I have had some limited success using a custom ConstAttributeFunctor subclass to accumlate attributes inside of a ' NodeVisitor::apply( osg::Geode&)' override, but I'm struggling with devising the best plan for mapping each vertex back to the appropriate attribute. My current solution is very messy and I doublt it's very robust. Is there a prescribed way to do this?
Thanks, Rob Radtke -----Original Message----- From: [email protected][mailto:[email protected]] On Behalf Of Robert Osfield
Sent: 12 March 2010 10:57 To: OpenSceneGraph UsersSubject: Re: [osg-users] Need help to understand this code snippet withaccept
and operator Hi Fred, The TriangleIndexFunctor is a template helper class for making it easier to access the triangles held in osg::Drawable. The actual geometry primitives held within different osg::Drawable subclasses could be of any type and any arrangement, so casting osg::Drawable to osg::Geometry etc. and then accessing the primitives directly and then working out how to interpret the triangles from this is rather complex, tedious and prone to poor performance unless you are very careful. To address the tight coupling of accessors to implementations, the osg::Drawable has an accept(osg::Drawable::PrimitiveFunctor&) method exists to allow a subclass from osg::Drawable to pass details on the geometry primitives that it has to the functor in a generic way - thus hiding the local implementation details of that Drawable and enabling your own custom PrimitiveFunctor to work with a wide range of Drawable without needing to know the implementation details. While this achieves good decoupling the PrimitiveFunctor still has to handle all the different types of primitives - polygons, tri strips, quads, qaud strips, lines etc, which is still pretty complicated to implement. To address the complexity of handling all the different types of primitives the TriangleIndexFunctor template class exists to decompose all the descriptions of generic primitives into the simple triangles marked by their corner indices. For you the app developer all you then need to do is create you little functor that implements the void operator()(const int v1, const int v2, const int v3) method as per the example, and then pass the resulting templated class to the drawable to get all the triangle information. The use of templates also ensures good performance. If it wasn't for PrimtiiveFunctor and TriangleFunctor/TriangleIndexFunctor accessing geometry would be very tedious and error prone task - lots of casts, switch statements and book keeping. These helper classes might seem a bit convoluted at first look, they really make life much easier. Go have a search through the OSG code base, there are plenty of examples of TriangleFunctor/TriangleIndexFunctor in action - especially in src/osgUtil. Cheers, Robert.
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