional camera-based view model, with the added
functionality of being able to generate stereo views transparently.
The Physical Environment
Influences the View
Letting the application control all viewing parameters is not
reasonable in systems in which the physical environment dictates some
of the view parameters.
One example of this is a head-mounted display (HMD), where the
optics
of the head-mounted display directly determine the field of view that
the application should use. Different HMDs have different optics,
making it unreasonable for application developers to hard-wire such
parameters or to allow end users to vary that parameter at will.
Another example is a system that automatically computes view
parameters
as a function of the user's current head position. The specification of
a world and a predefined flight path through that world may not exactly
specify an end-user's view. HMD users would expect to look and thus see
to their left or right even when following a fixed path through the
environment-imagine an amusement park ride with vehicles that follow
fixed paths to present content to their visitors, but visitors can
continue to move their heads while on those rides.
Depending on the physical details of the end-user's environment, the
values of the viewing parameters, particularly the viewing and
projection matrices, will vary widely. The factors that influence the
viewing and projection matrices include the size of the physical
display, how the display is mounted (on the user's head or on a table),
whether the computer knows the user's head location in three space, the
head mount's actual field of view, the display's pixels per inch, and
other such parameters. For more information, see "View Model Details."
Separation of Physical and
Virtual
The Java 3D view model separates the virtual environment, where
the
application programmer has placed objects in relation to one another,
from the physical environment, where the user exists, sees computer
displays, and manipulates input devices.
Java 3D also defines a fundamental correspondence between the
user's
physical world and the virtual world of the graphic application. This
physical-to-virtual-world correspondence defines a single common space,
a space where an action taken by an end user affects objects within the
virtual world and where any activity by objects in the virtual world
affects the end user's view.
The Virtual World
The virtual world is a common space in which virtual objects exist.
The
virtual world coordinate system exists relative to a high-resolution
Locale-each Locale object defines the origin of virtual world
coordinates for all of the objects attached to that Locale. The Locale
that contains the currently active ViewPlatform object defines the
virtual world coordinates that are used for rendering. Java3D
eventually transforms all coordinates associated with scene graph
elements into this common virtual world space.
The Physical World
The physical world is just that-the real, physical world. This is
the
space in which the physical user exists and within which he or she
moves his or her head and hands. This is the space in which any
physical trackers define their local coordinates and in which several
calibration coordinate systems are described.
The physical world is a space, not a common coordinate system
between
different execution instances of Java 3D. So while two different
computers at two different physical locations on the globe may be
running at the same time, there is no mechanism directly within
Java 3D
to relate their local physical world coordinate systems with each
other. Because of calibration issues, the local tracker (if any)
defines the local physical world coordinate system known to a
particular instance of Java 3D.
The Objects That Define the
View
Java 3D distributes its view model parameters across several
objects,
specifically, the View object and its associated component objects, the
PhysicalBody object, the PhysicalEnvironment object, the Canvas3D
object, and the Screen3D object. Figure
1 shows graphically the central role of the View object and the
subsidiary role of its component objects.
Figure 1 – View Object, Its Component
Objects, and Their
Interconnection
The view-related objects shown in Figure
1
and their roles are as follows. For each of these objects, the portion
of the API that relates to modifying the virtual world and the portion
of the API that is relevant to non-head-tracked standard display
configurations are derived in this chapter. The remainder of the
details are described in "View Model
Details."
- ViewPlatform: A leaf
node that locates a view within a
scene graph. The ViewPlatform's parents specify its location,
orientation, and scale within the virtual universe. See "ViewPlatform: A Place in the Virtual World,"
for more
information.
- View: The main view object.
It contains many pieces of
view state.
- Canvas3D: The 3D version
of the Abstract Windowing
Toolkit
(AWT) Canvas object. It represents a window in which Java 3D will
draw
images. It contains a reference to a Screen3D object and information
describing the Canvas3D's size, shape, and location within the Screen3D
object.
- Screen3D: An object that
contains information describing
the display screen's physical properties. Java 3D places
display-screen
information in a separate object to prevent the duplication of screen
information within every Canvas3D object that shares a common screen.
