The wall which is currently being drawn when your display function is called. Possible values are CAVE_FRONT_WALL, CAVE_LEFT_WALL, CAVE_RIGHT_WALL, CAVE_FLOOR_WALL, CAVE_BACK_WALL, CAVE_CEILING_WALL, CAVE_SCREEN[0-7]_WALL, CAVE_SIMULATOR_WALL, CAVE_SIMULATOR1_WALL, and CAVE_SIMULATOR2_WALL. This variable is only considered to be valid within the CAVEDisplay callback; in non-rendering processes, and in the CAVEFrameFunction or CAVEInitApplication callbacks its value is undefined.