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< title > introduction to the OpenSceneGraph< / title >
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< h2 > Introduction to the OpenSceneGraph< / h2 >
< p > Welcome to OpenSceneGraph project!
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< p > The OpenSceneGraph is an Open Source (LGPL), Cross Platform (Widows, Linux, Mac OSX, FreeBSD, Irix, Solaris),
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Standard C++ and OpenGL based graphics development library, uses range from visual simulation, games,
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virtual reality, sceintific visualization and graphics research. This pages introduces
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what scene graphs are, why graphics developers use them, and details about the OpenSceneGraph,
project, how to learn how to use it and contribute to the OpenSceneGraph community.
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< i > Robert Osfield,
Project Lead.
April 2002.< / i >
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< h2 > What is a Scene Graph?< / h2 >
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< p > Its a tree! Quite simply one the best and most reusable data structures invented.Typically drawn schematically as
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root at the top, leaves at the bottom. It all starts with a topmost root node which
encompasses your whole virtual world, be it 2D or 3D. The world is then broken down
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into hierachy of nodes representing either a spatial grouping of objects,
setting the position of objects, animating objects,. or define a logical relationship between objects such as to manage
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the various states of a traffic light.The leaves of the graph represent the phyical objects
themselves, the drawable geometry and their material properties.
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< p > A scene graph isn't a complete game or simulation engine, although may be one of the main
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components of such an engine, it's primary focus is representing your 3d worlds, and rendering it efficiently. Physics models, collision detection and audio are left to other development libraries that a user will
integrate with.. The fact that scene graphs don't typically integrate all these features
is actually a really good thing, it aids interoprability with clients own applications
and tools they wish to use and allows them to serve many varied markets from games, visual
simulation, virtual reality, scientific and commerical visulasation, training thruogh to modelling
programs.
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< h2 > Why use a Scene Graph - Performance, Productivity, Portability and Scalability.< / h2 >
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< p > < i > Performance< / i > - scene graphs provide an excellent framework for maximize graphics
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performance. A good scene graph employs two key techinques - culling of the objects that won't
be seen on screen, and state sorting of properties such as textures and materials
so that all similar objects are drawn together. Without culling the CPU, buses and
GPU will all become swamped by many times the amount of data tham they acual require.to represent
you work accurately. The hierachical structure of the scene graph makes this culling
process.very efficient.with whole town being culled with just a few operations! Without state storting, the the buses and GPU will thrash between
states, stalling the graphics and destroying graphisc throughout. As GPU's get faster and faster, the cost of
stalling the graphics is also going up, so scene graph are become ever more important.
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< p > < i > Productivity< / i > - scene graphs take much of the hardwork required to develop
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high perftomance graphics applications. The scene graphs manage all the graphics for you,
reducing what would be thousands of lines of OpenGL down to a few simple calls..Furthermoe,
one of most powerfukl concepts in Object Orientated programming is that of object
compotsition, enshrined in < i > Composite Design Pattern< / i > , which fits the scene graph
tree strucutre perfectly which makes it highly flexible and reusable design - in real terms
this means that it can be easily adapted it to solve your problems. Scene graph also often come
additional utilitie libraries which range for helping users set up and manage graphics
windows to import of 3d modes and images. All this together allows the user to achieve
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a great deal with very little coding. A dozen lines of code can be enough to load your data
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and create an interactive viewer!
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< i > Portability< / i > - scene graphs encapsulate much of the lower level tasks of rendering
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graphics and reading and writing data, reducing or even eradicating the platform specific
coding that you require in your own application. If the underlying scene graph is portable
then moving from platform to platform can be a simple as recompiling your source code.
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< i > Scalability< / i > - along with being able to dynamic manage the complexity of scenes
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automatically to account for differences in graphics performance across a range of machines,
scene graphs also make it much easier to manage complex hardware configurations, such as clusters
of graphics machines, or multiprocessor/multipipe systems such as SGI's Onyx. A good scene graph
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will allow the developer to concentrate on developing their own application while the rendering
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framework of the scene graph handles the differnt underlying hardware configurations.