- PhysicalBody: An object that
contains calibration information
describing the user's physical body.
- PhysicalEnvironment: An
object that contains calibration
information describing the physical world, mainly information that
describes the environment's six-degrees-of freedom tracking hardware,
if present.
Together, these objects describe the geometry of viewing rather than
explicitly providing a viewing or projection matrix. The Java 3D
renderer uses this information to construct the appropriate viewing and
projection matrices. The geometric focus of these view objects provides
more flexibility in generating views-a flexibility needed to support
alternative display configurations.
ViewPlatform: A Place in the
Virtual World
A ViewPlatform leaf node defines a coordinate system, and thus a
reference frame with its associated origin or reference point, within
the virtual world. The ViewPlatform serves as a point of attachment for
View objects and as a base for determining a renderer's view.
Figure
2
shows a portion of a scene graph containing a ViewPlatform node. The
nodes directly above a ViewPlatform determine where that ViewPlatform
is located and how it is oriented within the virtual world. By
modifying the Transform3D object associated with a TransformGroup node
anywhere directly above a ViewPlatform, an application or behavior can
move that ViewPlatform anywhere within the virtual world. A simple
application might define one TransformGroup node directly above a
ViewPlatform, as shown in Figure
2.
A VirtualUniverse may have many different ViewPlatforms, but a
particular View object can attach itself only to a single ViewPlatform.
Thus, each rendering onto a Canvas3D is done from the point of view of
a single ViewPlatform.
Figure 2 – A Portion of a Scene Graph
Containing a ViewPlatform Object
Moving through the Virtual
World
An application navigates within the virtual world by modifying a
ViewPlatform's parent TransformGroup. Examples of applications that
modify a ViewPlatform's location and orientation include browsers,
object viewers that provide navigational controls, applications that do
architectural walkthroughs, and even search-and-destroy games.
Controlling the ViewPlatform object can produce very interesting and
useful results. Our first simple scene graph (see "Introduction," Figure 1)
defines a scene graph for a simple application that draws an object in
the center of a window and rotates that object about its center point.
In that figure, the Behavior object modifies the TransformGroup
directly above the Shape3D node.
An alternative application scene graph, shown in Figure
3,
leaves the central object alone and moves the ViewPlatform around the
world. If the shape node contains a model of the earth, this
application could generate a view similar to that seen by astronauts as
they orbit the earth.
Had we populated this world with more objects, this scene graph
would allow navigation through the world via the Behavior node.
Figure 3 – A Simple Scene Graph with View
Control
Applications and behaviors manipulate a TransformGroup through its
access methods. These methods allow an application to retrieve and
set the Group node's Transform3D object. Transform3D Node methods
include getTransform and setTransform.
Dropping in on a Favorite
Place
A scene graph may contain multiple ViewPlatform
objects. If a user detaches a View object
from a ViewPlatform and then
reattaches that View to a different ViewPlatform, the image on the
display will now be rendered from the point of view of the new
ViewPlatform.
Associating Geometry with a
ViewPlatform
Java 3D does not have any built-in semantics for displaying a
visible
manifestation of a ViewPlatform within the virtual world (an avatar).
However, a developer can construct and manipulate an avatar using
standard Java 3D constructs.
A developer can construct a small scene graph consisting of a
TransformGroup node, a behavior leaf node, and a shape node and insert
it directly under the BranchGroup node associated with the ViewPlatform
object. The shape node would contain a geometric model of the avatar's
head. The behavior node would change the TransformGroup's transform
periodically to the value stored in a View object's UserHeadToVworld
parameter (see "View Model
Details").
The avatar's virtual head, represented by the shape node, will now move
around in lock-step with the ViewPlatform's TransformGroup and any
relative position and orientation changes of the user's actual physical
head (if a system has a head tracker).
Generating a View
Java 3D generates viewing matrices in one of a few different
ways,
depending on whether the end user has a head-mounted or a room-mounted
display environment and whether head tracking is enabled. This section
describes the computation for a non-head-tracked, room-mounted
display-a standard computer display. Other environments are described
in "View Model Details."
