反射剖面精确拟合的次表面散射计算方法

近年来,随着电影、游戏、虚拟现实应用等对真实感要求的不断提高,针对人体组织、牛奶等半透明材质的实时渲染变得越发重要.针对当前大部分次表面散射计算方法难以正确估计散射范围的问题,提出了一种全新的次表面散射计算方法用以精确表示最大散射距离.首先,针对暴力蒙特卡洛光子追踪结果进行模拟,以得到反射剖面结果.其次通过多项式模型进行反射剖面拟合,计算精确着色点处的最大散射范围.最后,提出了一种新的重要性采样方案以减少蒙特卡洛所需的采样数,进一步提高计算效率.此外,方法所需的参数仅由着色点上的反射率以及材质平均自由程提供,以便于灵活调整渲染效果.实验证明,所提模型避免了之前对于散射范围的错误估计,对材质反射率复杂的区域具有更好的渲染精度,且渲染速率满足实时要求.     

一种动态物体次表面散射实时绘制方法

在给出非均匀材质建模描述方法的基础上,通过对光线类型和光的次表面散射物理过程进行分解,分别给出了图像空间非均匀材质单次散射和多次散射的近似计算方法。延迟着色思想的融入,满足了动态物体实时绘制的需要,并可较好地适用于分层非均匀半透明材质。     

Separable Subsurface Scattering

In this paper, we propose two real‐time models for simulating subsurface scattering for a large variety of translucent materials, which need under 0.5 ms per frame to execute. This makes them a practical option for real‐time production scenarios. Current state‐of‐the‐art, real‐time approaches simulate subsurface light transport by approximating the radially symmetric non‐separable diffusion kernel with a sum of separable Gaussians, which requires multiple (up to 12) 1D convolutions. In this work we relax the requirement of radial symmetry to approximate a 2D diffuse reflectance profile by a single separable kernel. We first show that low‐rank approximations based on matrix factorization outperform previous approaches, but they still need several passes to get good results. To solve this, we present two different separable models: the first one yields a high‐quality diffusion simulation, while the second one offers an attractive trade‐off between physical accuracy and artistic control. Both allow rendering of subsurface scattering using only two 1D convolutions, reducing both execution time and memory consumption, while delivering results comparable to techniques with higher cost. Using our importance‐sampling and jittering strategies, only seven samples per pixel are required. Our methods can be implemented as simple post‐processing steps without intrusive changes to existing rendering pipelines.     

空间动态可变材质的交互式全局光照明绘制

提出了一种空间动态可变材质的交互式全局光照明绘制算法.如果在绘制过程中允许用户对物体的材质作修改,并且对一个物体的不同部分的材质作不同的修改,则称为空间动态可变材质.由于最终出射的辐射亮度和材质呈非线性关系,因此现有许多交互式全局光照明算法不允许用户修改物体的材质.如果一个物体各部分的材质可以不相同,那么材质对最终的出射的辐射亮度的影响更为复杂,目前没有任何交互式全局光照明绘制算法能够在绘制过程中对一个物体不同部分的材质作不同的修改.将一个空间动态可变材质区域划分成许多子区域来近似模拟,每个子区域内部材质处处相同.光在场景传播过程中可能先后被不同的子区域反射,并以此将最终出射的辐射亮度分为许多部分.用一组基材质来线性表示所有的材质,这组基材质被赋予场景中的所有子区域,从而得到不同的基材质的分布.预计算所有这些基材质分布下的各部分最终出射的辐射亮度.绘制时根据各子区域材质在基材质上的系数组合相应的预计算数据,就能交互式绘制全局光照明效果.     

