Difference Between Clipping and Culling

Clipping and culling are two computer graphics techniques used to improve the efficiency and realism of rendering. Although both techniques involve removing parts of a scene that are not visible, their approaches and results differ.

Read this article to find out more about Clipping and Culling and how they are different from each other.

What is Clipping?

Clipping is a basic computer graphics technique for removing objects, polygons, or fragments that are outside the viewing frustum or viewport. In 3D space, the viewing frustum is a pyramid-shaped area that defines the portion of the scene visible from the camera's perspective. Outside the viewing frustum, geometry is not visible to the viewer and does not need to be rendered.

Clipping improves the performance of real-time graphics applications such as games, simulations, and virtual reality by optimizing the rendering pipeline.

Types of Clipping

Polygon clipping, line clipping, and point clipping are all examples of clipping techniques. Each of these methods employs a unique algorithm to detect and remove geometry outside the viewing frustum.

Polygon Clipping

The process of removing polygons that are outside the viewing frustum is known as polygon clipping. The Sutherland-Hodgman algorithm is the most widely used polygon clipping algorithm. This algorithm operates by intersecting each polygon edge with the viewing frustum and generating new vertices where the edges intersect the frustum.

Line Clipping

Line clipping is the removal of lines that are outside of the viewing frustum. The most common line clipping algorithm is the Cohen-Sutherland algorithm. This algorithm divides the viewing frustum into nine regions and checks whether each endpoint of the line is in the same region as the frustum. If not, the algorithm clips the line to the region's boundary.

Point Clipping

The process of removing points that are outside the viewing frustum is known as point clipping. Individual pixels or fragments that are outside the frustum are removed using this technique. The point clipping algorithm simply determines whether each point lies within the frustum and discards any that do not.

What is Culling?

Culling is the process of removing objects and primitives that cannot possibly be visible. Culling is performed in the early stages of the graphics pipeline to ensure that rejected invisible objects are not fetched, transformed, rasterized, and shaded. It is used to improve the rendering process by reducing the number of objects that need to be processed.

The viewing frustum is a three-dimensional volume that represents the portion of the scene seen by the camera. Six planes intersect at the camera's location and extend outward to form a pyramid-shaped volume, which is defined by the frustum. Outside of these planes, any object or portion of the scene is considered to be outside of the frustum and can be culled.

Types of Culling

Culling techniques of various types can be used to improve rendering performance.

View frustum culling

The most common type of culling involves examining each object in the scene to see if it intersects with the viewing frustum. Objects that are completely outside of the frustum can be removed from the rendering process without risk.

Occlusion Culling

This method entails determining which objects in a scene are obscured, or hidden, by other objects. An object can be culled if it is completely obscured.

Backface Culling

The faces of objects facing away from the camera are culled using this technique. There is no need to render these faces because they will never be seen by the camera.

LOD culling

LOD culling is a technique that involves using lower-detail versions of objects that are far away from the camera. The higher-detail versions of the object are only rendered when they are visible to the camera.

Difference between Clipping and Culling

The following table highlights the major differences between Clipping and Culling −

Characteristics	Clipping	Culling
Definition	Clipping is a basic computer graphics technique for removing objects, polygons, or fragments that are outside the viewing frustum or viewport.	Culling is the process of removing objects and primitives that cannot possibly be visible.
Applied to	Clippings are applied to polygons that are individual or fragmented.	Culling is applied to groups of objects or to entire objects.
Types	Polygon Clipping, View Frustum Clipping, and near and far plane clipping are some of the types of clipping.	Back-face culling, occlusion culling, and object culling are some of the types of culling.
Effect on geometry	It is used to modify the fragments and polygon shapes to fit within the view frustum.	It doesn't modify the geometry; it simply removes it from the rendering process.
Usage	It is used to save time.	It is used to remove unnecessary entries from the graphics.
Process Type	It is a selective process type.	It is an eliminative process type.

Conclusion

In a nutshell, clipping and culling are two crucial techniques used in computer graphics to improve rendering efficiency and realism. While both techniques involve removing parts of a scene that are not visible, their approaches and results differ. Clipping is the process of removing geometry that is outside of the viewing frustum or viewport, whereas culling is the process of removing objects or parts of objects that are not visible to the camera.