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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<HTML>
<HEAD>
<TITLE>Jogl - User's Guide</TITLE>
</HEAD>
<BODY>
<H1>Jogl - User's Guide</H1>
<P>
<UL>
<LI> Overview
<LI> Creating a GLDrawable
<LI> Writing a GLEventListener
<LI> Using the Composable Pipeline
<LI> Multithreading Issues
<LI> Pbuffers
<LI> GLU
<LI> More Resources
<LI> Platform notes
<UL>
<LI> All Platforms
<LI> Windows
<LI> Solaris, Linux (X11 platforms)
<LI> Macintosh OS X
</UL>
</UL>
<H2> Overview </H2>
<P>
Jogl is a Java programming language binding for the OpenGL 3D graphics
API. It supports integration with the Java platform's AWT and Swing
widget sets while providing a minimal and easy-to-use API that handles
many of the issues associated with building multithreaded OpenGL
applications. Jogl provides access to the latest OpenGL routines
(OpenGL 2.0 with vendor extensions) as well as platform-independent
access to hardware-accelerated offscreen rendering ("pbuffers"). Jogl
also provides some of the most popular features introduced by other
Java bindings for OpenGL like GL4Java, LWJGL and Magician, including a
composable pipeline model which can provide faster debugging for
Java-based OpenGL applications than the analogous C program.
</P>
<P>
Jogl was designed for the most recent version of the Java platform and
for this reason supports only J2SE 1.4 and later. It also only
supports truecolor (15 bits per pixel and higher) rendering; it does
not support color-indexed modes. Certain areas of the public APIs are
more restrictive than in other bindings; for example, the GLCanvas and
GLJPanel classes are final, unlike in GL4Java, and the GLContext class
is no longer exposed in the public API. These changes have been made
to keep the public API simple and because most of the programming
errors that have been seen with earlier Java/OpenGL interfaces, in
particular GL4Java, have been related to subclassing the OpenGL widget
classes and performing manual OpenGL context management. Several
complex and leading-edge OpenGL demonstrations have been successfully
ported from C/C++ to Jogl without needing direct access to any of
these APIs. However, all of these classes and concepts are accessible
at the Java programming language level in implementation packages, and
in fact the Jogl binding is itself written almost completely in the
Java programming language. There are roughly 150 lines of handwritten
C code in the entire Jogl source base (100 of which work around bugs
in older OpenGL drivers on Windows); the rest of the native code is
autogenerated during the build process by a new tool called GlueGen,
the source code of which is in the Jogl source tree. Documentation for
GlueGen is forthcoming.
</P>
<H2> Creating a GLDrawable </H2>
<P>
Jogl provides two basic widgets into which OpenGL rendering can be
performed. The GLCanvas is a heavyweight AWT widget which supports
hardware acceleration and which is intended to be the primary widget
used by applications. The GLJPanel is a fully Swing-compatible
lightweight widget which supports hardware acceleration but which is
not as fast as the GLCanvas because it reads back the frame buffer in
order to draw it using Java2D. The GLJPanel is intended to provide
100% correct Swing integration in the circumstances where a GLCanvas
can not be used. See <a href =
"http://java.sun.com/products/jfc/tsc/articles/mixing/">this
article</a> on <a href = "http://java.sun.com/products/jfc/tsc/">The
Swing Connection</a> for more information about mixing lightweight and
heavyweight widgets.
</P>
<P>
Both the GLCanvas and GLJPanel implement a common interface called
GLDrawable so applications can switch between them with minimal code
changes. The GLDrawable interface provides
<UL>
<LI> access to the GL and GLU objects for calling OpenGL routines
<LI> the mechanism for registering GLEventListeners for performing
OpenGL rendering
<LI> a <CODE>display()</CODE> method for forcing OpenGL rendering to
be performed
<LI> exclusion methods (<CODE>setRenderingThread()</CODE>,
<CODE>setNoAutoRedrawMode()</CODE>) for controlling the
multithreading behavior of the widget
<LI> AWT- and Swing-independent abstractions for getting and setting
the size of the widget and adding and removing event listeners
<LI> a platform-independent mechanism for creating
hardware-accelerated offscreen surfaces (pbuffers) for performing
advanced rendering techniques
</UL>
</P>
<P>
GLCanvas and GLJPanel instances are created using the factory methods
in GLDrawableFactory. These factory methods allow the user to request
a certain set of OpenGL parameters in the form of a GLCapabilities
object, to customize the format selection algorithm by specifying a
GLCapabilitiesChooser, to share textures and display lists with other
GLDrawables, and to specify the display device on which the GLDrawable
will be created.
