OpenSceneGraph Introduction

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Introduction

osgART brings ARToolKit tracking to the OpenSceneGraph graphics frameworks. Because OpenSceneGraph wraps OpenGL's underlying power in high-level contructs, building an AR-based application using osgART allows you to achieve higher-level interaction more rapidly. Most of the code in your application will be interacting with OpenSceneGraph, and so you will need to understand OpenSceneGraph's programming metaphors.

A good way to learn about OpenSceneGraph is to read the free introductory text by Paul Martz. It is available for free download.

OpenSceneGraph Quick Start Guide

The remainder of this article will introduce you to some of the concepts and features of OpenSceneGraph. Almost anything that can be done in a piece regular OpenSceneGraph code can be done in AR using osgART, as well as opening up a whole new class of interaction paradigms through ARToolKit.

The basic OpenSceneGraph boilerplate

Creating a window and rendering a model into it using OpenSceneGraph requires a very small amount of code. Because most OpenSceneGraph programs build on this code, it is often referred to as boilerplate.

This first tutorial simple explains how to create a window in which to draw your 3D graphics using . All other tutorials will build on this one.

In OSG you have several options for window management. You can use one of the inbuilt classes to create the window for you, you can use another library to do it, or your can create the window yourself using whatever mechanisms your operating supports.

In this case we will use the osgProducer Viewer class to create the window for us.

Osg window with cow.png

#include <osgProducer/Viewer>
#include <osgDB/ReadFile>
 
int main( int argc, char **argv )
{
    osg::ArgumentParser arguments(&argc,argv);
    osgProducer::Viewer viewer(arguments);
    viewer.setUpViewer(osgProducer::Viewer::STANDARD_SETTINGS);
 
    osg::Node* _cow = osgDB::readNodeFile("cow.osg");
 
    viewer.setSceneData( _cow );
    viewer.realize();
    while( !viewer.done() )
    {
        // wait for all cull and draw threads to complete.
        viewer.sync();
        viewer.update();
        viewer.frame();
    }   
    viewer.sync();
    viewer.cleanup_frame();
    viewer.sync();
    return 0;
}

All our osgART examples build on this code by adding sections to connect to a camera, initialise an AR tracker, and load in pattern files, as well as any interaction logic.

Loading models

OSG is supplied with plugins that load many different file formats. The most useful currently supported formats are:

  • 3D Studio (.3ds)
  • Lightwave (.lwo)
  • Wavefront (.obj)
  • OSG native ASCII (.osg)
  • OSG native binary (.ive)

OSGExp is a plugin for 3D Studio Max that saves scenes from Max directly into the OSG native formats (OSG and IVE). IVE is a binary format that loads very quickly, can store textures internally, and is the recommended format. Something to watch out for is that binary IVE files may not necessarily be compatible with new versions of OSG, so keep you source Max files.

A model in 3D Studio Max.
The same model exported and viewed in the OSG viewer.

If you have existing models in a format that OSG supports and want to convert it to the native OSG format, you can use the osgconv utility included with OSG.

Terrain rendering

OSG has built-in support for terrain rendering. It is possible to generate databases of high-detail terrain tiles that are loaded in when needed. This is known as paged LOD (level of detail).

osgdem is a utility that comes with OSG which can generate these databases.

Using sound

OpenSceneGraph does not have built in support for sound (it's primarily a renderer, not a full-featured game engine). However, it is quite straightforward to add sound support using one of the many available sound libraries. Some possibilities are DirectSound, fmod and OpenAL.

A wrapper has been written of OpenGL that makes it easy to use with OpenSceneGraph. The wrapper is called osgAL and it allows SoundNodes to be added to the scene graph.

Animation

There are several types of animation that a 3D application can use.

The most basic is simply movement over time, achieved by incrementally translating, rotating and scaling an object.

Then there is showing a series of frames.

Keyframe animation interpolates between frames to give smooth movement.

Skeletal animation uses a mesh rigged to a skeleton. As the skeleton moves the mesh moves realistically with it.


OSG has support for:

  • Interpolating movement over time (osg::AnimationPath)
  • Series of frames (osg::Sequence)

There is no official support for keyframe or skeletal animation. There is work being done on a nodekit called osgCharacter that provides skeletal animation. Currently there is only a Maya exporter.

Render to texture

Many graphical effects nowadays are created using render-to-texture (RTT) techniques.

RTT is achieved using the osg::CameraNode class. The subgraph below the camera node in the scene graph is “seen” by the camera. A texture can be attached to the camera node so that the camera’s view is rendered into the texture, rather than the frame buffer. This happens before the main scene is rendered, making the texture available for use within the scene. For example, you could use this technique to render the view from a security camera into a texture, then use that texture on the surface of a television monitor.

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