非经典参与介质的渲染方法研究综述

Survey of Non-classical Participating Media Rendering

参与介质在自然界中广泛存在,也是包括影视特效、电子游戏、仿真系统等大多数图形应用中的主要场景元素之一,对其外观的模拟和再现,可以极大地提升场景的真实感和沉浸感.但是由于参与介质本身结构以及光线在参与介质中的传播过程都非常复杂,所以迄今为止,对参与介质渲染的研究都一直是图形领域的热点和难点.为了处理的方便和计算的高效,传统的参与介质渲染方法都基于两点假设:独立散射假设和局部连续假设.这两点假设也是经典的辐射传输方程成立的关键.但实际上,自然界中的很多参与介质都不满足这两点假设,因此导致现有的参与介质渲染方法生成的图片效果和真实世界的效果存在一定的差异.近年来,研究者们提出了各种非经典参与介质渲染方法,试图打破上述的两点假设,用更符合物理客观规律的方式来处理参与介质,从而进一步提升参与介质渲染的物理真实感.从相干介质渲染技术和离散介质渲染技术两方面展开对现有的面向非经典参与介质的渲染方法进行分析和讨论,重点阐述经典和非经典参与介质渲染方法的区别,以及现有非经典参与介质渲染方法的原理、优势和不足.最后,展望一些开放性问题并进行总结.本综述希望能启发相关领域的研究人员进一步攻克非经典参与介质渲染技术中的关键问题和技术难点,也为工业界改进现有渲染器以提高参与介质渲染的真实感提供参考.

Participating media are ubiquitous in nature and are also major elements in many rendering applications such as special effects,digital games,and simulation systems.Physically-based simulation and reproduction of their appearance can significantly boost the realism and immersion of 3D virtual scenes.However,both the underlying structures of participating media and the light propagation in them are very complex.Therefore,rendering with participating media is a difficult task and hot topic in computer graphics so far.In order to facilitate the treatment in rendering and lower the computational cost,classical methods for participating media rendering are always based on two assumptions:independent scattering and local continuity.These two assumptions are also the building blocks of classical radiative transfer equation(RTE).However,most participating media in nature do not satisfy these two assumptions.This results in the noticeable discrepancy between rendered images and real images.In recent years,these two assumptions have been relaxed by incorporating more physically accurate methods to model participating media,thus significantly improving the physical realism of participating media rendering.This survey analyzes and discusses existing non-classical participating media rendering techniques from two aspects:correlated media rendering and discrete media rendering.The differences between classical and non-classical participating media rendering are discussed.The principles,advantages,and limitations behind each method are also described.Finally,some future directions are provided around non-classical participating media rendering that are worth delving into.It is hoped that this survey can inspire researchers to study non-classical participating media rendering by addressing some critical issues.It is also hoped that this survey can be a guidance for engineers from industry to improve their renderers by considering non-classical participating media rendering.