Other Applications of Shadow Maps
Treating the shadow map as defining a volume of space, separating light from dark, can also help in determining what parts of objects to shadow. Gollent describes how CD Projekt’s terrain shadowing system computes for each area a maximum height that is still occluded, which can then be used to shadow not only terrain but also trees and other elements in the scene. To find each height, a shadow map of the visible area is rendered for the sun. Each terrain heightfield location is then checked for visibility from the sun. If in shadow, the height where the sun is first visible is estimated by increasing the world height by a fixed step size until the sun comes into view and then performing a binary search. In other words, we march along a vertical line and iterate to narrow down the location where it intersects the shadow map’s surface that separates light from dark. Neighboring heights are interpolated to find this occlusion height at any location. We will see more use of ray marching through areas of light and dark in Chapter 14.
One last method worth a mention is rendering screen-space shadows. Shadow maps often fail to produce accurate occlusions on small features, because of their limited resolution. This is especially problematic when rendering human faces, because we are particularly prone to noticing any visual artifacts on them. For example, rendering glowing nostrils (when not intended) looks jarring. While using higher-resolution shadow maps or a separate shadow map targeting only the area of interest can help, another possibility is to leverage the already-existing data. In most modern rendering engines the depth buffer from the camera perspective, coming from an earlier prepass, is available during rendering. The data stored in it can be treated as a heightfield. By iteratively sampling this depth buffer, we can perform a ray-marching process (Section 6.8.1) and check if the direction toward the light is unoccluded. While costly, as it involves repeatedly sampling the depth buffer, doing so can provide high-quality results for closeups in cut scenes, where spending extra milliseconds is often justified. The method was proposed by Sousa at al. and is commonly used in many game engines today .
To summarize this whole chapter, shadow mapping in some form is by far the most common algorithm used for shadows cast onto arbitrary surface shapes.
Cascaded shadow maps improve sampling quality when shadows are cast in a large area, such as an outdoor scene.
Finding a good maximum distance for the near plane via SDSM can further improve precision.
Percentage-closer filtering (PCF) gives some softness to the shadows,
percentage-closer soft shadows (PCSS) and its variants give contact hardening,
and the irregular z-buffer can provide precise hard shadows.
Filtered shadow maps provide rapid soft-shadow computation and work particularly well when the occluder is far from the receiver, as with terrain.
Finally, screen-space techniques can be used for additional precision, though at a noticeable cost.
In this chapter, we have focused on key concepts and techniques currently used in applications. Each has its own strengths, and choices depend on world size, composition (static content versus animated), material types (opaque, transparent, hair, or smoke), and number and type of lights (static or dynamic; local or distant; point, spot, or area), as well as factors such as how well the underlying textures can hide any artifacts. GPU capabilities evolve and improve, so we expect to continue seeing new algorithms that map well to the hardware appear in the years ahead. For example, the sparse-texture technique described in Section 19.10.1 has been applied to shadowmap storage to improve resolution . In an inventive approach, Sintorn, Kampe, and others explore the idea of converting a two-dimensional shadow map for a light into a three-dimensional set of voxels (small boxes; see Section 13.10). An advantage of using a voxel is that it can be categorized as lit or in shadow, thus needing minimal storage. A highly compressed sparse voxel octree representation stores shadows for a huge number of lights and static occluders. Scandolo et al. combine their compression technique with an interval-based scheme using dual shadow maps, giving still higher compression rates. Kasyan uses voxel cone tracing (Section 13.10) to generate soft shadows from area lights.
Our focus in this chapter is on basic principles and what qualities a shadow algorithm needs?predictable quality and performance?to be useful for interactive rendering. We have avoiding an exhaustive categorization of the research done in this area of rendering, as two texts tackle the subject.