| Index | Introduction | Contents | Install | Dependencies | Demos | Data | Viewer | Stereo | Plan | Reference Guides |
The OpenSceneGraph is an Open Source (LGPL), Cross Platform (Widows, Linux, Mac OSX, FreeBSD, Irix, Solaris), Standard C++ and OpenGL based graphics development library, uses range from visual simulation, games, virtual reality, scientific visualization and graphics research. This pages introduces 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.
Robert Osfield, Project Lead. April 2002.
A scene graph isn't a complete game or simulation engine, although may
be one of the main 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 commercial visualization, training through to modeling programs.
Performance - scene graphs provide an excellent framework for maximize graphics performance. A good scene graph employs two key techniques - 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 than they actually require to represent you work accurately. The hierarchical structure of the scene graph makes this culling process very efficient with whole town being culled with just a few operations! Without state sorting, the the buses and GPU will thrash between states, stalling the graphics and destroying graphics 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.
Productivity - scene graphs take much of the hard work required to develop high performance 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 powerful concepts in Object Orientated programming is that of object composition, enshrined in Composite Design Pattern, which fits the scene graph tree structure 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 utility 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 a great deal with very little coding. A dozen lines of code can be enough to load your data and create an interactive viewer!
Portability - scene graphs encapsulate much of the lower level tasks of rendering 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.
Scalability - along with being able to dynamic manage the complexity of scenes 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 will allow the developer to concentrate on developing their own application while the rendering framework of the scene graph handles the different underlying hardware configurations.
Performance - supports view frustum culling, small feature culling, Level 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 implementation of Continuos Level of Detail (CLOD) meshes on top the scene graph, these allow the visualization 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.
Productivity - 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 interpretable. 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.
Portability - The core scene graph has also been designed to be have minimal platform specific dependency, 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 and most recently Solaris! Being completely windowing system independent makes it easy for users to add their own window specific libraries and applications on top. In the distribution there is already the osgGLUT library, and in the Bazaar found at openscenegrph.org/download/ once can find examples of how applications written on top Qt, MFC, WxWindows and SDL. Users have also integrated it with Motif, and X.
Scalability - the scene graph not only runs from portables all the way up to Onyx Infinite Reality Monsters, it supports the multiple graphics subsystems found on machines like the a mulitpipe Onyx. This is possible since the core scene 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 the Onyx and graphics clusters
The project is current in alpha, which means parts of the API are still
to be developed, or subject to change, but the vast majority of the scene
graph is there, and a beta will be published within the next few months,
with a 1.0 release in late summer. Despite the alpha development status,
the project has already earned the reputation the leading open source scene
graph, and is establishing itself a viable alternative to the commercial
scene graphs. Numerous companies, university researchers and graphics enthusiasts
have already adopted their projects, and are from all over the world.
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.
The development distribution contains the libraries (.dll's /.so's), demo executables, include files, and source to the demos. This is suitable for using the developers using the OpenSceneGraph.
The source distribution contains the all the source and include files required to build the OpenSceneGraph from scratch, and is ideal if you want to learn more about how the scene graph works, how to extend it, and to track down and fix any problems that you come across.
If you are using a source distribution then read the installation
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 dependent upon such as glut, check the
dependencies
list for further details.
A programming guide will be available in form of a OpenSceneGraph book which is being written by Don Burns and Robert Osfield, parts of it will be available online.
Although not directly related to the OpenSceneGraph, once can learn about scene graph technology from such sources as the Open Inventor Mentor, and Performer Programming Guides. The later is the closest in design concepts to the OpenSceneGraph, although Performer manuals is in C alas. Also of use as a background to some of the techniques used is a SIGGRAPH Vis-Sim course.
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 naming 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 - `Red
Book` and OpenGL reference guide 'Blue Book'. The main OpenGL
website is also a good source of links and further information.
Professional support is also available in the form of confidential online, phone and onsite support and consultancy, for details contact Robert Osfield at robert@openscenegraph.com.