Clipplanes

Clipplanes, or clipping planes, are planar boundaries used in 3D graphics to discard geometry that lies outside a defined volume. The primary purpose is to limit rendering to a subset of space, reducing unnecessary work and ensuring that only visible or relevant geometry is processed and rasterized. The most common example is the camera’s view frustum, which uses near and far clipping planes to exclude objects behind the camera or beyond a certain distance.

A clip plane is typically described by a plane equation ax + by + cz + d = 0. The space on one side of the plane is considered inside, while the other side is outside. In graphics pipelines, clipping is usually performed after transforming vertices into clip space, where coordinates are (x, y, z, w). The standard clipping volume requires -w ≤ x ≤ w, -w ≤ y ≤ w, and -w ≤ z ≤ w. Polygons that straddle a clipping plane are intersected with the plane, producing new vertices at the intersection points, so that only the portion inside the volume proceeds through the pipeline.

Clipping can be categorized into near/far clipping (part of the view frustum) and user-defined clipping planes. In older APIs, clip planes could be set per object or scene. In modern graphics, clipping against the view frustum is automatic, while user-defined clipping can be implemented via shader-based techniques that assign clip distances or discard fragments.

Clipping algorithms include Sutherland–Hodgman for polygon clipping against convex planes and Cyrus–Beck for convex polyhedra; in hardware, clipping is typically performed by the GPU as part of the fixed-function or programmable stages before perspective division. Clipplanes play a role in effects such as portal rendering, mirror and water surfaces, and custom view culling in complex scenes.