introduction to the OpenSceneGraph

Introduction to the OpenSceneGraph

Welcome to OpenSceneGraph project!

In this document I'll introduce what a scene graph is, what the OpenSceneGraph project encompasses, and provide tips on how to learn how to use it and to contact or contribute to the OpenSceneGraph community.

Robert Osfield, Project Lead. April 2002.

What is a Scene Graph?

Its a tree! Quite simply one the best and most reusable datastructures invented.Typically drawn schematically as 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 into hierachy of nodes represent either a spatial grouping of objects, set the position of or animate objects,. or define a logical relationship between objects such as to manage the various states of a traffic light.The leaves of the graph represent the phyical objects themselves, the drawable geometry and their material properties.

Why use a Scene Graph - Performance, Productivity, Portability and Scalability.

Performance - scene graphs provide an excellent framework for maximize graphics 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.

Productivity - scene graphs take much of the hardwork required to develop 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 Composite Design Pattern, 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 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 rendering framework of the scene graph handles the differnt underlying hardware configurations.