Back-face culling

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In computer graphics, back-face culling determines whether a polygon of a graphical object is visible, depending on the position of the camera. It is a step in the graphical pipeline that uses the test

Ax+By+Cz+D < 0

to test if the polygon is visible or not. If this test is true, the normal vector of the polygon is pointed away from the camera, meaning that the polygon is facing away and does not need to 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.

A related technique is clipping, which determines whether polygons are within the camera's field of view at all.

It is important to note that this technique only works with single-sided polygons, which are only visible from one side. Double-sided polygons are rendered from both sides, and thus have no back-face to cull.