Culling

The term Culling (English cull : selection, cutting away, waste ) refers to a principle of the 3D computer graphics , is tested wherein if an observed object is currently visible to discard non-visible objects as early as possible and thereby a performance gain in to achieve the representation, see visibility problem .

Depending on the method used, a distinction is made between backface culling , frontface culling , frustum culling and occlusion culling .

Backface culling

The back-face culling is a technique of 3D computer graphics, with a majority of non-visible triangles (or more generally of polygons) of objects in a scene is removed. The lower number of areas to be drawn increases the display speed.

In the case of a triangular surface, one of its two sides is defined as the front side by the normal vector . If there are only closed, massive objects in the scene, the viewer always looks at such fronts. ${\ displaystyle N_ {P}}$

In a cube, for example, a maximum of 3 sides are visible at the same time, the other sides are facing away from the viewer. It is therefore superfluous to draw the back faces , as these are covered by other triangles.

So, assuming opaque, closed objects, a simple test to determine whether a side of the object can be seen or not is to compare the direction of the normal vector of the triangle with the direction of view . ${\ displaystyle N_ {P}}$ ${\ displaystyle N}$

Normal of the triangle ( cross product of and ): ${\ displaystyle V_ {1}}$ ${\ displaystyle V_ {2}}$

${\ displaystyle N_ {p} = V_ {1} \ times V_ {2}}$

Check whether normal in the viewing direction ( scalar product of and ): ${\ displaystyle N_ {p}}$ ${\ displaystyle N}$

${\ displaystyle N_ {p} \ cdot N}$

where and are oriented triangle edges and the line of sight vector. ${\ displaystyle V_ {1}}$ ${\ displaystyle V_ {2}}$ ${\ displaystyle N}$

If the result is , then the surface can be seen (from the front), in the case of a result not (or only from the back). This assumes that the surface normals on a body point away from the body. If an area is visible, the further steps of the graphics pipeline are then run through. If the surface cannot be seen, it does not go through any further steps and is therefore discarded. ${\ displaystyle \ leq 0}$ ${\ displaystyle> 0}$

The back-face culling should be as efficient as possible to the overall process, be applied as early as possible. Often times, it's the first step in a graphics pipeline.

Front face culling

Closely related to backface culling is frontface culling , in which the actually visible front sides are discarded and only the back sides of an object are drawn. This technique is required for special effects (for example the shadow volumes).

Frustum culling

Frustration

With frustum culling ( Frustum , German: truncated pyramid) it is tested whether a viewed object is in the field of vision. Frustum culling means something like truncated cone selection .

The frustum is a truncated pyramid , the base of which is the rear clipping plane , i.e. the area behind which nothing should be drawn anymore. The cutting plane of the pyramid, the front clipping plane, corresponds to the position of the observer (who is looking in the direction of the pyramid base). Everything that lies in the pyramid between the cutting plane and the tip is practically behind the viewer. The side walls of the pyramid are the levels that make up the edge of the screen.

In order to be in the field of view, a point must be within the so-called frustum. In order not to have to calculate each point of an object individually, simple envelope objects ("bounding volume") such as cuboids or spheres are placed around the objects and a cut test is carried out between the envelope object and the visible volume (frustum) for faster calculation . If the intersection of the volume of the frustum and the volume of the shell object is empty, the object is not visible on the screen. If the volume of the shell object is completely in the frustum, the object is completely visible. If there is another intersection, the object may only be partially visible.

Occlusion culling

Occlusion Culling (English occlusion = masking) is a method to covert (partial) objects scene to determine. The polygons hidden by other objects do not have to be treated and drawn further, which increases the speed of calculations for the entire scene. Therefore, this method is used for scenes with a lot of polygons.

Occlusion Culling is natively from NVIDIA on the GeForce 3 supports.

Web links

Backface culling in the Delphi OpenGL Wiki