This tutorial survey paper reviews several different models for light interaction with volume densities of absorbing, glowing, reflecting, and/or scattering material. They are, in order of increasing realism, absorption only, emission only, emission and absorption combined, single scattering of external illumination without shadows, single scattering with shadows, and multiple scattering. For each model the paper provides the physical assumptions, describes the applications for which it is appropriate, derives the differential or integral equations for light transport, presents calculation methods for solving them, and shows output images for a data set representing a cloud. Special attention is given to calculation methods for the multiple scattering model. >

Optical models for direct volume rendering

In this paper, we propose a new approach to numerical smoke simulation for computer graphics applications. The method proposed here exploits physics unique to smoke in order to design a numerical method that is both fast and efficient on the relatively coarse grids traditionally used in computer graphics applications (as compared to the much finer grids used in the computational fluid dynamics literature). We use the inviscid Euler equations in our model, since they are usually more appropriate for gas modeling and less computationally intensive than the viscous Navier-Stokes equations used by others. In addition, we introduce a physically consistent vorticity confinement term to model the small scale rolling features characteristic of smoke that are absent on most coarse grid simulations. Our model also correctly handles the inter-action of smoke with moving objects.

/pdf/visual-simulation-of-smoke-5e6o2k5lr8.pdf

Visual simulation of smoke

This paper presents a new method for computing global illumination in scenes with participating media. The method is based on bidirectional Monte Carlo ray tracing and uses photon maps to increase efficiency and reduce noise. We remove previous restrictions limiting the photon map method to surfaces by introducing a volume photon map containing photons in participating media. We also derive a new radiance estimate for photons in the volume photon map. The method is fast and simple, but also general enough to handle nonhomogeneous media and anisotropic scattering. It can efficiently simulate effects such as multiple volume scattering, color bleeding between volumes and surfaces, and volume caustics (light reflected from or transmitted through specular surfaces and then scattered by a medium). The photon map is decoupled from the geometric representation of the scene, making the method capable of simulating global illumination in scenes containing complex objects. These objects do not need to be tessellated; they can be instanced, or even represented by an implicit function. Since the method is based on a bidirectional simulation, it automatically adapts to illumination and view. Furthermore, because the use of photon maps reduces noise and aliasing, the method is suitable for rendering of animations. CR Descriptors: I.3.7 [Computer Graphics]: ThreeDimensional Graphics and Realism — color, shading, shadowing, and texture; raytracing; I.6.8 [Simulation and Modeling]: Types of Simulation — Monte Carlo. Additional keywords: participating media, light transport, global illumination, multiple scattering, volume caustics, nonhomogeneous media, anisotropic scattering, rendering, photo-realism, photon tracing, photon map, ray marching. ∗mental images GmbH & Co. KG, Fasanenstraße 81, D-10623 Berlin, Germany. E-mail: {henrik,per}@mental.com.

/pdf/efficient-simulation-of-light-transport-in-scenes-with-2eylbwtmem.pdf

Efficient simulation of light transport in scenes with participating media using photon maps

Developing a visually convincing model of fire, smoke, and other gaseous phenomenais among the most difficult and attractive problems in computer graphics We have created new methods of animating a wide range of gaseous phenomena, including the particularly subtle problem of modelling “wispy” smoke and steam, using far fewer primitives than before One significant innovation is the reformulation and solution of the advection-diffusion equation for densities composed of “warped blobs” These blobs more accurately model the distortions that gases undergo when advected by wind fields We also introduce a simple model for the flame of a fire and its spread Lastly, we present an efficient formulation and implementation of global illumination in the presence of gases and fire Our models are specifically designed to permit a significant degree of user control over the evolution of gaseous phenomena

/pdf/depicting-fire-and-other-gaseous-phenomena-using-diffusion-3zd9wvdygi.pdf

Depicting fire and other gaseous phenomena using diffusion processes

Multiple scattering in participating media is generally a complex phenomenon. In the limit of an optically thick medium, i.e., when the mean free path of each photon is much smaller than the medium size, the effects of multiple scattering can be approximated by a diffusion process. We introduce this approximation from the radiative transfer literature to the computer graphics community and propose several numerical methods for its solution. We implemented both a multi-grid finite differences scheme and a finite-element blob method.

/pdf/multiple-scattering-as-a-diffusion-process-rnib82ndjk.pdf

Multiple scattering as a diffusion process

:
We present a new algorithm for simulating the effect of light travelling through volume objects. Such objects (haze, fog, clouds…) are usually modelized by voxel grids which define their density distribution in a discrete tridimensional space. The method we propose is a two-pass Monte-Carlo ray-tracing algorithm that does not make any restrictive assumptions neither about the characteristics of the objects (both arbitrary density distributions and phase functions are allowed) nor about the physical phenomena included in the rendering process (multiple scattering is accounted for). The driving idea of the algorithm is to use the phase function for Monte-Carlo sampling, in order to modify the direction of the ray during scattering.

A Rendering Algorithm for Discrete Volume Density Objects

We propose a method for solving the global illumination problem with no restrictive assumptions concerning the behaviour of light either on surface or volume objects in the scene. Surface objects are defined either by facets or parametric patches and volume objets are defined by voxel grids which define arbitrary density distributions in a discrete tridimensional space. The rendering technique is a Monte-Carlo ray-tracing based radiosity which unifies the processing of objects in a scene, whether they are surfacic or volumic. The main characteristics of our technique are the use of separated Markov chains to prevent the explosion of the number of rays and an optimal importance sampling to speed-up the convergence.

An Importance Driven Monte-Carlo Solution to the Global Illumination Problem

Gas diffusion in a microporous media is strongly analogous to light transport in a scene. Interaction of light with surfaces has a counterpart in Knudsen diffusion and light dispersion in participating media is very similar to gaseous bulk diffusion. On behalf of this fact, a Monte-Carlo simulation method has been designed to compute gas diffusivities and related properties in 3D images of porous media, making use of techniques originated in the domain of image rendering, such as tesselation, spatial partitioning, distance images, optimized intersection tests as well as in the domain of physical simulations, such as microscopic/macroscopic random walk algorithms. Putting all these methods together results in a very efficient algorithm allowing to determine accurately transport properties in real porous media from imaging techniques such as X-ray tomography.

Philippe Blasi

Papers

A Rendering Algorithm for Discrete Volume Density Objects

An Importance Driven Monte-Carlo Solution to the Global Illumination Problem

Application of Rendering Techniques to Monte-Carlo Physcal Simulation of Gas Diffusion