In the absence of head tracking, the ViewPlatform's origin specifies
the virtual eye's location and orientation within the virtual world.
However, the eye location provides only part of the information needed
to render an image. The renderer also needs a projection matrix. In the
default mode, Java 3D uses the projection policy, the specified
field-of-view information, and the front and back clipping distances to
construct a viewing frustum.
Composing Model and Viewing
Transformations
Figure
4
shows a simple scene graph. To draw the object labeled "S,"
Java 3D
internally constructs the appropriate model, view platform, eye, and
projection matrices. Conceptually, the model transformation for a
particular object is computed by concatenating all the matrices in a
direct path between the object and the VirtualUniverse. The view matrix
is then computed-again, conceptually-by concatenating all the matrices
between the VirtualUniverse object and the ViewPlatform attached to the
current View object. The eye and projection matrices are constructed
from the View object and its associated component objects.
Figure 4 – Object and ViewPlatform
Transformations
In our scene graph, what we would normally consider the
model transformation would consist of the following three
transformations: LT1T2. By
multiplying LT1T2
by a vertex in the shape object, we would transform that vertex into
the virtual universe's coordinate system. What we would normally
consider the view platform transformation would be (LTv1)-1
or Tv1-1L-1.
This presents a problem since coordinates in the virtual universe are
256-bit fixed-point values, which cannot be used to represent
transformed points efficiently.
Fortunately, however, there is a solution to this problem. Composing
the model and view platform transformations gives us
- Tv1-1L-1LT1T2
= Tv1-1IT1T2
= Tv1-1T1T2,
the matrix that takes vertices in an object's local coordinate
system
and places them in the ViewPlatform's coordinate system. Note that the
high-resolution Locale transformations cancel each other out, which
removes the need to actually transform points into high-resolution
VirtualUniverse coordinates. The general formula of the matrix that
transforms object coordinates to ViewPlatform coordinates is Tvn-1...Tv2-1Tv1-1T1T2...Tm.
As mentioned earlier, the View object contains the remainder of the
view information, specifically, the eye matrix, E,
that takes points in the View-Platform's local coordinate system and
translates them into the user's eye coordinate system, and the
projection matrix, P, that projects objects in the
eye's coordinate system into clipping coordinates. The final
concatenation of matrices for rendering our shape object "S" on the
specified Canvas3D is PETv1-1T1T2.
In general this is PETvn-1...Tv2-1Tv1-1T1T2...Tm.
The details of how Java 3D constructs the matrices E
and P in different end-user configurations are
described in "View Model Details."
Multiple Locales
Java 3D supports multiple high-resolution Locales. In some
cases,
these
Locales are close enough to each other that they can "see" each other,
meaning that objects can be rendered even though they are not in the
same Locale as the ViewPlatform object that is attached to the View.
Java 3D automatically handles this case without the application
having
to do anything. As in the previous example, where the ViewPlatform and
the object being rendered are attached to the same Locale, Java 3D
internally constructs the appropriate matrices for cases in which the
ViewPlatform and the object being rendered are not attached
to the same Locale.
Let's take two Locales, L1 and L2, with the View attached to a
ViewPlatform in L1. According to our general formula, the modeling
transformation-the transformation that takes points in object
coordinates and transforms them into VirtualUniverse coordinates-is LT1T2...Tm.
In our specific example, a point in Locale L2 would be transformed into
VirtualUniverse coordinates by L2T1T2...Tm.
The view platform transformation would be (L1Tv1Tv1...Tvn)-1
or Tvn-1...Tv2-1Tv1-1L1-1.
Composing these two matrices gives us
- Tvn-1...Tv2-1Tv1-1L1-1L2T1T2...Tm.
Thus, to render objects in another Locale, it is sufficient to
compute L1-1L2
and use that as the starting matrix when composing the model
transformations. Given that a Locale is represented by a single
high-resolution coordinate position, the transformation L1-1L2
is a simple translation by L2 - L1.
