Back-face culling
In computer graphics, back-face culling determines whether a polygon of a graphical object is visible. It is a step in the graphical pipeline that tests whether the points in the polygon appear in clockwise or counter-clockwise order when projected onto the screen. If the user has specified that front-facing polygons have a clockwise winding, but the polygon projected on the screen has a counter-clockwise winding then it has been rotated to face away from the camera and will not be drawn.
The process makes rendering objects quicker and more efficient by reducing the number of polygons for the program to draw. For example, in a city street scene, there is generally no need to draw the polygons on the sides of the buildings facing away from the camera; they are completely occluded by the sides facing the camera.
In general back-face culling can be assumed to produce no visible artifact in a rendered scene if it contains only closed and opaque geometry. In scenes containing transparent polygons, rear facing polygons may become visible through the process of alpha composition. In wire-frame rendering, back-face culling can be used to partially address problem of hidden line removal, but only for closed convex geometry.
A related technique is clipping, which determines whether polygons are within the camera's field of view at all.
Another similar technique is Z-culling, also known as occlusion culling, which attempts to skip the drawing of polygons which are covered from the viewpoint by other visible polygons.
One method of implementing back-face culling is by discarding all triangles where the dot product of their surface normal and the camera-to-triangle vector is greater than or equal to zero
where P is the view point, V0 is the first vertex of a triangle and N is its normal, defined as a cross product of two vectors representing sides of the triangle adjacent to V0
Since cross product is non-commutative, defining the normal in terms of cross product allows to specify normal direction relative to triangle surface using vertex order(winding):
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