The apply() method on SoAction SoAction SoAction always calls beginTraversal() to begin traversal of the graph. The default behavior of beginTraversal() is simply to call SoAction::traverse(). If you need to initialize the action every time it is applied, you can implement a beginTraversal() method for your action class. For example, the SoGetBoundingBoxAction SoGetBoundingBoxAction SoGetBoundingBoxAction sets the bounding box to be empty each time before it begins traversing the scene graph. Afterwards, it calls SoAction::traverse(). Note that beginTraversal() is passed a node, which is either the root of the graph the action is being applied to or the head of a path. Regardless of whether the action is applied to a node, a path, or a path list, the traverse() method handles the traversal appropriately.

For certain classes, you may also want to perform certain operations after traversal, or you may want to change the traversal behavior itself. For example, the render action performs multiple passes for antialiasing.