Again, it is not actually necessary to transform points into
high-resolution VirtualUniverse coordinates.
In general, Locales that are close enough that the difference in
their
high-resolution coordinates can be represented in double precision by a
noninfinite value are close enough to be rendered. In practice, more
sophisticated culling techniques can be used to render only those
Locales that really are "close enough."
A Minimal Environment
An application must create a minimal set of Java 3D objects
before
Java
3D can render to a display device. In addition to a Canvas3D object,
the application must create a View object, with its associated
PhysicalBody and PhysicalEnvironment objects, and the following scene
graph elements:
- A VirtualUniverse object
- A high-resolution Locale object
- A BranchGroup node object
- A TransformGroup node object with associated transform
- A ViewPlatform leaf node object that defines the position and
orientation within the virtual universe for generating views
View Model Details
An application programmer writing a 3D
graphics program that will deploy on a variety of platforms must
anticipate the likely end-user environments and must carefully
construct the view transformations to match those characteristics using
a low-level API. This appendix addresses many of the issues an
application must face and describes the sophisticated features that
Java 3D's advanced view model provides.
An Overview of the
Java 3D
View Model
Both camera-based and Java 3D-based view models allow a programmer
to
specify the shape of a view frustum and, under program control, to
place, move, and reorient that frustum within the virtual environment.
However, how they do this varies enormously. Unlike the camera-based
system, the Java 3D view model allows slaving the view frustum's
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.
Java 3D must handle two rather different head-tracking
situations.
In one case, we rigidly attach a tracker's base,
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 sensor, 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.
Physical Environments and
Their Effects
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.
The following two examples show how end-user environments can
significantly affect how an application must construct viewing
transformations.
A Head-Mounted Example
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.
A Room-Mounted Example
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.
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.
Impact of Head Position and
Orientation on the Camera
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.
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).
Java 3D's view model incorporates the appropriate abstractions
to
compensate automatically for such variability in end-user hardware
environments.
The Coordinate Systems
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.
Room-Mounted Coordinate
Systems
The room-mounted coordinate system is divided into the virtual
coordinate system and the physical coordinate system. Figure
5
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.
The Virtual Coordinate
Systems
The Virtual World Coordinate System
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 +x to the right, +y up, and +z toward
the viewer.
The ViewPlatform Coordinate System
The ViewPlatform coordinate system is the local coordinate system of
the ViewPlatform leaf node to which the View is attached.
Figure 5 – Display Rigidly Attached to the
Tracker Base
The Coexistence Coordinate System
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 3D defines a special coordinate
system,
called coexistence coordinates, that is defined to exist in both the
physical world and the virtual world.
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.
Modifying the transform that maps the coexistence coordinate system
into the virtual world coordinate system changes what the end user can
see. The Java 3D application programmer moves the end user within
the
virtual world by modifying this transform.
The Physical Coordinate
Systems
The Head Coordinate System
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.
The Image Plate Coordinate System
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 XY
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.
The Head Tracker Coordinate System
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.
The Tracker Base Coordinate System
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.
Head-Mounted Coordinate
Systems
Head-mounted coordinate systems divide the same virtual coordinate
systems and the physical coordinate systems. Figure
6
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 3D's
coordinate system differs in another way. It includes two image plate
coordinate systems, one for each of an end-user's eyes.
The Left Image Plate and Right Image Plate Coordinate Systems
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 XY plane. Note that the left image plate's XY
plane does not necessarily lie parallel to the right image plate's XY
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.
Figure 6 – Display Rigidly Attached to the
Head Tracker (Sensor)
The Screen3D Object
A Screen3D object represents one independent display device. The most
common environment for a Java 3D application is a desktop computer
with
or without a head tracker. Figure
7 shows a scene graph fragment for a display environment designed
for such an end-user environment. Figure
8 shows a display environment that matches the scene graph
fragment in Figure
7.
Figure 7 – A Portion of a Scene Graph
Containing a Single Screen3D
Object
Figure 8 – A Single-Screen Display
Environment
A multiple-projection wall display presents a more exotic environment.
