基于物理的次表面散射绘制方法

发布时间：2018-09-18 11:31

【摘要】：随着图形学技术的发展以及计算机运算能力的提升,真实感渲染技术在影视动漫、视频游戏、模拟仿真等领域的应用越来越广泛。具有高度真实感的材质在整个渲染过程中具有重要作用,目前像PRman、Arnold等主流的电影级商业渲染器都具有自己的材质系统,艺术家通过组合各种材质并调节它们的参数,就可以使用这些渲染器渲染出照片级真实感的图片,最终这些令人震撼的画面制作成电影呈现给了观众。商业渲染器提供的材质可以分为两类,其中可用于模拟玻璃、塑料、不锈钢等硬质表面的基本材质因为实现难度低且发展较早已经日趋成熟,而像皮肤这种特殊的具有次表面散射特点的材质因为实现难度高且发展起步晚一直是衡量一个真实感渲染器优劣的重要因素。另一方面,因为所有非传导的有机体都具有次表面散射特性,所以这种材质在影视和游戏中的应用非常广泛。使用纯物理模拟的次表面散射材质因为收敛速度太慢而无法实际应用,然后近似模拟的次表面散射材质便应运而生。但这些近似方法各有优缺点,有的渲染速度快但是流程复杂效果一般,有的流程简单效果一流但是渲染速度较慢,而目前关于实时次表面散射的研究很少,徐昆提出的方法只能适用于物体的光照环境不变的情形。本文深入研究了两种不同的次表面散射算法,并以renderman规范渲染器为平台,就如何在该类渲染器下实现次表面散射材质进行了深入研究,最终给出一套完整实现,并对实验结果进行了对比和分析。然后,结合前面方法,提出了一种基于GPU的实时次表面散射算法,该方法分为两步,第一步利用BSSRDF的重要性采样方法得到每个着色点所依赖的采样点纹理坐标,并将它们存入贴图中,第二步计算每个着色点时,根据上一步得到的纹理坐标将物体表面的直接光照进行融合即得到着色点颜色。本文的主要工作有：1,研究基于点云的快速分层次表面散射,对在REYES架构渲染器中实现该方法时的点云分布进行优化,结合多层次表面散射算法,提出了一种带有表面粗糙度的快速分层次表面散射算法,可以模拟光滑的玉石。2,提出了一种在renderman规范渲染器中实现基于BSSRDF重要性采样次表面散射算法的方法,提出了一种简单高效的BVH树构建方法用于光线与场景求交,并对之前渲染器中射线与四边形求交算法进行改进。3,提出了一种通过将BSSRDF的重要性采样点预计算并存入贴图的方式实现基于GPU的实时次表面散射的算法,该方法首次允许物体的光照环境可以实时改变。
[Abstract]:With the development of graphics technology and the improvement of computer computing ability, realistic rendering technology is more and more widely used in video animation, video games, simulation and other fields. Materials with a high degree of realism play an important role in the whole rendering process. At present, mainstream commercial renderings such as PRman,Arnold have their own material systems, and artists combine various materials and adjust their parameters. You can use these renderings to render photo-level realistic images, and eventually these stunning images are made into movies and presented to the audience. The commercial renderer offers two types of materials, one of which can be used to simulate hard surfaces such as glass, plastic, stainless steel and so on, because of the low difficulty of implementation and the early development of the basic materials, they have become more and more mature. The special material with subsurface scattering such as skin has been an important factor to evaluate the quality of a realistic renderer because of its high implementation difficulty and late development. On the other hand, because all non-conducting organisms have subsurface scattering properties, this material is widely used in film, television and games. Due to the slow convergence rate of the subsurface scattering material simulated by pure physics, it can not be applied in practice, and then the subsurface scattering material which is similar to the simulated subsurface scattering material emerges as the times require. However, these approximate methods have their own advantages and disadvantages. Some of them have fast rendering speed but generally complex process effects, while some simple process effects are first class but rendering speed is relatively slow. However, there are few researches on real-time subsurface scattering at present. The method proposed by Xu Kun can only be applied to the case where the illumination environment of an object is invariant. In this paper, two different subsurface scattering algorithms are studied in depth, and how to realize the subsurface scattering material under the renderman standard renderer is studied deeply, and a complete set of implementation is given. The experimental results are compared and analyzed. Then, a real-time subsurface scattering algorithm based on GPU is proposed, which is divided into two steps. In the first step, the texture coordinates of each coloring point are obtained by using the importance sampling method of BSSRDF. They are stored in the map. In the second step, when each coloring point is calculated, the coloring point color is obtained by merging the direct illumination of the object surface according to the texture coordinates obtained from the previous step. The main work of this paper is: 1. The fast hierarchical surface scattering based on point cloud is studied. The distribution of point cloud in REYES architecture renderer is optimized, and the multi-layer surface scattering algorithm is combined. In this paper, a fast hierarchical surface scattering algorithm with surface roughness is proposed, which can simulate smooth jade. A method is proposed to realize the subsurface scattering algorithm based on BSSRDF importance sampling in renderman renderer. In this paper, a simple and efficient BVH tree construction method is proposed for the intersection of light and scene. By improving the intersection algorithm of ray and quadrilateral in the previous renderer, a real-time subsurface scattering algorithm based on GPU is proposed by pre-calculating the important sampling points of BSSRDF and storing them into the map. For the first time, the method allows the illumination of objects to change in real time.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TP391.41

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