A practical path guiding method for participating media

Rendering translucent materials is costly:light transport algorithms need to simulate a large number of scattering events inside the material before reaching convergence.The cost is especially high for materials with a large albedo or a small mean-freepath,where higher-order scattering effects dominate.In simple terms,the paths get lost in the medium.Path guiding has been proposed for surface rendering to make convergence faster by guiding the sampling process.In this paper,we introduce a path guiding solution for translucent materials.We learn an adaptive approximate representation of the radiance distribution in the volume and use it to sample the scattering direction,combining it with phase function sampling by resampled importance sampling.The proposed method significantly improves the performance of light transport simulation in participating media,especially for small lights and media with refractive boundaries.Our method can handle any homogeneous participating medium,with high or low scattering,with high or low absorption,and from isotropic to highly anisotropic.

A survey on rendering homogeneous participating media

Participating media are frequent in real-world scenes,whether they contain milk,fruit juice,oil,or muddy water in a river or the ocean.Incoming light interacts with these participating media in complex ways:refraction at boundaries and scattering and absorption inside volumes.The radiative transfer equation is the key to solving this problem.There are several categories of rendering methods which are all based on this equation,but using different solutions.In this paper,we introduce these groups,which include volume density estimation based approaches,virtual point/ray/beam lights,point based approaches,Monte Carlo based approaches,acceleration techniques,accurate single scattering methods,neural network based methods,and spatially-correlated participating media related methods.As well as discussing these methods,we consider the challenges and open problems in this research area.

Rendering discrete participating media using geometrical optics approximation

We consider the scattering of light in participating media composed of sparsely and randomly distributed discrete particles.The particle size is expected to range from the scale of the wavelength to several orders of magnitude greater,resulting in an appearance with distinct graininess as opposed to the smooth appearance of continuous media.One fundamental issue in the physically-based synthesis of such appearance is to determine the necessary optical properties in every local region.Since these properties vary spatially,we resort to geometrical optics approximation(GOA),a highly efficient alternative to rigorous Lorenz–Mie theory,to quantitatively represent the scattering of a single particle.This enables us to quickly compute bulk optical properties for any particle size distribution.We then use a practical Monte Carlo rendering solution to solve energy transfer in the discrete participating media.Our proposed framework is the first to simulate a wide range of discrete participating media with different levels of graininess,converging to the continuous media case as the particle concentration increases.

Prediction of the coupled heat radiation and conduction parameters and boundary condition using the unscented Kalman filter

The boundary emissivity, thermal conductivity, and boundary time-varying heat flux in the participating media are retrieved in this work. In the forward model, the coupled radiation-conduction heat transfer in the participating medium is resolved by the finite volume method combined with discrete ordinates method. The inverse model utilizes the temperature signals at the appropriate position of the medium as the known information and uses the unscented Kalman filter(UKF) as an optimization tool to reconstruct the boundary emissivity, thermal conductivity, and boundary time-varying heat flux. It is found that when the emissivity, thermal conductivity, and boundary time-varying heat flux are reconstructed simultaneously, only the temperature information of two locations is required. The influence of the measurement noise, sampling interval, absorption coefficient,process noise covariance, measurement noise covariance on the accuracy and stability of the retrieval results is investigated in detail. All the reconstruction results indicate that the UKF technique is effective and robust for estimating the photothermal parameters and boundary condition of the radiation-conduction heat transfer problems.