Such environments have multiple screens, typically three or more. Figure
9 shows a scene graph fragment representing such a system, and Figure
10 shows the corresponding display environment.
Figure 9 – A Portion of a Scene Graph
Containing Three Screen3D
Objects
Figure 10 – A Three-Screen Display
Environment
A multiple-screen environment requires more care during the
initialization and calibration phase. Java 3D must know how the
Screen3Ds are placed with respect to one another, the tracking device,
and the physical portion of the coexistence coordinate system.
Viewing in Head-Tracked Environments
The "Generating a View" section
describes how Java 3D generates a view for a standard flat-screen
display with no head tracking. In this section, we describe how
Java 3D
generates a view in a room-mounted, head-tracked display
environment-either a computer monitor with shutter glasses and head
tracking or a multiple-wall display with head-tracked shutter glasses.
Finally, we describe how Java 3D generates view matrices in a
head-mounted and head-tracked display environment.
A Room-Mounted Display with
Head Tracking
When head tracking combines with a room-mounted
display environment (for example, a standard flat-screen display), the
ViewPlatform's origin and orientation serve as a base for constructing
the view matrices. Additionally, Java 3D uses the end-user's head
position and orientation to compute where an end-user's eyes are
located in physical space. Each eye's position serves to offset the
corresponding virtual eye's position relative to the ViewPlatform's
origin. Each eye's position also serves to specify that eye's frustum
since the eye's position relative to a Screen3D uniquely specifies that
eye's view frustum. Note that Java 3D will access the PhysicalBody
object to obtain information describing the user's interpupilary
distance and tracking hardware, values it needs to compute the
end-user's eye positions from the head position information.
A Head-Mounted Display with
Head Tracking
In a head-mounted environment, the ViewPlatform's origin and
orientation also serves as a base for constructing view matrices. And,
as in the head-tracked, room-mounted environment, Java 3D also
uses the
end-user's head position and orientation to modify the ViewPlatform's
position and orientation further. In a head-tracked, head-mounted
display environment, an end-user's eyes do not move relative to their
respective display screens, rather, the display screens move relative
to the virtual environment. A rotation of the head by an end user can
radically affect the final view's orientation. In this situation, Java
3D combines the position and orientation from the ViewPlatform with the
position and orientation from the head tracker to form the view matrix.
The view frustum, however, does not change since the user's eyes do not
move relative to their respective display screen, so Java 3D can
compute the projection matrix once and cache the result.
If any of the parameters of a View object are updated, this will
effect
a change in the implicit viewing transform (and thus image) of any
Canvas3D that references that View object.
Compatibility Mode
A camera-based view model allows application programmers to think
about
the images displayed on the computer screen as if a virtual camera took
those images. Such a view model allows application programmers to
position and orient a virtual camera within a virtual scene, to
manipulate some parameters of the virtual camera's lens (specify its
field of view), and to specify the locations of the near and far
clipping planes.
Java 3D allows applications to enable compatibility mode for
room-mounted, non-head-tracked display environments or to disable
compatibility mode using the following methods. Camera-based viewing
functions are available only in compatibility mode. The setCompatibilityModeEnable
method turns compatibility mode on or off. Compatibility mode is
disabled by default.
Note: Use of these view-compatibility
functions will disable some of Java 3D's view model features and
limit
the portability of Java 3D programs. These methods are primarily
intended to help jump-start porting of existing applications.
Overview of the
Camera-Based View Model
The traditional camera-based view model, shown in Figure
11,
places a virtual camera inside a geometrically specified world. The
camera "captures" the view from its current location, orientation, and
perspective. The visualization system then draws that view on the
user's display device. The application controls the view by moving the
virtual camera to a new location, by changing its orientation, by
changing its field of view, or by controlling some other camera
parameter.
The various parameters that users control in a
camera-based view model specify the shape of a viewing volume (known as
a frustum because of its truncated pyramidal shape) and locate that
frustum within the virtual environment. The rendering pipeline uses the
frustum to decide which objects to draw on the display screen. The
rendering pipeline does not draw objects outside the view frustum, and
it clips (partially draws) objects that intersect the frustum's
boundaries.
