+ * A layer has either LAYER_2D or LAYER_3D mode. LAYER_2D has better performance,
+ * the depth test is disabled, and a child actor hides its parent actor.
+ * LAYER_3D uses the depth test, thus a close actor hides a farther one.
+ * LAYER_2D is the default mode and recommended for general cases.
+ * See Layer::Behavior and SetBehavior() for more information.
+ *
+ * Layer is a type of Actor, thus can have parent or children actors.
+ * A layer influences rendering of its all descendant actors,
+ * until another layer appears in the actor tree and manages its own subtree.
+ *