Back-face culling
Encyclopedia
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, 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.
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.
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.
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.
One method of implementing back-face culling is by discarding all polygons where the dot product
of their surface normal
and the camera-to-polygon vector is greater than or equal to zero.
Computer graphics
Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....
, back-face culling determines whether a polygon
Polygon
In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...
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, 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.
The process makes rendering
Rendering (computer graphics)
Rendering is the process of generating an image from a model , by means of computer programs. A scene file contains objects in a strictly defined language or data structure; it would contain geometry, viewpoint, texture, lighting, and shading information as a description of the virtual scene...
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
Clipping (computer graphics)
Any procedure which identifies that portion of a picture which is either inside or outside a picture is referred to as a clipping algorithm or clipping.The region against which an object is to be clipped is called clipping window.-Examples:...
, 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.
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.
One method of implementing back-face culling is by discarding all polygons where the dot product
Dot product
In mathematics, the dot product or scalar product is an algebraic operation that takes two equal-length sequences of numbers and returns a single number obtained by multiplying corresponding entries and then summing those products...
of their surface normal
Surface normal
A surface normal, or simply normal, to a flat surface is a vector that is perpendicular to that surface. A normal to a non-flat surface at a point P on the surface is a vector perpendicular to the tangent plane to that surface at P. The word "normal" is also used as an adjective: a line normal to a...
and the camera-to-polygon vector is greater than or equal to zero.
Further reading
- Geometry Culling in 3D Engines, by Pietari Laurila