Schattenvolumen

Schattenvolumen is a technique in real-time 3D computer graphics for rendering sharp-edged shadows. It works by constructing geometric volumes, called shadow volumes, around occluders (objects that block light) with respect to a given light source. The region inside a shadow volume is the part of the scene that is not directly lit by that light.

Construction and operation: For each occluder, the silhouette edges as seen from the light are identified. These edges are connected and extruded along the light direction to form the shadow volume, potentially capped at the occluder and extended to a far plane or to infinity. The resulting closed volume defines the set of points that are in shadow. During rendering, the stencil buffer is used to determine which fragments lie inside shadow volumes. A typical workflow renders the normal scene, then renders the shadow volumes with special stencil and depth tests, and finally shades the scene only where the stencil value indicates no shadow volume containment. Two common variants are used: the z-pass method, which counts front and back faces as the depth buffer is updated, and the z-fail method (Carmack’s method), which counts volumes even if they are clipped by the near plane. For directional lights, extrusion is performed along the light direction; for point lights, extrusion extends along rays from the light.

Advantages and limitations: Schattenvolumen render sharp, geometry-driven shadows that are independent of the shading model and do not rely on shadow maps. They handle arbitrary geometry and complex occluders. Limitations include higher geometric and rendering cost for scenes with many occluders, the need for robust edge detection and clipping, potential artifacts at near planes or seams, and their unsuitability for soft shadows or very large, dynamic scenes.

Historische Einordnung: Shadow Volumes wurden in den 1970er Jahren eingeführt und haben die Entwicklung alternativer Shadow-Rendering-Techniken beeinflusst, bleiben aber im modernen Real-Time-Rendering überwiegend durch Shadow Mapping ersetzt oder in Hybridformen genutzt.