</P>
<P>
A GLCapabilities object specifies the OpenGL parameters for a
newly-created widget, such as the color, alpha,, z-buffer and
accumulation buffer bit depths and whether the widget is
double-buffered. The default capabilities are loosely specified but
provide for truecolor RGB, a reasonably large depth buffer,
double-buffered, with no alpha, stencil, or accumulation buffers.
</P>
<P>
An application can override the default pixel format selection
algorithm by providing a GLCapabilitiesChooser to the
GLDrawableFactory. The chooseCapabilities method will be called with
all of the available pixel formats as an array of GLCapabilities
objects, as well as the index indicating the window system's
recommended choice; it should return an integer index into this
array. The DefaultGLCapabilitiesChooser uses the window system's
recommendation when it is available, and otherwise attempts to use a
platform-independent selection algorithm.
</P>
<H2> Writing a GLEventListener </H2>
<P>
Applications implement the GLEventListener interface to perform OpenGL
drawing. When the methods of the GLEventListener are called, the
underlying OpenGL context associated with the drawable is already
current. The listener fetches the GL object out of the GLDrawable and
begins to perform rendering.
</P>
<P>
The <CODE>init()</CODE> method is called when a new OpenGL context is
created for the given GLDrawable. Any display lists or textures used
during the application's normal rendering loop can be safely
initialized in <CODE>init()</CODE>. It is important to note that
because the underlying AWT window may be destroyed and recreated while
using the same GLCanvas and GLEventListener, the GLEventListener's
<CODE>init()</CODE> method may be called more than once during the
lifetime of the application. The init() method should therefore be
kept as short as possible and only contain the OpenGL initialization
required for the <CODE>display()</CODE> method to run properly. It is
the responsibility of the application to keep track of how its various
OpenGL contexts share display lists, textures and other OpenGL objects
so they can be either be reinitialized or so that reinitialization can
be skipped when the <CODE>init()</CODE> callback is called.
</P>
<P>
The <CODE>display()</CODE> method is called to perform per-frame
rendering. The <CODE>reshape()</CODE> method is called when the
drawable has been resized; the default implementation automatically
resizes the OpenGL viewport so often it is not necessary to do any
work in this method. The <CODE>displayChanged()</CODE> method is
designed to allow applications to support on-the-fly screen mode
switching, but support for this is not yet implemented so the body of
this method should remain empty.
</P>
<P>
It is strongly recommended that applications always refetch the GL and
GLU objects out of the GLDrawable upon each call to the
<CODE>init()</CODE>, <CODE>display()</CODE> and <CODE>reshape()</CODE>
methods and pass the GL object down on the stack to any drawing
routines, as opposed to storing the GL in a field and referencing it
from there. The reason is that multithreading issues inherent to the
AWT toolkit make it difficult to reason about which threads certain
operations are occurring on, and if the GL object is stored in a field
it is unfortunately too easy to accidentally make OpenGL calls from a
thread that does not have a current context. This will usually cause
the application to crash. For more information please see the section
on multithreading.
</P>
<H2> Using the Composable Pipeline </H2>
<P>
Jogl supports the "composable pipeline" paradigm introduced by the
Magician Java binding for OpenGL. The DebugGL pipeline calls
<CODE>glGetError</CODE> after each OpenGL call, reporting any errors
found. It can greatly speed up development time because of its
fine-grained error checking as opposed to the manual error checking
usually required in OpenGL programs written in C. The TraceGL prints
logging information upon each OpenGL call and is helpful when an
application crash makes it difficult to see where the error occurred.
</P>
<P>
To use these pipelines, call <CODE>GLDrawable.setGL</CODE> at the
beginning of the <CODE>init</CODE> method in your GLEventListener. For
example,
<PRE>
class MyListener implements GLEventListener {
public void init(GLDrawable drawable) {
drawable.setGL(new DebugGL(drawable.getGL()));
// ...
}
// ...
}
</PRE>
</P>
<H2> Multithreading Issues </H2>
<P>
Jogl was designed to interoperate with the AWT, an inherently
multithreaded GUI toolkit. OpenGL, in contrast, was originally
designed in single-threaded C programming environments. For this
reason Jogl provides a framework in which it is possible to write
correct multithreaded OpenGL applications using the GLEventListener
paradigm.
</P>
<P>
If an application written using Jogl interacts in any way with the
mouse or keyboard, the AWT is processing these events and the
multithreaded aspects of the program must be considered.
</P>
<P>
OpenGL applications usually behave in one of two ways: either they
repaint only on demand, for example when mouse input comes in, or they
repaint continually, regardless of whether user input is coming in. In
the repaint-on-demand model, the application can merely call
<CODE>GLDrawable.display()</CODE> manually at the end of the mouse or
keyboard listener to cause repainting to be done. Alternatively if the
application knows the concrete type of the GLDrawable it can call
repaint() to have the painting scheduled for a later time.