Though a view frustum's specification may have many items in common
with those of a physical camera, such as placement, orientation, and
lens settings, some frustum parameters have no physical analog. Most
noticeably, a frustum has two parameters not found on a physical
camera: the near and far clipping planes.
Figure 11 – The Camera-Based View Model
The location of the near and far clipping planes allows the application
programmer to specify which objects Java 3D should not draw.
Objects
too far away from the current eyepoint usually do not result in
interesting images. Those too close to the eyepoint might obscure the
interesting objects. By carefully specifying near and far clipping
planes, an application programmer can control which objects the
renderer will not be drawing.
From the perspective of the display device, the virtual camera's
image
plane corresponds to the display screen. The camera's placement,
orientation, and field of view determine the shape of the view frustum.
Using the Camera-Based View
Model
The camera-based view model allows Java 3D to bridge the gap
between
existing 3D code and Java 3D's view model. By using the
camera-based
view model methods, a programmer retains the familiarity of the older
view model but gains some of the flexibility afforded by Java 3D's
new
view model.
The traditional camera-based view model is supported in Java 3D
by
helping methods in the Transform3D object. These methods were
explicitly designed to resemble as closely as possible the view
functions of older packages and thus should be familiar to most 3D
programmers. The resulting Transform3D objects can be used to set
compatibility-mode transforms in the View object.
Creating a Viewing Matrix
The Transform3D object provides a lookAt utility
method
to create a
viewing matrix. This method specifies the position and orientation of
a viewing transform. It works similarly to the equivalent function in
OpenGL. The inverse of this transform can be used to control the
ViewPlatform object within the scene graph. Alternatively, this
transform can be passed directly to the View's VpcToEc
transform via the compatibility-mode viewing functions. The setVpcToEc
Creating a Projection
Matrix
The Transform3D object provides three methods for
creating a projection matrix: frustum, perspective,
and ortho. All three map points from eye coordinates
(EC) to clipping coordinates (CC). Eye coordinates are defined such
that (0, 0, 0) is at the eye and the projection plane is at z
= -1.
The frustum method
establishes a perspective projection with the eye at the apex of a
symmetric view frustum. The transform maps points from eye coordinates
to clipping coordinates. The clipping coordinates generated by the
resulting transform are in a right-handed coordinate system (as are all
other coordinate systems in Java 3D).
The arguments define the frustum and its associated perspective
projection: (left, bottom, -near)
and (right, top, -near)
specify the point on the near clipping plane that maps onto the
lower-left and upper-right corners of the window, respectively. The -far
parameter specifies the far clipping plane. See Figure
12.
The perspective method establishes a perspective
projection with the eye at the apex of a symmetric view frustum,
centered about the Z-axis,
with a fixed field of view. The resulting perspective projection
transform mimics a standard camera-based view model. The transform maps
points from eye coordinates to clipping coordinates. The clipping
coordinates generated by the resulting transform are in a right-handed
coordinate system.
The arguments define the frustum and its associated perspective
projection: -near and -far specify the near
and far clipping planes; fovx specifies the field of view
in the X dimension, in radians; and aspect
specifies the aspect ratio of the window. See Figure
13.
Figure 12 – A Perspective Viewing Frustum
Figure 13 – Perspective View Model Arguments
The ortho method
establishes a parallel projection. The orthographic projection
transform mimics a standard camera-based video model. The transform
maps points from eye coordinates to clipping coordinates. The clipping
coordinates generated by the resulting transform are in a right-handed
coordinate system.
The arguments define a rectangular box used for projection: (left,
bottom, -near) and (right, top,
-near)
specify the point on the near clipping plane that maps onto the
lower-left and upper-right corners of the window, respectively. The -far
parameter specifies the far clipping plane. See Figure
14.
Figure 14 – Orthographic View Model
The setLeftProjection
and setRightProjection methods are used to set the
projection matrices for the left eye and right eye, respectively, when
in compatibility mode.
