OpenGL newbie question: what is back face culling?

Question

What exactly is back face culling in OpenGL? Can you give mi a specific example with e.g. one triangle?

Answer 1

If you look carefully you can see examples of this in a lot of video games. Any time the camera accidentally moves through an object - typically a moving object like a character - notice how the world continues to render correctly. That's because the back sides of the triangles that form the skin of the character are not rendered; they are effectively transparent. If this were not the case then every time the camera accidentally moved inside an object either the screen would go black (because the interior of the object is not lit) or you'd get a bizarre perspective on what the skin of the object looks like from the inside.

Answer 2

Back face culling is where the triangles pointing away from the camera/viewpoint are not considered for drawing.

Wikipedia defines this as:

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, if the polygon projected on the screen has a counter-clockwise winding it has been rotated to face away from the camera and will not be drawn.

Other systems use the face normal and do the dot product with the view direction.

It is a relatively quick way of deciding whether to draw a triangle or not. Consider a cube. At any one time 3 of the sides of the cube are going to be facing away from the user and hence not visible. Even if these were drawn they would be obscured by the three "forward" facing sides. By performing back face culling you are reducing the number of triangles drawn from 12 to 6 (2 per side).

Back face culling works best with closed "solid" objects such as cubes, spheres, walls.

Some systems don't have this as they consider faces to be double sided and hence visible from either direction.