基于梯度图的微结构表面全局光照实时绘制

针对带有微结构表面的几何模型全局光照计算复杂、难以达到实时性要求的问题,提出一种基于高度梯度图分析的全局光照实时绘制方法.首先,定义微结构高度梯度图,并据此构建可见点的局部最高点集合.其次,给出面向微结构表面对象实时绘制的全局光照计算模型,将光照计算近似分解为环境光入射、光源直接光照和一次交互漫反射这3种分量的计算.在环境光计算过程中,提出一种自适应环境光遮挡计算,借助局部最高点集合计算遮挡角.在直接光照中,给出一种微结构阴影的修正方法,搜索入射光方向的最近局部最高点剖面;通过比较剖面内光线投影与局部最高点的遮挡角,近似确定由微结构造成的阴影区域.最后,根据可见点的局部最高点集合确定一次交互漫反射的采样范围,进行渗色处理.整个全局光照计算方法在图像空间完成,较好地利用了延迟着色的思想和GPU并行计算的特点.算法可以在使用低精度几何模型时表现出带有微结构表面的高精度模型的全局光照效果,且适用于动态场景和可变形物体的全局光照计算.     

Spherical Gaussian-based Lightcuts for Glossy Interreflections

It is still challenging to render directional but non-specular reflections in complex scenes. The SG-based (Spherical Gaussian) many-light framework provides a scalable solution but still requires a large number of glossy virtual lights to avoid spikes as well as reduce clamping errors. Directly gathering contributions from these glossy virtual lights to each pixel in a pairwise way is very inefficient. In this paper, we propose an adaptive algorithm with tighter error bounds to efficiently compute glossy interreflections from glossy virtual lights. This approach is an extension of the Lightcuts that builds hierarchies on both lights and pixels with new error bounds and new GPU-based traversal methods between light and pixel hierarchies. Results demonstrate that our method is able to faithfully and efficiently compute glossy interreflections in scenes with highly glossy and spatial varying reflectance. Compared with the conventional Lightcuts method, our approach generates lightcuts with only one-fourth to one-fifth light nodes therefore exhibits better scalability. Additionally, after being implemented on GPU, our algorithms achieve a magnitude of faster performance than the previous method.     

多重散射的天空光照效果建模与实时绘制

依据大气散射的物理原理,提出了一种考虑多重散射的天空光照效果建 模与实时绘制方法。该方法首先以太阳和天空光作为光源建立了多重散射的天空光照效果模 型,然后综合多种大气粒子密度, 采用合理的分段采样策略,对天空颜色模型的积分进行简 化,以减少积分近似计算所带来的误差;通过对简化后的模型进行分析提出了采用二维纹理 与三维纹理对光学深度预计算的方法,避免了运行时计算光学深度积分的问题;最后该算法 在GPU 的片段处理器上执行,实现了天空光照效果模型的实时绘制,可以满足各种实时应 用需求。     

Image-Space Subsurface Scattering for Interactive Rendering of Deformable Translucent Objects

The authors present an algorithm for real-time realistic rendering of translucent materials such as marble, wax, and milk. Their method captures subsurface scattering effects while maintaining interactive frame rates. The main idea of this work is that it employs the dipole diffusion model with a splatting approach to evaluate the integral over the surface area for computing illumination due to multiple scattering.     

A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering

Interactive volume rendering in its standard formulation has become an increasingly important tool in many application domains. In recent years several advanced volumetric illumination techniques to be used in interactive scenarios have been proposed. These techniques claim to have perceptual benefits as well as being capable of producing more realistic volume rendered images. Naturally, they cover a wide spectrum of illumination effects, including varying shading and scattering effects. In this survey, we review and classify the existing techniques for advanced volumetric illumination. The classification will be conducted based on their technical realization, their performance behaviour as well as their perceptual capabilities. Based on the limitations revealed in this review, we will define future challenges in the area of interactive advanced volumetric illumination.     

基于概率模型的树木间接光照效果的实时模拟

太阳光照射到树冠上之后,会在叶片间进行相互的反射、折射,形成复杂的间接光照效果。在进行树木真实感绘制时,对这种间接光照的快速模拟是非常困难的。传统的光线跟踪、辐射度等算法均比较耗时,很难满足实时计算的要求。采用一种概率模型来快速估算树木叶片间的间接光照效果,实现了带间接光照效果的树木实时绘制。为树冠构建一个包围体,并假定该包围体中叶子的位置和法向基本符合均匀分布;为叶子的正反面分别定义反射系数和透射系数,并定义一个衰减函数来描述光线穿越叶子层后的衰减量;基于这种均匀概率分布的包围体模型,并利用叶片材质属性,就可以为每片叶子计算出叶子表面的出射光。对于中心在[P]处的叶子来说,可以快速计算出太阳光经过其邻近叶子到达该叶子表面的间接光照效果。为了进一步提高绘制效率,并未采用传统的包含大量面片的树木模型,而是采用了一种利用Billboards结构进行树木枝叶表达的简洁的树木三维模型。将上述概率模型与这种表达方式进行了有机结合,完成了带间接光照效果的树木实时真实感绘制。     