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< h2 > So what about the OpenSceneGraph project?< / h2 >
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The OpenSceneGraph is an Open Source Scene Graph, and our goal is make the benifits
of scene gaph technology available to all. Our scene gaph is still in development,
but has already gained a great deal of respect amoungst the development community
for its high performance, cleaness of design and portability. Written entitely in
Standard C++ and OpenGL, it makes full use of STL and Design Patterns, and leverages
the open source development model to provide a development library that is legacy
free and well focused on the solving the task. The OpenSceneGraph delivers on
the four key benifits of scene graph techonolgy outlined above using the following
features:
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< p > < i > Performance< / i > - supports view frustum culling, small feature culling,
Leval Of Details (LOD') nodes, state sorting, vertex arrays and display list
as part of the core scene graph, these together make the OpenSceneGraph one
highest performance scene graph available. User feedback is that performance
surpasses much more established scene graphs such as Performer, VTee,
Vega Scene Graph and Jave3D! The OpenSceneGraph also supports easy customization
of the drawing process, which has allowed implemention of Continous Level of
Detail (CLOD) meshes ontop the scene graph, these allow the visualisation of
massive terrain databases interactively, examples of this approach can be
found at both Vterrain.org and TerrainEngine.com which both integrate with the
OpenSceneGraph.
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< p > < i > Productivity< / i > - by combining lessons learned from established scene graph like
Performer and Open Inventor, with modern software engineering
methodologies like Design Patterns and a great deal of feedback early on
in the development cycle, it has been possible to design a design that clean
and highly interporable. This has made it easy for user to adopt to the
OpenSceneGraph and to integrate with their own applications. With a full
feature set in the core scene graph, utilities to set up the scene graph and
viewers and a wide range of loaders it is possible to create an application
and bring in user data with a very small amount of code.
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< i > Portability< / i > - The core scene graph has also been designed to be have minimal
platform speciific dependancy, requiring little more than Standard C++ and OpenGL.
The has allowed the scene graph to be rapidly ported on wide range of platforms -
originally developed on IRIX, then ported to Linux, then to Windows, then FreeBSD, then Mac OSX
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and most recently Solaris! Being completely windowing system independant makes it easy
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for users to add their own window specific libraries and applications on top.
In the distribution there is aleady the osgGLUT library, and in the Bazaar found
at openscenegrph.org/download/ once can find examples of how applications written
ontop Qt, MFC, WxWindows and SDL. Users have also integrated it with Motif, and X.
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< i > Scalability< / i > - the scene graph not only runs from portables all the way up
to Onyx Infinite Reality Monsters, it supports the multiple graphics subsystems
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found on machines like the a mulitpipe Onyx. This is possible since the core scene
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graph supports multiple graphics context for both OpenGL DisplayLists and texture objects,
and the cull and draw traversals have been designed to cache rendering data locally and
use the scene gaph almost entirely as a read only operation. This allows multiple
cull-draw pairs to run on multiple CPU's which are bound to multiple graphics subsystems.
This has been demonstrated using the OpenSceneGraph in conjunction with sgi's OpenGL
multipipe SDK. We also have osgMP in development which will be cross platform and
transparently support multiple multipipe systems like th Onyx and gaphics clusters.
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All the source to is published under the GNU Library General Public License (LGPL)
which allows both open source and closed source projects to use, modify and
distribute it freely as long its usage complies with the LGPL. The project has been
developed over the last four years, initliated by Don Burns, and then taken over
by Robert Osfield who continues to lead project today, there are many other
contributors to the library, for a full list check out the AUTHORS file. Both Robert
and Don now work on the OpenSceneGraph in a professional capaciity providing consultancy
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and bespoke developments ontop the library, and are also colloborating on the book.Work on
the core scene graph and support of public mailing list remains unpaid as are the
contributions of the rest of the communinity, but this hasn't impacted the quality of the
source or support which once you get stuck in you grow to appreciate.
