open scene graph - handy snippets

quick notes

• osg, openGl use a right handed coordinate system.
• turn off cameras auto compute of near/far when doing orthographic views:
      pCam->setComputeNearFarMode(osg::CullSettings::DO_NOT_COMPUTE_NEAR_FAR);

obtaining the viewing frustum

osgViewer::Viewer	pViewer = ...;
osg::Polytope		frustum;

frustum.setToUnitFrustum();
frustum.transformProvidingInverse(
	pViewer->getCameraManipulator->getInverseMatrix() *
	pViewer->getCamera()->getProjectionMatrix());