278 lines
15 KiB
HTML
278 lines
15 KiB
HTML
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="GENERATOR" content="Mozilla/4.77 [en] (X11; U; Linux 2.4.3-20mdk i686) [Netscape]">
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<title>introduction to the OpenSceneGraph</title>
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</head>
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<body bgcolor="#FFFFFF">
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<img SRC="OpenSceneGraphBanner_Distribution.jpg" BORDER=0 height=77 width=640>
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<table>
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<tr>
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<td><a href="index.html">Index</a></td>
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<td><a href="introduction.html">Introduction</a></td>
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<td><a href="contents.html">Contents</a></td>
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<td><a href="install.html">Install</a></td>
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<td><a href="dependencies.html">Dependencies</a></td>
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<td><a href="demos.html">Demos</a></td>
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<td><a href="data.html">Data</a></td>
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<td><a href="sgv.html">Viewer</a></td>
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<td><a href="stereo.html">Stereo</a></td>
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<td><a href="documentation.html">Reference Guides</a></td>
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</tr>
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</table>
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<h2>
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<u>Introduction to the OpenSceneGraph</u></h2>
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Welcome to OpenSceneGraph project!
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<p>The OpenSceneGraph is an Open Source (LGPL), Cross Platform (Widows,
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Linux, Mac OSX, FreeBSD, Irix, Solaris), Standard C++ and OpenGL based
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graphics development library, uses range from visual simulation, games,
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virtual reality, scientific visualization and graphics research. This pages
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introduces what scene graphs are, why graphics developers use them, and
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details about the OpenSceneGraph, project, how to learn how to use it and
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contribute to the OpenSceneGraph community.
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<p><i>Robert Osfield, Project Lead. April 2002.</i>
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<br>
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<hr>
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<h3>
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<u>What is a Scene Graph?</u></h3>
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Its a tree! Quite simply one the best and most reusable data structures
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invented. Typically drawn schematically as root at the top, leaves at the
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bottom. It all starts with a topmost root node which encompasses your whole
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virtual world, be it 2D or 3D. The world is then broken down into hierarchy
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of nodes representing either a spatial grouping of objects, setting the
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position of objects, animating objects,. or define a logical relationship
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between objects such as to manage the various states of a traffic light.
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The leaves of the graph represent the physical objects themselves, the
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drawable geometry and their material properties.
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<p>A scene graph isn't a complete game or simulation engine, although may
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be one of the main components of such an engine, it's primary focus is
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representing your 3d worlds, and rendering it efficiently. Physics models,
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collision detection and audio are left to other development libraries that
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a user will integrate with.. The fact that scene graphs don't typically
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integrate all these features is actually a really good thing, it aids interoprability
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with clients own applications and tools they wish to use and allows them
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to serve many varied markets from games, visual simulation, virtual reality,
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scientific and commercial visualization, training through to modeling programs.
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<br>
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<hr>
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<h3>
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<u>Why use a Scene Graph - Performance, Productivity, Portability and Scalability</u>.</h3>
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<ol>
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<li><p>
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<i>Performance</i> - scene graphs provide an excellent framework for
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maximize graphics performance. A good scene graph employs two key techniques
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- culling of the objects that won't be seen on screen, and state sorting
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of properties such as textures and materials so that all similar objects
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are drawn together. Without culling the CPU, buses and GPU will all become
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swamped by many times the amount of data than they actually require to
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represent you work accurately. The hierarchical structure of the scene
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graph makes this culling process very efficient with whole town being culled
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with just a few operations! Without state sorting, the the buses and GPU
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will thrash between states, stalling the graphics and destroying graphics
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throughout. As GPU's get faster and faster, the cost of stalling the graphics
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is also going up, so scene graph are become ever more important.
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</p></li>
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<li><p>
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<i>Productivity</i> - scene graphs take much of the hard work required
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to develop high performance graphics applications. The scene graphs manage
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all the graphics for you, reducing what would be thousands of lines of
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OpenGL down to a few simple calls. Furthermoe, one of most powerful concepts
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in Object Orientated programming is that of object composition, enshrined
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in <i>Composite Design Pattern</i>, which fits the scene graph tree structure
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perfectly which makes it highly flexible and reusable design - in real
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terms this means that it can be easily adapted it to solve your problems.
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Scene graph also often come additional utility libraries which range for
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helping users set up and manage graphics windows to import of 3d modes
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and images. All this together allows the user to achieve a great deal with
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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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</p></li>
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<li><p>
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<i>Portability</i> - scene graphs encapsulate much of the lower level
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tasks of rendering graphics and reading and writing data, reducing or even
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eradicating the platform specific coding that you require in your own application.
