Conference Paper

Path Integration for Light Transport in Volumes.

Conference: Proceedings of the 14th Eurographics Workshop on Rendering Techniques, Leuven, Belgium, June 25-27, 2003
Source: DBLP

ABSTRACT Simulating the transport of light in volumes such as clouds or objects with subsurface scattering is computationally expensive. We describe an approximation to such transport using path integration. Unlike the more commonly used diffusion approximation, the path integration approach does not explicitly rely on the assumption that the material within the volume is dense. Instead, it assumes the phase function of the volume material is strongly forward scattering and uniform throughout the medium, an assumption that is often the case in nature. We show that this approach is useful for simulating subsurface scattering and scattering in clouds.

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    ABSTRACT: Central to all Monte Carlo-based rendering algorithms is the construction of light transport paths from the light sources to the eye. Existing rendering approaches sample path vertices incrementally when constructing these light transport paths. The resulting probability density is thus a product of the conditional densities of each local sampling step, constructed without explicit control over the form of the final joint distribution of the complete path. We analyze why current incremental construction schemes often lead to high variance in the presence of participating media, and reveal that such approaches are an unnecessary legacy inherited from traditional surface-based rendering algorithms. We devise joint importance sampling of path vertices in participating media to construct paths that explicitly account for the product of all scattering and geometry terms along a sequence of vertices instead of just locally at a single vertex. This leads to a number of practical importance sampling routines to explicitly construct single-and double-scattering subpaths in anisotropically-scattering media. We demonstrate the benefit of our new sampling techniques, integrating them into several path-based rendering algorithms such as path tracing, bidirectional path tracing, and many-light methods. We also use our sampling routines to generalize deterministic shadow connections to connection subpaths consisting of two or three random decisions, to efficiently simulate higher-order multiple scattering. Our algorithms significantly reduce noise and increase performance in renderings with both isotropic and highly anisotropic, low-order scattering.
    ACM Transactions on Graphics (TOG). 11/2013; 32(6).
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    ABSTRACT: Monte Carlo path tracing is a simple and effective way to solve the volume rendering equation. However, propagating light paths through participating media can be very costly because of the need to simulate potentially many scattering events. This paper presents a simple technique to accelerate path tracing of homogeneous participating media. We use a traditional path tracer for scattering near the surface but switch to a new approach for handling paths that penetrate far enough inside the medium. These paths are determined by sampling from a set of precomputed probability distributions, which avoids the need to simulate individual scattering events or perform ray intersection tests with the environment. We demonstrate cases where our approach leads to accurate and more efficient rendering of participating media, including subsurface scattering in translucent materials.
    Proceedings of the Sixth Eurographics / Ieee VGTC conference on Volume Graphics; 09/2007

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