public int addOccluders(Spatial scene) {
            if (scene != null) process(scene);
            return casterCount;
        }
Check the rootScene against camera frustum and if intersects process it recursively.
The global OccludersExtractor variables need to be initialized first.
Variables are updated and used in {@link ShadowUtil#updateShadowCamera} at last.

@param scene the root of the scene to check (may be null)
@return the number of shadow casters found
jme3-core/src/main/java/com/jme3/shadow/ShadowUtil.java