279 lines
14 KiB
HTML
279 lines
14 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="images/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
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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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<ol>
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<li>
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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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</li>
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<li>
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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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</li>
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<li>
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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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</li>
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<li>
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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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</li>
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</ol>
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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 an Open Source Scene Graph, and our goal is make the benifits
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of scene gaph technology available to all. Our scene gaph is still in development,
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but has already gained a great deal of respect amoungst the development community
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for its high performance, cleaness of design and portability. Written entitely in
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Standard C++ and OpenGL, it makes full use of STL and Design Patterns, and leverages
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the open source development model to provide a development library that is legacy
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free and well focused on the solving the task. The OpenSceneGraph delivers on
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the four key benifits of scene graph techonolgy outlined above using the following
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features:
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</p>
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<ol>
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<li>
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<p><i>Performance</i> - supports view frustum culling, small feature culling,
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Leval Of Details (LOD') nodes, state sorting, vertex arrays and display list
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as part of the core scene graph, these together make the OpenSceneGraph one
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highest performance scene graph available. User feedback is that performance
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surpasses much more established scene graphs such as Performer, VTee,
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Vega Scene Graph and Jave3D! The OpenSceneGraph also supports easy customization
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of the drawing process, which has allowed implemention of Continous Level of
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Detail (CLOD) meshes ontop the scene graph, these allow the visualisation of
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massive terrain databases interactively, examples of this approach can be
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found at both Vterrain.org and TerrainEngine.com which both integrate with the
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OpenSceneGraph.
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</p>
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</li>
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</li>
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<li>
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<p><i>Productivity</i> - by combining lessons learned from established scene graph like
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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 clean
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and highly interporable. This has made it easy for user to adopt to the
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OpenSceneGraph and to integrate with their own applications. With a full
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feature set in the core scene graph, utilities to set up the scene graph and
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viewers and a wide range of loaders it is possible to create an application
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and bring in user data with a very small amount of code.
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</p>
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</li>
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<li>
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<p>
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<i>Portability</i> - The core scene graph has also been designed to be have minimal
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platform speciific dependancy, requiring little more than Standard C++ and OpenGL.
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The has allowed the scene graph to be rapidly ported on wide range of platforms -
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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.
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In the distribution there is aleady the osgGLUT library, and in the Bazaar found
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at openscenegrph.org/download/ once can find examples of how applications written
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ontop Qt, MFC, WxWindows and SDL. Users have also integrated it with Motif, and X.
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</p>
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</p>
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</li>
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<li>
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</p>
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<i>Scalability</i> - the scene graph not only runs from portables all the way up
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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,
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and the cull and draw traversals have been designed to cache rendering data locally and
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use the scene gaph almost entirely as a read only operation. This allows multiple
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cull-draw 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 sgi's OpenGL
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multipipe SDK. We also have osgMP in development which will be cross platform and
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transparently support multiple multipipe systems like th Onyx and gaphics clusters.
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</p>
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</li>
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</ol>
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<p>
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All the source to is published under the GNU Library General Public License (LGPL)
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which allows both open source and closed source projects to use, modify and
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distribute it freely as long its usage complies with the LGPL. The project has been
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developed over the last four years, initliated by Don Burns, and then taken over
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by Robert Osfield who continues to lead project today, there are many other
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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 providing 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 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 the leading
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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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</p>
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<hr>
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<h2>Getting started</h2>
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<p>
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The first thing is to select the distribution which suits you, there are binary, development and
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source code distributions, these can be loaded from the
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<a href="http://www.openscenegraph.org/download">http://www.openscenegraph.org/download</a> page.
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The latest developments area available as via a nightly tarball or via cvs.
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</p>
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<p>
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The binary distribution contains just the libraries (.dll's /.so's) and demo executables.
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This is suitable for using the OpenSceneGraph with an application that has already been compiled
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but depends at runtime on the OpenSceneGraph.
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</p>
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<p>
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The development distribution contains the libraries (.dll's /.so's), demo executabls, include files, and source to
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the demos. This is suitable for using the developers using the OpenSceneGraph.
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</p>
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<p>
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The source distribution contains the all the source and include files required to build the OpenSceneGraph from
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sratch, and is ideal if you want to learn more about how the scene gaph works, how to extend it, and to track
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down and fix any problems that you come across.
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</p>
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<p>
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If you are using a source disitribution then read the <a href="install.html">installation</a> instructions
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for how to get the OpenSceneGraph compiling and installed on your system. You may also need to download
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libraries that parts of the OpenSceneGraph is dependant upon such as glut, check the
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<a href="dependencies.html">dependencies</a> list for futher details.
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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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<p>
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The OpenSceneGraph distribution comes with a reference guide for each of the componet libraries - osg, osgDB,
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osgUtil, osgText and osgGLUT, a set 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 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>
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which may help answer a few of the common querries.
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<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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</p>
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<p>
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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>.
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The later is the closest in design concepts to the OpenSceneGraph, although Performer manuals is in C alas.
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</p>
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<p>
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The OpenSceneGraph uses OpenGL and does with a deliberately thin layer, making it easy to control the underlying
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OpenGL and to extend it with OpenGL extensions. The close tie with OpenGL is also reflected in the nameing
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of many of the OpenGL state related classes, the the parameters that they encapsulate and means that knowledge
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of OpenGL itself will go a long way 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 Book`</a>
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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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</p>
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<hr>
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<h2>Support and discussion - the <i>openscenegraph-news</i> mailing list</h2>
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<p>
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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,
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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>
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<p>
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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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</body>
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</html>
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