Efficient Rendering of Local Subsurface Scattering

A novel approach is presented to efficiently render local subsurface scattering effects. We introduce an importance sampling scheme for a practical subsurface scattering model. It leads to a simple and efficient rendering algorithm, which operates in image space, and which is even amenable for implementation on graphics hardware. We demonstrate the applicability of our technique to the problem of skin rendering, for which the subsurface transport of light typically remains local. Our implementation shows that plausible images can be rendered interactively using hardware acceleration.     

Specular Lobe‐Aware Filtering and Upsampling for Interactive Indirect Illumination

Although geometry‐aware filtering and upsampling have often been used for interactive or real‐time rendering, they are unsuitable for glossy surfaces because shading results strongly depend on the bidirectional reflectance distribution functions. This paper proposes a novel weighting function of cross bilateral filtering and upsampling to measure the similarity of specular lobes. The difficulty is that a specular lobe is represented with a distribution function in directional space, whereas conventional cross bilateral filtering evaluates similarities using the distance between two points in a Euclidean space. Therefore, this paper first generalizes cross bilateral filtering for the similarity of distribution functions in a non‐Euclidean space. Then, the weighting function is specialized for specular lobes. Our key insight is that the weighting function of bilateral filtering can be represented with the product integral of two distribution functions corresponding to two pixels. In addition, we propose spherical Gaussian‐based approximations to calculate this weighting function analytically. Our weighting function detects the edges of glossiness, and adapts to all‐frequency materials using only a camera position and G‐buffer. These features are not only suitable for path tracing, but also deferred shading and non‐ray tracing–based methods such as voxel cone tracing.     

Flux‐Limited Diffusion for Multiple Scattering in Participating Media

For the rendering of multiple scattering effects in participating media, methods based on the diffusion approximation are an extremely efficient alternative to Monte Carlo path tracing. However, in sufficiently transparent regions, classical diffusion approximation suffers from non‐physical radiative fluxes which leads to a poor match to correct light transport. In particular, this prevents the application of classical diffusion approximation to heterogeneous media, where opaque material is embedded within transparent regions. To address this limitation, we introduce flux‐limited diffusion, a technique from the astrophysics domain. This method provides a better approximation to light transport than classical diffusion approximation, particularly when applied to heterogeneous media, and hence broadens the applicability of diffusion‐based techniques. We provide an algorithm for flux‐limited diffusion, which is validated using the transport theory for a point light source in an infinite homogeneous medium. We further demonstrate that our implementation of flux‐limited diffusion produces more accurate renderings of multiple scattering in various heterogeneous datasets than classical diffusion approximation, by comparing both methods to ground truth renderings obtained via volumetric path tracing.     

Efficient Glossy Global Illumination with Interactive Viewing

The ability to perform interactive walkthroughs of global illumination solutions including glossy effects is a challenging open problem. In this paper we overcome certain limitations of previous approaches. We first introduce a novel, memory- and compute-efficient representation of incoming illumination, in the context of a hierarchical radiance clustering algorithm. We then represent outgoing radiance with an adaptive hierarchical basis, in a manner suitable for interactive display. Using appropriate refinement and display strategies, we achieve walkthroughs of glossy solutions at interactive rates for non-trivial scenes. In addition, our implementation has been developed to be portable and easily adaptable as an extension to existing, diffuse-only, hierarchical radiosity systems. We present results of the implementation of glossy global illumination in two independent global illumination systems.     

