A hierarchical volumetric shadow algorithm for single scattering

Abstract

Figure 1: Rendering volumetric shadows in participating media requires integrating scattering over view rays. Left: The visibility component of this integral has a special structure: once a light ray hits an occluder, that light ray does not contribute to the integral along any view ray past the occluder. Middle: Our method exploits this structure by computing the integrals in an epipolar coordinate system, in which light rays (dashed grey) and view rays (solid black) are orthogonal and the integration can be performed asymptotically efficiently using a partial sum tree. Right: This enables us to compute high-quality scattering integrals much faster than the previous state of the art. AbstractVolumetric effects such as beams of light through participating media are an important component in the appearance of the natural world. Many such effects can be faithfully modeled by a single scattering medium. In the presence of shadows, rendering these effects can be prohibitively expensive: current algorithms are based on ray marching, i.e., integrating the illumination scattered towards the camera along each view ray, modulated by visibility to the light source at each sample. Visibility must be determined for each sample using shadow rays or shadow-map lookups. We observe that in a suitably chosen coordinate system, the visibility function has a regular structure that we can exploit for significant acceleration compared to brute force sampling. We propose an efficient algorithm based on partial sum trees for computing the scattering integrals in a single-scattering homogeneous medium. On a CPU, we achieve speedups of 17-120x over ray marching.

Instant convolution shadows for volumetric detail mapping

“…In contrast, we calculate the visibility while traversing the scene which eliminates the need for precomputation. Baran et al [2010] present a real-time method with volumetric shadows for recreating the effect of light beams from a point light. The spatial positioning of light beams through a volume is defined by objects blocking the light source.…”

Adaptive sampling approach for volumetric shadows in dynamic scenes

“…The incremental integration method is proposed by Baran et al [18]. They used epipolar rectification to reduce computation of scattering integral and accelerate this computation with a partial sum tree.…”

Virtual ray lights for rendering scenes with participating media

“…Our 2D uv domain shares similarities with epipolar slices. This geometric interpretation has previously been used to accelerate volumetric shadows Baran et al 2010], and sampling and reconstruction of motion blur and depth-of-field [Hachisuka et al 2008a]. It would be interesting to consider more sophisticated, but perhaps biased reconstruction of the 2D uv slice, or whether occluders could be directly rasterized into this warped domain.…”