@Override
    public Material getMaterial(Vector3f worldLocation) {
        // get the material from one of the children. They all share the same material
        if (children != null) {
            for (int i = children.size(); --i >= 0;) {
                Spatial child = children.get(i);
                if (child instanceof TerrainQuad) {
                    return ((TerrainQuad)child).getMaterial(worldLocation);
                } else if (child instanceof TerrainPatch) {
                    return ((TerrainPatch)child).getMaterial();
                }
            }
        }
        return null;
    }
Generate the entropy values for the terrain for the "perspective" LOD
calculator. This routine can take a long time to run!
@param progressMonitor optional
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/TerrainQuad.java