MesoGAN: Generative Neural Reflectance Shells

We introduce MesoGAN, a model for generative 3D neural textures. This new graphics primitive represents mesoscale appearance by combining the strengths of generative adversarial networks (StyleGAN) and volumetric neural field rendering. The primitive can be applied to surfaces as a neural reflectance shell; a thin volumetric layer above the surface with appearance parameters defined by a neural network. To construct the neural shell, we first generate a 2D feature texture using StyleGAN with carefully randomized Fourier features to support arbitrarily sized textures without repeating artefacts. We augment the 2D feature texture with a learned height feature, which aids the neural field renderer in producing volumetric parameters from the 2D texture. To facilitate filtering, and to enable end-to-end training within memory constraints of current hardware, we utilize a hierarchical texturing approach and train our model on multi-scale synthetic datasets of 3D mesoscale structures. We propose one possible approach for conditioning MesoGAN on artistic parameters (e.g. fibre length, density of strands, lighting direction) and demonstrate and discuss integration into physically based renderers.     

Interactive rendering of globally illuminated scenes including anisotropic and inhomogeneous participating media

Although new graphics hardware has accelerated the rendering process, the realistic simulation of scenes including participating media remains a difficult problem. Interactive results have been achieved for isotropic media as well as for single scattering. In this paper, we present an interactive global illumination algorithm for the simulation of scenes that include participating media, even anisotropic and/or inhomogeneous media. The position of the observer is important in order to render inhomogeneous media according to the transport equation. Previous work normally needed to be ray-based in order to compute this equation properly. Our approach is capable of achieving real time using two 3D textures on a simple desktop PC. For anisotropic participating media we combine density estimation techniques and graphics hardware capabilities.     

可变材质的实时全局光照明绘制

现有的基于预计算的全局光照明绘制算法都假设场景中物体的材质固定不变,这样,从入射光照到出射的辐射亮度之间的传输变换就是线性变换.通过对这种线性变换的预计算,可以在动态光源下实现全局光照明的实时绘制.但是,当材质可以改变时,这种线性变换不再成立,因此,现有算法无法直接用于动态材质的场景.提出了一种方法:在修改场景中的物体材质时,可以实时得到场景在直接光照和间接光照下的绘制效果.将最终到达视点的辐射亮度根据其之前经过的反射次数及相应的反射材质分为多个部分,每个部分和先后反射的材质的乘积成正比,从而把该非线性问题转化为线性问题.又将所有可选的材质都表示为一组基的线性组合.将这组基作为材质赋予场景中的物体,就有各种不同的组合方式,预计算每种组合下所有部分的出射辐射亮度.在绘制时,根据各物体材质投影到基上的系数线性组合预计算的数据就能实时得到最终的全局光照明的绘制结果.该方法适用于几何场景、光照和视点都不发生变化的场景.使用双向反射分布函数来表示物体的材质,不考虑折射或者半透明的情况.该实现最多包含两次反射,并可以实时绘制得到一些很有趣的全局光照明效果,比如渗色、焦散等等.     

双层介质光线散射效果的实时绘制

实时绘制光线散射效果是计算机图形学中的一个难点.利用光线散射的特性,提出一种多项式近似的方法;通过解析双层介质光线散射公式,能实现实时地绘制双层介质中不同光源分布、不同介质特性下的光线散射效果.     

海量模型实时交互可视化技术综述

综述了海量模型实时交互可视化的最新研究进展,详细讨论了其中涉及的主要支撑技术,并对目前国内外典型的交互绘制系统进行比较和分析;最后,对今后的发展方向进行了展望。     

Spatio‐Temporal Filtering of Indirect Lighting for Interactive Global Illumination

We introduce a screen‐space statistical filtering method for real‐time rendering with global illumination. It is inspired by statistical filtering proposed by Meyer et al. to reduce the noise in global illumination over a period of time by estimating the principal components from all rendered frames. Our work extends their method to achieve nearly real‐time performance on modern GPUs. More specifically, our method employs the candid covariance‐free incremental PCA to overcome several limitations of the original algorithm by Meyer et al., such as its high computational cost and memory usage that hinders its implementation on GPUs. By combining the reprojection and per‐pixel weighting techniques, our method handles the view changes and object movement in dynamic scenes as well.