OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Specifying the Face Winding for Generated Primitives

As with any filled primitive in OpenGL, the order the vertices are issued determines the facedness of the primitive. Since we don’t issue the vertices directly in this case, but rather have the primitive generator do it on our behalf, we need to tell it the face winding of our primitives. In the layout directive, specify cw for clockwise vertex winding or ccw for counterclockwise vertex winding.