Analytic and Data-Driven Methods for Volumetric Scattering


We focus on rendering the effects of scattering in participating media like mist, fog or haze. While volumetric scattering is extremely difficult to simulate computationally, its qualitative effects are relatively simple, usually involving blurring, such as the glows around light sources. We focus on deriving novel analytic results, in particular a point spread function describing the glow around light sources, or the spreading of a beam through a medium. These can then be used to develop efficient rendering algorithms. We have demonstrated real-time rendering with the effects shown in the images above.
Primary Current Participants

Craig Donner
Bo Sun (Graduated Aug 2008)
Simon Premoze (Completed Mar 2005)
Srinivasa Narasimhan (now at CMU)
Shree Nayar
Ravi Ramamoorthi


We have so far derived two results, both making use of analytic point spread functions for efficient rendering of volumetric scattering. In our first approach, we use analytic expressions for the spread of a beam due to multiple scattering in a participating medium. This can then be used in an efficient rendering algorithm for inhomogeneous media. In our second approach, which we believe to be a significant theoretical and practical contribution to real time rendering, we derive new analytic expressions for the airlight or glow around a light source due to single scattering. The results are expressed as a combination of an analytic function depending only on the physical parameters of the problem, and a purely numerical 2D lookup table, which can be precomputed and stored once. This enables efficient implementation on modern programmable graphics hardware. Furthermore, the approach can be extended to handle the effects of scattering on surface radiance and the effects of complex lighting and materials, which are often omitted even in numerical Monte Carlo methods, as being too expensive computationally.

More recently, we have also shown how to acquire the scattering properties of a variety of substances by dilution in water to the single scattering regime. This represents the first database of volumetric scattering, which we hope will be analogous to databases of measured BRDFs and BTFs. We have also taken a brand new approach to modeling BSSRDFs using exhaustive numerical simulations to derive a general empirical BSSRDF model that allows us to efficiently represent and simulate the full gamut of materials, rather than just the single-scattering and diffusion regimes. Finally, we have developed a new layered heterogeneous reflectance model for human skin, which gave us the cover image of the inaugural Siggraph Asia conference.
A Practical Analytic Single Scattering Model for Real Time Rendering Siggraph 05, pages 1040-1049
We develop an analytic airlight model by explicitly integrating the single scattering equations for light transport due to a point source in a homogeneous medium.

Paper:     PDF    Video (74M)
Practical Rendering of Multiple Scattering Effects in Participating Media EGSR 04, pages 363-374
We use an expression for the spreading of a beam in a participating medium, and develop an efficient and general rendering algorithm

Paper:     PDF   
Acquiring Scattering Properties of Participating Media by Dilution SIGGRAPH 06, pages 1003-1012
We present a simple device and technique for robustly estimating the properties of a broad class of participating media that can be either (a) diluted in water such as juices or beverages, (b) dissolved in water such as powders and sugar/salt crystals, or (c) suspended in water, such as impurities.

Paper:     PDF   
A Layered, Heterogeneous Reflectance Model for Acquiring and Rendering Human Skin SIGGRAPH Asia 2008.
We introduce a layered, heterogeneous spectral reflectance model for human skin. The model captures the inter-scattering of light among layers, each of which may have an independent set of spatially-varying absorption and scattering parameters. To obtain parameters for our model, we use a novel acquisition method that begins with multi-spectral photographs. We create complex skin visual effects such as veins, tattoos, rashes, and freckles.

Paper:     PDF (8M)
An Empirical BSSRDF Model SIGGRAPH 09.
Current scattering models are tuned to the two extremes of thin media and single scattering, or highly scattering materials modeled using the diffusion approximation. The vast intermediate range of materials has no efficient approximation. In this work, we simulate the full space of BSSRDFs, analogous to work on measured BRDF databases. We show new types of scattering behavior, fitting an analytic model and tabulating its parameters. This allows new efficient rendering and reflectance models for a variety of BSSRDFs.

Paper:     PDF

Last modified: Wed May 13 15:33:24 PDT 2009