Research On Physically Based Modeling And Applications Of Complex Materials

Photorealism has long been pursued by the film and game industry.Although physically based rendering is capable of synthesizing realistic images,modern rendering still faces the challenge that there are many complex natural phenomena it cannot simulate.In this thesis,we focus on two fundamental ingredients of physically based rendering —materials and participating media.To be specific,our work includes physically based modeling of special effect pigments and spatially-correlated media.We also conduct a statistical analysis on measured materials.The work of this thesis is divided into the following aspects:1.An appearance model for special effect pigments has to take both high-frequency spatial details(e.g.,glints)and wave-optical effects(e.g.,iridescence)due to thin-film interference into account.However,either phenomenon is challenging to characterize and simulate in a physically accurate way.Capturing these fascinating effects simultaneously is even harder as the normal distribution function and the reflectance term are highly correlated.We propose a multi-scale BRDF model for reproducing the main visual effects of special effect pigments,enabling a smooth transition from fine-scale surface details to large-scale iridescent patterns.We demonstrate that the wavelengthdependent reflectance inside the pixel's footprint follows a Gaussian distribution,and is closely related to the distribution of the thin-film's thickness.We efficiently determine the mean and the variance of this Gaussian distribution assuming that the thin-film's thickness is uniformly distributed.To validate its effectiveness,rendered images of the proposed model are compared against photographs of actual materials.2.Measured materials are hard to understand because thay are tabulated BRDFs.The inconvenience of the raw data has limited the practicality of measured materials.Toalleviate this issue,we propose to analyze the MERL material dataset using directional statistics.First,we separate the material data into diffuse and specular parts according to the Phong model.Then we extract the albedo and scattering matrix for each material.Based on that,we present an efficient framework for editing the appearance of measured materials.In our framework,diffuse part and specular part are edited independently.This is important for measured BRDFs since editing without distinguishing the two parts makes the results less predictive.We use a novel method to cluster the materials by their scattering matrices and select the representative material of every level.With this at hand,the roughness,i.e.,shape of the specular highlights can be adjusted among different levels.Finally,we reconstruct the edited materials which can be directly used in both online and offline rendering.3.Transmission of radiation through spatially-correlated media has shown deviations from the classical exponential law of uncorrelated media.We propose a general,physically based method for modeling such correlated media.We describe spatial correlations by introducing the Fractional Gaussian Field(FGF),a powerful mathematical tool that has proven useful in many areas but remains under-explored in Computer Graphics.With the FGF,we study the effects of correlations in a unified manner,considering both high-frequency,noise-like fluctuations and k-th order fractional Brownian motion(f Bm)with a stochastic continuity property.As a result,we are able to reproduce a wide variety of appearances stemming from different types of spatial correlations.Compared to previous work,our method is the first that addresses both short-range and long-range correlations.We show that our method can simulate different extents of randomness in spatially-correlated media,resulting in a smooth transition in a range of appearances from exponential fall off to complete transparency.We further demonstrate how our method can be integrated into an energy-conserving RTE framework.