Efficient Physically-Based Simulation of Light Transport in Participating MediaVincent Pegoraro[paper] [slides] [bibtex] Abstract: Despite their numerous applications, accurately and effectively simulating the interaction of light with volumetric materials remains very challenging due to the intricacy of the various radiative transfer processes. In this thesis, we consider several propagation models and introduce efficient methods for the simulation of physically-based light transport in participating media. We first present a novel method for physically-based rendering of flames that accounts for their unique characteristics. Instead of relying on ad-hoc models, the spectral properties of the various species are computed using theoretical foundations in the domains of molecular chemistry and radiative physics. Combined with a model of nonlinear energy transfer and with tone-mapping techniques simulating the visual adaptation of a human observer, the model allows one to predict the appearance of various types of fire, including both the common yellow flames dominated by soot radiation as well as the previously unaddressed colorful flames where radiation from other chemical compounds prevails. We subsequently present an innovative method that symbiotically combines both control variates and importance sampling in a sequential Monte Carlo framework. The radiance estimates computed in the medium during the rendering process are cached in a 5-D adaptive hierarchical structure that defines dynamic predicate functions for both variance reduction techniques and guarantees well-behaved PDFs, yielding continually increasing efficiencies thanks to a marginal computational overhead. The concepts are also extended to handle general single-bounce surface global illumination effects via an adaptive per-pixel structure with applications to both off-line and progressive interactive rendering. While remaining unbiased, the technique is effective within a single pass as both estimation and caching are done online, exploiting the coherency in illumination while being independent of the actual scene representation. The method is relatively easy to implement and to tune via a single parameter, and we demonstrate its practical benefits with important gains in convergence rate and competitive results with state of the art techniques. Finally, we introduce a novel analytical approach to solving single scattering from punctual light sources in homogeneous media as an alternative to traditional ray-marching and slice-based numerical methods inherently prone to under-sampling artifacts. We start by showing how to derive the first closed-form solution to the air-light integral for isotropic media and light sources. The technique is then extended to both anisotropic phase functions and light distributions via a fast semi-analytical solution relying on a dual-formulation of the air-light integral. Additionally, we derive the very first closed-form solution to the air-light integral for a general formulation of angular distributions of phase functions and light sources, pioneering the analytical computation of exact solutions to complex scattering phenomena. The technique practically relies neither on precomputation nor on storage, and we provide a robust and efficient implementation allowing for an explicit control on the accuracy of the results. We finally demonstrate its quantitative and qualitative benefits over both previous numerical and analytical approaches, and evaluate the performance characteristics of the method that allows real-time/interactive frame rates to be achieved on graphics hardware. |