128 lines
6.9 KiB
HTML
128 lines
6.9 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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<html>
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<head>
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<title>introduction to the OpenSceneGraph</title>
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</head>
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<body bgcolor="white">
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<img src="OpenSceneGraphBanner_Distribution.jpg" width=640 height=77 border=0>
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<h2>Introduction to the OpenSceneGraph</h2>
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<p>Welcome to OpenSceneGraph project!
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</p>
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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 a
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what scene graphs are, why graphics developers use them, and details about the OpenSceneGraph,
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project, how to learn how to use it and contribute to the OpenSceneGraph community.
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</p>
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<i>Robert Osfield,
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Project Lead.
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April 2002.</i>
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<hr>
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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
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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,
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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
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themselves, the drawable geometry and their material properties.
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</p>
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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
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integrate with.. The fact that scene graphs don't typically integrate all these features
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is actually a really good thing, it aids interoprability with clients own applications
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and tools they wish to use and allows them to serve many varied markets from games, visual
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simulation, virtual reality, scientific and commerical visulasation, training thruogh to modelling
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programs.
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</p>
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<hr>
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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
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be seen on screen, and state sorting of properties such as textures and materials
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so that all similar objects are drawn together. Without culling the CPU, buses and
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GPU will all become swamped by many times the amount of data tham they acual require.to represent
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you work accurately. The hierachical structure of the scene graph makes this culling
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process.very efficient.with whole town being culled with just a few operations! Without state storting, the the buses and GPU will thrash between
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states, stalling the graphics and destroying graphisc throughout. As GPU's get faster and faster, the cost of
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stalling the graphics is also going up, so scene graph are become ever more important.
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</p>
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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,
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reducing what would be thousands of lines of OpenGL down to a few simple calls..Furthermoe,
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one of most powerfukl concepts in Object Orientated programming is that of object
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compotsition, enshrined in <i>Composite Design Pattern</i>, which fits the scene graph
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tree strucutre perfectly which makes it highly flexible and reusable design - in real terms
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this means that it can be easily adapted it to solve your problems. Scene graph also often come
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additional utilitie libraries which range for helping users set up and manage graphics
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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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</p>
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<p>
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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
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coding that you require in your own application. If the underlying scene graph is portable
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then moving from platform to platform can be a simple as recompiling your source code.
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</p>
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</p>
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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,
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scene graphs also make it much easier to manage complex hardware configurations, such as clusters
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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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</p>
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<hr>
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<h2>So what about the OpenSceneGraph project?</h2>
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<p>
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The OpenSceneGraph is Open Source, all the source to is published under the
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GNU Library General Public License (LGPL) which allows both open source and closed source
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projects to use, modify and distribute it freely as long its usage complies with the
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LGPL. The project has been developed over the last four years, initliated by Don Burns, and
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then taken over by Robert Osfield who leads the continues to lead project, there are many
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other contributors to the library, for a full list check out the AUTHORS file. Both Robert
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and Don now work on the OpenSceneGraph in a professional capaciity provide consultancy
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and bespoke developments ontop the library, and are also colloborating on the book.Work on
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the core scene graph and support of public mailing list remains unpaid as are the
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contributions of the rest of the communinity, but this hasn't impacted the quality of the
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source or support which once you get stuck in you grow to appreciate.
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</p>
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<p>
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The project itself is written entitely in Standard C++ and OpenGL, makes full use of STL
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and Design Patterns, and leverages the open source development model to provide clean, highly portable,
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and flexible scene graph. 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
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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 as one of the leading
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open source scene graphs, 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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</p>
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<hr>
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<h2>How to learn to use the OpenSceneGraph</h2>
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<hr>
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<h2>Support and discussion - the OpenSceneGraph-news mailing list</h2>
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</body>
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</html> |