</P>
<P>
In the continuous repaint model, the application typically has a main
loop which is calling <CODE>GLDrawable.display()</CODE> repeatedly, or
is using the Animator class, which does this internally. In both of
these cases the OpenGL rendering will be done on this thread rather
than the internal AWT event queue thread which dispatches mouse and
keyboard events.
</P>
<P>
Both of these models (repaint-on-demand and repaint continually) still
require the user to think about which thread keyboard and mouse events
are coming in on, and which thread is performing the OpenGL rendering.
OpenGL rendering <B>may not</B> occur directly inside the mouse or
keyboard handlers, because the OpenGL context for the drawable is not
current at this point (hence the warning about storing a GL object in
a field, where it can be fetched and accidentally used by another
thread). However, a mouse or keyboard listener may invoke
<CODE>GLDrawable.display()</CODE>.
</P>
<P>
It is generally recommended that applications perform as little work
as possible inside their mouse and keyboard handlers to keep the GUI
responsive. However, since OpenGL commands can not be run from
directly within the mouse or keyboard event listener, the best
practice is to store off state when the listener is entered and
retrieve this state during the next call to
<CODE>GLEventListener.display()</CODE>.
</P>
<P>
Furthermore, it is recommended that if there are long computational
sequences in the GLEventListener's <CODE>display</CODE> method which
reference variables which may be being simultaneously modified by the
AWT thread (mouse and keyboard listeners) that copies of these
variables be made upon entry to <CODE>display</CODE> and these copies
be referenced throughout display() and the methods it calls. This will
prevent the values from changing while the OpenGL rendering is being
performed. Errors of this kind show up in many ways, including certain
kinds of flickering of the rendered image as certain pieces of objects
are rendered in one place and other pieces are rendered elsewhere in
the scene. Restructuring the display() method as described has solved
all instances of this kind of error that have been seen with Jogl to
date.
</P>
<P>
Prior to Jogl 1.1 b10, the Jogl library attempted to give applications
strict control over which thread or threads performed OpenGL
rendering. The <CODE>setRenderingThread()</CODE>,
<CODE>setNoAutoRedrawMode()</CODE> and <CODE>display()</CODE> APIs
were originally designed to allow the application to create its own
animation thread and avoid OpenGL context switching on platforms that
supported it. Unfortunately, serious stability issues caused by
multithreading bugs in either vendors' OpenGL drivers or in the Java
platform implementation have arisen on three of Jogl's major supported
platforms: Windows, Linux and Mac OS X. In order to address these
bugs, the threading model in Jogl 1.1 b10 and later has changed.
</P>
<P>
All GLEventListener callbacks and other internal OpenGL context
management are now performed on one thread. (In the current
implementation, this thread is the AWT event queue thread, which is a
thread internal to the implementation of the AWT and which is always
present when the AWT is being used. Future versions of Jogl may change
the thread on which the OpenGL work is performed.) When the
<CODE>GLDrawable.display()</CODE> method is called from user code, it
now performs the work synchronously on the AWT event queue thread,
even if the calling thread is a different thread. The
<CODE>setRenderingThread()</CODE> optimization is now a no-op. The
<CODE>setNoAutoRedraw()</CODE> API still works as previously
advertised, though now that all work is done on the AWT event queue
thread it no longer needs to be used in most cases. (It was previously
useful for working around certain kinds of OpenGL driver bugs.)
</P>
<P>
Most applications will not see a change in behavior from this change
in the Jogl implementation. Applications which use thread-local
storage or complex multithreading and synchronization may see a change
in their control flow requiring code changes. While it is strongly
recommended to change such applications to work under the new
threading model, the old threading model can be used by specifying the
system property <CODE>-Djogl.1thread=auto</CODE> or
<CODE>-Djogl.1thread=false</CODE>. The "auto" setting is equivalent to
the behavior in 1.1 b09 and before, where on ATI cards the
single-threaded mode would be used. The "false' setting is equivalent
to disabling the single-threaded mode. "true" is now the default
setting.
</P>
<H2> Pbuffers </H2>
<P>
Jogl exposes hardware-accelerated offscreen rendering (pbuffers) with
a minimal and platform-agnostic API. Several recent demos have been
successfully ported from C/C++ to Java using Jogl's pbuffer APIs.
However, the pbuffer support in Jogl remains one of the more
experimental aspects of the package and the APIs may need to change in
the future.
</P>
<P>
To create a pbuffer, create a GLCanvas and (assuming it reports that
it can create an offscreen drawable) make a pbuffer using the
<CODE>createOffscreenDrawable</CODE> API. Because of the multithreaded
nature of the AWT, the pbuffer is actually created lazily. However,
even if multiple pbuffers are created, and the order in which they are
rendered is significant, handling the lazy instantiation can be
straightforward: the display(GLDrawable) method of one pbuffer's
GLEventListener can directly call another pbuffer's display() method.
