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Java 3D API - Immediate-Mode Rendering
Immediate-Mode Rendering
Java 3D is fundamentally a scene graph-based API. Most of
the constructs in the API are biased toward retained mode and
compiled-retained mode rendering. However, there are some applications
that want both the control and the flexibility that immediate-mode
rendering offers.
Immediate-mode applications can either use or ignore Java 3D's
scene
graph structure. By using immediate mode, end-user applications have
more freedom, but this freedom comes at the expense of performance. In
immediate mode, Java 3D has no high-level information concerning
graphical objects or their composition. Because it has minimal global
knowledge, Java 3D can perform only localized optimizations on
behalf
of the application programmer.
Two Styles of Immediate-Mode
Rendering
Use of Java 3D's immediate mode falls into one of two categories:
pure
immediate-mode rendering and mixed-mode rendering in which immediate
mode and retained or compiled-retained mode interoperate and render to
the same canvas. The Java 3D renderer is idle in pure immediate
mode,
distinguishing it from mixed-mode rendering.
Pure Immediate-Mode
Rendering
Pure immediate-mode rendering provides for those applications and
applets that do not want Java 3D to do any automatic rendering of
the
scene graph. Such applications may not even wish to build a scene graph
to represent their graphical data. However, they use Java 3D's
attribute objects to set graphics state and Java 3D's geometric
objects
to render geometry.
Note: Scene antialiasing is not supported
in pure immediate mode.
A pure immediate mode application must create a
minimal set of Java 3D
objects before rendering. 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, and, finally, a ViewPlatform leaf node object that defines
the position and orientation within the virtual universe that generates
the view (see Figure
1).
Figure 1 – Minimal Immediate-Mode Structure
Java 3D provides utility functions that create much of this
structure
on behalf of a pure immediate-mode application, making it less
noticeable from the application's perspective-but the structure must
exist.
All rendering is done completely under user control. It is necessary
for the user to clear the 3D canvas, render all geometry, and swap the
buffers. Additionally, rendering the right and left eye for stereo
viewing becomes the sole responsibility of the application.
In pure immediate mode, the user must stop the Java 3D
renderer, via
the Canvas3D object stopRenderer()
method, prior to adding the Canvas3D object to an active View object
(that is, one that is attached to a live ViewPlatform object).
Mixed-Mode Rendering
Mixing immediate mode and retained or compiled-retained mode requires
more structure than pure immediate mode. In mixed mode, the
Java 3D
renderer is running continuously, rendering the scene graph into the
canvas.
The basic Java 3D stereo rendering loop, executed for
each
Canvas3D, is as follows:
clear canvas (both eyes)
call preRender() // user-supplied method
set left eye view
render opaque scene graph objects
call renderField(FIELD_LEFT) // user-supplied method
render transparent scene graph objects
set right eye view
render opaque scene graph objects again
call renderField(FIELD_RIGHT) // user-supplied method
render transparent scene graph objects again
call postRender() // user-supplied method
synchronize and swap buffers
call postSwap() // user-supplied method
The basic Java 3D monoscopic rendering loop is as
follows:
clear canvas
call preRender() // user-supplied method
set view
render opaque scene graph objects
call renderField(FIELD_ALL) // user-supplied method
render transparent scene graph objects
call postRender() // user-supplied method
synchronize and swap buffers
call postSwap() // user-supplied method
In both cases, the entire loop, beginning with clearing the canvas and
ending with swapping the buffers, defines a frame. The application is
given the opportunity to render immediate-mode geometry at any of the
clearly identified spots in the rendering loop. A user specifies his or
her own rendering methods by extending the Canvas3D class and
overriding the preRender, postRender, postSwap,
and/or renderField methods.