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The project is current in alpha, which means parts of the API are still to be
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developerd, or subjec to change, but the vast majority of the scene graph is there, and a beta will
be published within the next few months, wiht a 1.0 release in late summer. Despite the
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alpha development status, the project has alrady earned the reputation the leading
open source scene graph, and is establishing itself a vialbe alternative to the commericial
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scene graphs. Numerous companies, university researchers and graphics enthusasts have
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already adopted their projects, and are from all over the world.
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< h2 > Getting started< / h2 >
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The first thing is to select the distribution which suits you, there are binary, development and
source code distributions, these can be loaded from the
< a href = "http://www.openscenegraph.org/download" > http://www.openscenegraph.org/download< / a > page.
The latest developments area available as via a nightly tarball or via cvs.
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The binary distribution contains just the libraries (.dll's /.so's) and demo executables.
This is suitable for using the OpenSceneGraph with an application that has already been compiled
but depends at runtime on the OpenSceneGraph.
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The development distribution contains the libraries (.dll's /.so's), demo executabls, include files, and source to
the demos. This is suitable for using the developers using the OpenSceneGraph.
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The source distribution contains the all the source and include files required to build the OpenSceneGraph from
sratch, and is ideal if you want to learn more about how the scene gaph works, how to extend it, and to track
down and fix any problems that you come across.
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If you are using a source disitribution then read the < a href = "install.html" > installation< / a > instructions
for how to get the OpenSceneGraph compiling and installed on your system. You may also need to download
libraries that parts of the OpenSceneGraph is dependant upon such as glut, check the
< a href = "dependencies.html" > dependencies< / a > list for futher details.
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< h2 > How to learn to use the OpenSceneGraph< / h2 >
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< p >
The OpenSceneGraph distribution comes with a reference guide for each of the componet libraries - osg, osgDB,
osgUtil, osgText and osgGLUT, a set of demos - the source of which can be found in src/Demos/.For questions
or help which can't be easily be answered by the reference guide and demo source, one should join the openscene
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gaph mailing list (details below).There is also the beginings of < a href = "http://www.c2.com/cgi/wiki?OpenSceneGraphFaq" > Wiki based FAQ< / a >
which may help answer a few of the common querries.
< p > A programming guide will be avaialbe in form of a OpenSceneGraph book which is being
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written by Don Burns and Robert Osfield, parts of it will be available.online.
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Although not directly releated to the OpenSceneGraph, once can learn about scene graph technolgy from
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such sources as the < a href = "http://www.sgi.com/software/inventor/manuals.html" > Open Inventor Mentor< / a > , and < a href = "http://www.cineca.it/manuali/Performer/ProgGuide24/html" > Performer Programming Guides< / a > .
The later is the closest in design concepts to the OpenSceneGraph, although Performer manuals is in C alas.
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The OpenSceneGraph uses OpenGL and does with a deliberately thin layer, making it easy to control the underlying
OpenGL and to extend it with OpenGL extensions. The close tie with OpenGL is also reflected in the nameing
of many of the OpenGL state related classes, the the parameters that they encapsulate and means that knowledge
of OpenGL itself will go a long way to understanding how to get the best out of the OpenSceneGraph. To this
end it is worth obtaining a copy of the OpenGL programming guide - < a href = "http://fly.cc.fer.hr/~unreal/theredbook/" > `Red Book`< / a >
and OpenGL reference guide 'Blue Book'. The main < a href = "http://www.opengl.org" > OpenGL website< / a > is also a good source of links and further information.
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< h2 > Support and discussion - the < i > openscenegraph-news< / i > mailing list< / h2 >
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For scene graph related questions, bug reports, bug fixes, and general design and development discussion one should
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join the < a href = "http://lists.sourceforge.net/mailman/listinfo/openscenegraph-news" > openscenegraph-news< / a > mailing list,
and check the the mailing list < a href = "http://www.geocrawler.com/redir-sf.php3?list=openscenegraph-news" > archives< / a > .
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Professional support is also available in the form of confidential online, phone and onsite support and
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consultancy, for details contact Robert Osfield at < a href = "mailto:robert@openscenegraph.com" > robert@openscenegraph.com< / a > .< / p >
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