See the source code for the Jogl demonstrations such as the
ProceduralTexturePhysics demo and the HDR demo for examples of this
usage.
</P>
<P>
Additionally, pbuffers are only created when the parent GLCanvas's
display(), init(), or reshape() methods are called; in other words, it
may be necessary to manually "prime" the GLCanvas by calling display()
on it until it creates all of its requested pbuffers. Again, please
see the demonstrations for concrete examples of this. We hope that it
may be possible to hide many of these details in the future.
</P>
<P>
A pbuffer is used by calling its display() method. Rendering, as
always, occurs while the pbuffer's OpenGL context is current. There
are render-to-texture options that can be specified in the
GLCapabilities for the pbuffer which can make it easier to operate
upon the resulting pixels. These APIs are however highly experimental
and not yet implemented on all platforms.
</P>
<H2> GLU </H2>
<P>
Jogl contains support for the GLU (OpenGL Utility Library) version
1.3. Jogl originally supported GLU by wrapping the C version of the
APIs, but over time, and thanks to the contributions of several
individuals, it now uses a pure-Java version of SGI's GLU library. The
pure Java port is enabled by default, and addresses stability issues
on certain Linux distributions as well as the lack of native GLU 1.3
support on the Windows platform. In case of problems with the Java
port, the C version of the GLU library may be used by specifying the
system property <CODE>-Djogl.glu.nojava</CODE> on the command
line. All of the same functionality is exposed with both the Java and
C versions of the GLU library; currently NURBS support is the only
missing feature on both sides. If you run into problems with the Java
port of the GLU library please file a bug using the Issue Tracker on
the Jogl home page.
</P>
<H2> More Resources </H2>
<P>
The Jogl forum on <A HREF="http://javagaming.org/">javagaming.org</A>
is the best place to ask questions about the library. Many users, as
well as the Jogl developers, read this forum frequently, and the
archived threads contain a lot of useful information (which still
needs to be distilled into documentation).
</P>
<P>
The <A HREF="http://jogl-demos.dev.java.net/">Jogl demos</A> provide
several examples of usage of the Jogl library.
</P>
<P>
Pepijn Van Eeckhoudt's ports of the NeHe demos<br>
Jogl port of GLExcess<br>
Greg Pierce's introduction to Jogl<br>
Link to release information about Jogl 1.1 on javagaming.org
</P>
<H2> Platform Notes </H2>
<H3> All Platforms </H3>
<P>
The following issues, among others, are outstanding on all platforms:
</P>
<UL>
<LI> A few remaining stability issues, mostly on older graphics cards.
</UL>
<H3> Windows </H3>
<P>
No outstanding issues at this time.
</P>
<H3> Solaris, Linux (X11 platforms) </H3>
<P>
No outstanding issues at this time.
</P>
<H3> Mac OS X </H3>
<P>
There are some problems with visual artifacts and stability problems
with some of the Jogl demos on Mac OS X. It appears that at least some
of these problems are due to bugs in Apple's OpenGL support. Bugs have
been filed about these problems and it is hoped they will be addressed
in the near future.
</P>
<P>
The Mac OS X port of Jogl, in particular the GL interface and its
implementation, can be used either with the provided GLCanvas widget
or with the Cocoa NSOpenGLView. In order to use it with Cocoa the
following steps should be taken:
<UL>
<LI> Instantiate a
<CODE>net.java.games.jogl.impl.macosx.MacOSXGLImpl</CODE> using the
public constructor taking no arguments.
<LI> Upon the first render of your application, or whenever the
available OpenGL routines might have changed (because a window moved
from one screen to another) call the publicly-accessible method
<CODE>MacOSXGLImpl.resetGLFunctionAvailability()</CODE>.
<LI> Only use the GL instance when the OpenGL context from the
NSOpenGLView is current.
</UL>
<B>NOTE:</B> the Cocoa interoperability has not yet been retested
since the GLCanvas was implemented. Please report any problems found
with using Jogl with an NSOpenGLView.
</P>
<P>
The following issues remain with the Mac OS X port:
<UL>
<LI> Due to the mechanism by which the Cocoa graphics system selects
OpenGL pixel formats, the GLCapabilitiesChooser mechanism can not be
implemented on Mac OS X as on other platforms. Currently the
underlying Cocoa pixel format selection is used on an
NSOpenGLPixelFormat derived from the settings in the GLCapabilities,
and the GLCapabilitiesChooser is ignored.
</UL>
</P>
</BODY>
</HTML>
|
