delim $$
The projection of a polygon to window coordinates is said to have clockwise winding if an imaginary object following the path from its first vertex, its second vertex, and so on, to its last vertex, and finally back to its first vertex, moves in a clockwise direction about the interior of the polygon. The polygon's winding is said to be counterclockwise if the imaginary object following the same path moves in a counterclockwise direction about the interior of the polygon. ggllFFrroonnttFFaaccee specifies whether polygons with clockwise winding in window coordinates, or counterclockwise winding in window coordinates, are taken to be front-facing. Passing GGLL__CCCCWW to _m_o_d_e selects counterclockwise polygons as front-facing; GGLL__CCWW selects clockwise polygons as front-facing. By default, counterclockwise polygons are taken to be front-facing.
GGLL__IINNVVAALLIIDD__OOPPEERRAATTIIOONN is generated if ggllFFrroonnttFFaaccee is executed between the execution of ggllBBeeggiinn and the corresponding execution of ggllEEnndd.