Fix the optimizer messing up LOD node's children (Fixes #4301)

components/sceneutil/optimizer.cpp

@@ -800,6 +800,13 @@ bool Optimizer::RemoveRedundantNodesVisitor::isOperationPermissible(osg::Node& n
            isOperationPermissibleForObject(&node);
 }
 
+void Optimizer::RemoveRedundantNodesVisitor::apply(osg::LOD& lod)
+{
+    // don't remove any direct children of the LOD because they are used to define each LOD level.
+    for (unsigned int i=0; i<lod.getNumChildren(); ++i)
+        traverse(*lod.getChild(i));
+}
+
 void Optimizer::RemoveRedundantNodesVisitor::apply(osg::Group& group)
 {
     if (typeid(group)==typeid(osg::Group) &&
@@ -1849,6 +1856,12 @@ bool Optimizer::MergeGroupsVisitor::isOperationPermissible(osg::Group& node)
             isOperationPermissibleForObject(&node);
 }
 
+void Optimizer::MergeGroupsVisitor::apply(osg::LOD &lod)
+{
+    // don't merge the direct children of the LOD because they are used to define each LOD level.
+    traverse(lod);
+}
+
 void Optimizer::MergeGroupsVisitor::apply(osg::Group &group)
 {
     if (group.getNumChildren() <= 1)

components/sceneutil/optimizer.hpp

@@ -339,6 +339,7 @@ class Optimizer
 
                 virtual void apply(osg::Group& group);
                 virtual void apply(osg::Transform& transform);
+                virtual void apply(osg::LOD& lod);
 
                 bool isOperationPermissible(osg::Node& node);
 
@@ -358,6 +359,7 @@ class Optimizer
             bool isOperationPermissible(osg::Group& node);
 
             virtual void apply(osg::Group& group);
+            virtual void apply(osg::LOD& lod);
         };
 
         class MergeGeometryVisitor : public BaseOptimizerVisitor