Mathematical Topics from 3D Graphics - 3D Math Primer for Graphics and Game Development - page 454

this topic, the convention is to order the vertices in a clockwise fashion
around the triangle when viewed from the front side. Thus, as shown in
Figure 10.41, we will normally remove any triangle whose vertices are or-
dered in a counterclockwise fashion on the screen. (Right-handers usually
employ the opposite conventions.)
Figure 10.41
Backface culling of
triangles with
vertices enumerated
counterclockwise in
screen space
The API will let you control backface culling. You may want to turn
backface culling off while rendering certain geometry. Or, if geometry has
been reflected, you may need to invert the culling, since reflection flips the
vertex order around the faces. Rendering using stencil shadows requires
rendering the frontfaces in one pass and the backfaces in another pass.
Figure 10.42
Backface culling in 3D