Unifying Geometric and Volumetric Light Scattering for Accurate Rendering of Dense Geometry

真实感渲染系统可以计算各种材质的物理精确图像。然而，当今渲染方面的一些最重要的挑战，特别是渲染人类，涉及到密集的元素--头上的头发，衬衫上的线--这些元素在普通距离下很容易看到。由于几何体密集地堆积在一起，因此在光线到达眼睛之前，需要完全模拟光线如何从头发反射到头发，或从线反射到线。然而，这些材质很难使用：这些部件太多，无法进行详细的模拟，但它们又不够小，无法作为连续介质建模。这项研究正在探索一种方法，可以在不牺牲精度的情况下同时使用这两种方法。正确绘制这些密集聚集是计算机图形学中一个重要的开放问题，需要基本的理论和算法进步来解决。在这种复杂的模型上直接模拟多次散射的成本高得令人望而却步，因此在实践中，它是用简单的启发式方法来近似的。然而，对于浅色材质，如金发或白色棉花，在单个交互中吸收的光很少，因此这些材质的视觉外观会受到多次散射的显著影响。为了得到正确的结果，需要一个完整的散射模拟。本研究项目旨在通过引入一个连续散射介质的整体聚集近似来有效地模拟密集几何结构中的散射。这允许在可能的情况下应用体积灯光传输的方法，同时在必要时仍使用详细的几何体。构建这种近似需要对几何渲染和体积渲染之间的关系有一个全新的理解。实现近似将需要开发新的算法，而应用它将需要对体绘制的最新技术进行改进。

Realistic rendering systems can compute physically accurate images of a wide range of materials. However, some of today's most important challenges in rendering, particularly rendering human beings, involve densely packed elements -- the hairs on a head, the threads in a shirt -- that are readily visible at ordinary distances. Because the geometry is densely packed together, accuracy requires a full simulation of how lightreflects from hair to hair, or from thread to thread, before it gets to the eye. However, these materials are difficult to work with: the parts are too numerous for a detailed simulation to be practical, but they are not small enough to be modeled as a continuous medium. This research is exploring methods that can use both approaches at once without sacrificing accuracy.Correctly rendering these dense aggregations is an important open problem in computer graphics, requiring fundamental theoretical and algorithmic advances to be solved. Directly simulating multiple scattering on such intricate models is prohibitively costly, so in practice it is approximated by simple heuristics. However, for light-colored materials such as blond hair or white cotton, very little light is absorbed in a single interaction, so the visual appearance of these materials is significantly affected by multiple scattering. For correct results a complete scattering simulation is required.This research project aims to efficiently simulate scattering in dense geometry by introducing an approximation of the whole aggregation as a continuous scattering medium. This allows methods for volumetric light transport to be applied when possible while still using the detailed geometry when necessary. Constructing this approximation requires a fundamentally new understanding of the relationship between geometric and volumetric rendering. Implementing the approximation will require new algorithms to be developed, and applying it will require improvements to the state of the art in volumetric rendering.