We can expect marginally improved loading times with this PR. Drawable, Transform and Node counts in stats panels are expected to remain unchanged - this PR does not add new scene graph optimisations, it just increases the speed with which we apply existing ones.
1. We add explicit `NodeVisitor::apply` overrides for commonly encountered node types to avoid additional virtual function calls per node associated with the default `apply` implementation.
2. We skip pushing `StateSet`s when `_mergeAlphaBlending` is enabled or the `StateSet` contains no relevant state.
3. We add a specialised variant of `CollectLowestTransformsVisitor::addTransform` accepting `MatrixTransform` to avoid matrix copies and multiplications.
With this PR we optimise a function that is called quite often when loading new cells.
We remove avoidable dynamic_casts.
We remove an unused pair.second element.
We convert a map to an unordered_map because its ordering is irrelevant in this case.
We avoid adding the root Skeleton node to the bones' node path.
Currently, we create a new ComputeLightSpaceBounds visitor per frame. Within this visitor, we require excessive memory allocations, mainly a new osg::RefMatrix per encountered Transform node.
With this PR we reuse a single ComputeLightSpaceBounds visitor across frames and enable the createOrReuseMatrix functionality to avoid allocating new matrices every frame. osgUtil::CullVisitor internally uses the same approach.
Currently, we run culling tests against all lights in the scene during LightListCallback::pushLightState. We can avoid most of these tests by removing off-screen lights at an earlier stage. We should benchmark the cumulative time spent within LightListCallback::pushLightState before and after this PR.
As we discovered in #3148, `Transform` nodes and their low level equivalence `pushModelViewMatrix` are somewhat costly involving a `Matrix::invert` operation per frame.
With this PR we avoid one `Transform` node for sun flashes and avoid another `pushModelViewMatrix` call in case the sun is fully visible.
With this PR we test out osg's shader define system for a somewhat harmless feature. As we can see, our code becomes more concise and efficient in this case. Most importantly, we no longer create unneeded vertex shader objects.