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If the underlying scene graph is portable then moving from platform to
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platform can be a simple as recompiling your source code.
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</p></li>
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<li><p>
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<i>Scalability</i> - along with being able to dynamic manage the complexity
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of scenes automatically to account for differences in graphics performance
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across a range of machines, scene graphs also make it much easier to manage
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complex hardware configurations, such as clusters of graphics machines,
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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
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while the rendering framework of the scene graph handles the different
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underlying hardware configurations.
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</p></li>
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</ol>
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<hr>
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<h3>
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<u>So what about the OpenSceneGraph project?</u></h3>
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The OpenSceneGraph is an Open Source Scene Graph, and our goal is make
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the benefits of scene graph technology available to all. Our scene graph
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is still in development, but has already gained a great deal of respect
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amongst the development community for its high performance, cleanness of
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design and portability. Written entirely in Standard C++ and OpenGL, it
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makes full use of STL and Design Patterns, and leverages the open source
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development model to provide a development library that is legacy free
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and well focused on the solving the task. The OpenSceneGraph delivers on
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the four key benefits of scene graph technology outlined above using the
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following features:
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<ol>
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<li><p>
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<i>Performance</i> - supports view frustum culling, small feature culling,
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Level Of Details (LOD') nodes, state sorting, vertex arrays and display
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list as part of the core scene graph, these together make the OpenSceneGraph
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one highest performance scene graph available. User feedback is that performance
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surpasses much more established scene graphs such as Performer, VTee, Vega
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Scene Graph and Jave3D! The OpenSceneGraph also supports easy customization
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of the drawing process, which has allowed implementation of Continuos Level
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of Detail (CLOD) meshes on top the scene graph, these allow the visualization
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of massive terrain databases interactively, examples of this approach can
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be found at both Vterrain.org and TerrainEngine.com which both integrate
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with the OpenSceneGraph.
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</p></li>
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<li><p>
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<i>Productivity</i> - by combining lessons learned from established
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scene graph like Performer and Open Inventor, with modern software engineering
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methodologies like Design Patterns and a great deal of feedback early on
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in the development cycle, it has been possible to design a design that
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clean and highly interpretable. This has made it easy for user to adopt
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to the OpenSceneGraph and to integrate with their own applications. With
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a full feature set in the core scene graph, utilities to set up the scene
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graph and viewers and a wide range of loaders it is possible to create
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an application and bring in user data with a very small amount of code.
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</p></li>
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<li><p>
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<i>Portability</i> - The core scene graph has also been designed to
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be have minimal platform specific dependency, requiring little more than
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Standard C++ and OpenGL. The has allowed the scene graph to be rapidly
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ported on wide range of platforms -
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originally developed on IRIX, then
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ported to Linux, then to Windows, then FreeBSD, then Mac OSX and most recently
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Solaris! Being completely windowing system independent makes it easy for
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users to add their own window specific libraries and applications on top.
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In the distribution there is already the osgGLUT library, and in the Bazaar
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found at openscenegrph.org/download/ once can find examples of how applications
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written on top Qt, MFC, WxWindows and SDL. Users have also integrated it
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with Motif, and X.
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</p></li>
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<li><p>
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<i>Scalability</i> - the scene graph not only runs from portables all the
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way up to Onyx Infinite Reality Monsters, it supports the multiple graphics
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subsystems found on machines like the a mulitpipe Onyx. This is possible
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since the core scene graph supports multiple graphics context for both
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OpenGL DisplayLists and texture objects, and the cull and draw traversals
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have been designed to cache rendering data locally and use the scene gaph
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almost entirely as a read only operation. This allows multiple cull-draw
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pairs to run on multiple CPU's which are bound to multiple graphics subsystems.
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This has been demonstrated using the OpenSceneGraph in conjunction with
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sgi's OpenGL multipipe SDK. We also have osgMP in development which will
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be cross platform and transparently support multiple multipipe systems
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like the Onyx and graphics clusters
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</p></li>
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</ol>
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All the source to is published under the GNU Library General Public License
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(LGPL) which allows both open source and closed source projects to use,
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modify and distribute it freely as long its usage complies with the LGPL.
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The project has been developed over the last four years, initiated by Don
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Burns, and then taken over by Robert Osfield who continues to lead project
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today, there are many other contributors to the library, for a full list
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check out the AUTHORS file. Both Robert and Don now work on the OpenSceneGraph
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in a professional capacity providing consultancy and bespoke developments
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on top the library, and are also collaborating on the book. Work on the
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core scene graph and support of public mailing list remains unpaid as are
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the contributions of the rest of the community, but this hasn't impacted
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the quality of the source or support which once you get stuck in you grow
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to appreciate.
