Editing Z-buffering (section)

==Z-culling==

In [[rendering (computer graphics)|rendering]], z-culling is early pixel elimination based on depth, a method that provides an increase in performance when rendering of hidden surfaces is costly. It is a direct consequence of z-buffering, where the depth of each pixel candidate is compared to the depth of the existing geometry behind which it might be hidden.

When using a z-buffer, a pixel can be culled (discarded) as soon as its depth is known, which makes it possible to skip the entire process of lighting and [[Texture mapping|texturing]] a pixel that would not be [[Visibility (geometry)|visible]] anyway. Also, time-consuming [[pixel shader]]s will generally not be executed for the culled pixels. This makes z-culling a good optimization candidate in situations where [[fillrate]], lighting, texturing, or pixel shaders are the main [[Bottleneck (engineering)|bottlenecks]].

While z-buffering allows the geometry to be unsorted, sorting [[Polygon (computer graphics)|polygons]] by increasing depth (thus using a reverse [[painter's algorithm]]) allows each screen pixel to be rendered fewer times. This can increase performance in fillrate-limited scenes with large amounts of overdraw, but if not combined with z-buffering it suffers from severe problems such as:
* polygons occluding one another in a cycle (e.g. triangle A occludes B, B occludes C, C occludes A)
* the lack of any canonical "closest" point on a triangle (i.e. no matter whether one sorts triangles by their [[centroid]] or closest point or furthest point, one can always find two triangles A and B such that A is "closer" but in reality B should be drawn first).
As such, a reverse painter's algorithm cannot be used as an alternative to z-culling (without strenuous re-engineering), except as an optimization to z-culling. For example, an optimization might be to keep polygons sorted according to x/y-location and z-depth to provide bounds, in an effort to quickly determine if two polygons might possibly have an occlusion interaction.