Hi Glenn, Just a quick reply as I'm just back from holiday season and rather cold on techy topics. Currently the OSG's culling is based around testing a internal nodes bounding sphere against the view frustum polytope and the geometry leaves bounding boxes. With shaders having the ability to move geometry around it obvious introduces a "where is it now?" issue for any of the present bounding volume code that tries to handle geometry with such shaders. Currently the OSG has an osg::Drawable::setInitialBound(..) feature but this is really crappy and has long deserved a complete overhaul in the face of widespread usage of shaders.
I don't believe tackling this issue by just providing some for of culling callback/control would be sufficient as it's not only culling traversals that need to test the whereabouts of geometry in the scene graph. What exactly a general purpose solution might be I can't say without putting some more time thinking about the topic. A good first step would be to improve the bounding box controls in osg::Drawable so it's better at managing user set bounding volumes. Robert. On 4 January 2013 15:44, Glenn Waldron <[email protected]> wrote: > Robert et al., > > The topic of culling vis-a-vis shaders has come up a few times recently. > > The issue is: if you transform or emit vertices in a vertex, geometry, or > tessellation shader, the new position information is not available to OSG's > culling mechanism. This may result in visible geometry being culled. > > One approach is to artificially inflate the bounds of your geometry. Not > very elegant. Cull callbacks won't help, since the visibility logic is in > CullStack::isCulled() which executes before any cull callbacks. > > What I would like to do is use a custom frustum for culling. In my app I > have just such a frustum, and I tried applying it the the current > osg::CullingSet; but that CullingSet is automatically kept in sync with the > Camera frustum and my changes get overwritten. > > I think we need a general-purpose solution, since this topic is bound to > come up again. Here are a couple ideas: > > 1) Allow the user to customize CullStack::isCulled() in some way, either > with a callback, or by making the isCulled() variants virtual. > > 2) Allow the user to apply a custom visibility frustum to the > osg::CullingSet. (Calling CullingSet::setFrustum works, but gets > automatically overwritten when there are subsequent Transforms in the scene > graph.) > > I look forward to your thoughts. > > > Glenn Waldron / @glennwaldron / osgEarth.org > > _______________________________________________ > osg-users mailing list > [email protected] > http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org > _______________________________________________ osg-users mailing list [email protected] http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