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<p>The project is current in alpha, which means parts of the API are still
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to be developed, or subject to change, but the vast majority of the scene
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graph is there, and a beta will be published within the next few months,
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with a 1.0 release in late summer. Despite the alpha development status,
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the project has already earned the reputation the leading open source scene
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graph, and is establishing itself a viable alternative to the commercial
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scene graphs. Numerous companies, university researchers and graphics enthusiasts
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have already adopted their projects, and are from all over the world.
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<br>
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<hr>
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<h3>
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<u>Getting started</u></h3>
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The first thing is to select the distribution which suits you, there are
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binary, development and source code distributions, these can be loaded
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from the
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<a href="http://www.openscenegraph.org/download">http://www.openscenegraph.org/download</a>
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page. The latest developments area available as via a nightly tarball or
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via cvs.
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<p>The binary distribution contains just the libraries (.dll's /.so's)
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and demo executables. This is suitable for using the OpenSceneGraph with
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an application that has already been compiled but depends at runtime on
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the OpenSceneGraph.
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<p>The development distribution contains the libraries (.dll's /.so's),
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demo executables, include files, and source to the demos. This is suitable
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for using the developers using the OpenSceneGraph.
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<p>The source distribution contains the all the source and include files
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required to build the OpenSceneGraph from scratch, and is ideal if you
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want to learn more about how the scene graph works, how to extend it, and
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to track down and fix any problems that you come across.
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<p>If you are using a source distribution then read the <a href="install.html">installation</a>
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instructions for how to get the OpenSceneGraph compiling and installed
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on your system. You may also need to download libraries that parts of the
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OpenSceneGraph is dependent upon such as glut, check the
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<a href="dependencies.html">dependencies</a>
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list for further details.
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<br>
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<hr>
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<h3>
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<u>How to learn to use the OpenSceneGraph</u></h3>
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The OpenSceneGraph distribution comes with a reference guide for each of
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the component libraries - osg, osgDB, osgUtil, osgText and osgGLUT, a set
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of demos - the source of which can be found in src/Demos/. For questions
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or help which can't be easily be answered by the reference guide and demo
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source, one should join the mailing list (details below). There is also
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the beginnings of <a href="http://www.c2.com/cgi/wiki?OpenSceneGraphFaq">Wiki
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based FAQ</a> which may help answer a few of the common queries.
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<p>A programming guide will be available in form of a OpenSceneGraph book
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which is being written by Don Burns and Robert Osfield, parts of it will
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be available online.
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<p>Although not directly related to the OpenSceneGraph, once can learn
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about scene graph technology from such sources as the <a href="http://www.sgi.com/software/inventor/manuals.html">Open
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Inventor Mentor</a>, and <a href="http://www.cineca.it/manuali/Performer/ProgGuide24/html">Performer
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Programming Guides</a>. The later is the closest in design concepts to
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the OpenSceneGraph, although Performer manuals is in C alas. Also of use
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as a background to some of the techniques used is a SIGGRAPH <a href="http://www.opengl.org/developers/code/sig99/advanced99/course_slides/vissim/index.htm">Vis-Sim
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course</a>.
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<p>The OpenSceneGraph uses OpenGL and does with a deliberately thin layer,
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making it easy to control the underlying OpenGL and to extend it with OpenGL
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extensions. The close tie with OpenGL is also reflected in the naming of
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many of the OpenGL state related classes, the the parameters that they
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encapsulate and means that knowledge of OpenGL itself will go a long way
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to understanding how to get the best out of the OpenSceneGraph. To this
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end it is worth obtaining a copy of the OpenGL programming guide - <a href="http://fly.cc.fer.hr/~unreal/theredbook/">`Red
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Book`</a> and OpenGL reference guide 'Blue Book'. The main <a href="http://www.opengl.org">OpenGL
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website</a> is also a good source of links and further information.
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<br>
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<hr>
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<h3>
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<u>Support and discussion - the <i>openscenegraph-news</i> mailing list</u></h3>
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For scene graph related questions, bug reports, bug fixes, and general
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design and development discussion one should join the <a href="http://lists.sourceforge.net/mailman/listinfo/openscenegraph-news">openscenegraph-news</a>
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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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<p>Professional support is also available in the form of confidential online,
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phone and onsite support and consultancy, for details contact Robert Osfield
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at <a href="mailto:robert@openscenegraph.com">robert@openscenegraph.com</a>.
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</body>
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</html>
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