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BRDF和GPU并行计算的全局光照实时渲染

基于光线追踪,将屏幕图像像素分解为投射光线与场景对象交点面片辐射亮度和纹理贴图的合成,每个面片的辐射亮度计算基于双向反射分布函数(BRDF)基的线性组合,并通过图形处理器(GPU)处理核心并行绘制进行加速,最后与并行计算的纹理映射结果进行合成。提出了一种基于BRDF和GPU并行计算的全局光照实时渲染算法,利用GPU并行加速,在提高绘制效率的前提下,实现动态交互材质的全局光照实时渲染。重点研究:对象表面对光线的多次反射用BRDF基的线性组合来表示,将非线性问题转换为线性问题,从而提高绘制效率;利用GPU并行加速,分别计算对象表面光辐射能量和纹理映射及其线性组合,进一步提高计算效率满足实时绘制需求。     

利用形状因子采样的实时全局光照绘制

为了对物体表面材质进行实时编辑,提出一种动态光照和任意视点条件下的实时全局光照算法.该算法预计算各面片的形状因子,并存储其中较大的形状因子值和相应的面片号,这些面片是光照贡献最大的面片.一次间接光照利用这些面片计算,并用亮度补偿策略增加计算精度,而二次及多次间接光照则用近似公式估算,整个光照计算过程在GPU中完成.实验结果表明,文中算法在视点改变、光照改变和材质改变情况下,对静态场景能获得逼真的实时全局光照绘制效果.     

实时全局重光照算法

提出了一个实时全局重光照算法．算法利用重建的几何模型和沿不同光照方向的预采样图像恢复材料的反射属性，通过这些属性得到了面片在不同采样图像上的间接光照和环境光照．并用低价基函数将它们拟合．重光照绘制时，将恢复的材料属性应用于光照明模型计算出直接光照分量问接光照和环境光照分量由低阶基函数获得．物体表面细节则通过求解表面法向和材料属性的扰动量重建.实验结果表明，算法有效地重建了全局光照明效果和表面细节，生成的阴影边缘清晰，且绘制速度迭到实时．     

三维物体表面材质实时编辑

艺术家进行设计时,常常需要一种可以交瓦地修改模型表面材质的工具.为了在环境光下对模型表面的材质进行实时编辑,提出一种基于预计算辐射传输的算法:首先预计算环境光相对于模型表面每一个顶点的可见性;然后在绘制时实时计算物体表面的双向反射分布函数(BRDF);最后通过查找环境光相对于模型表面每一个顶点的可见性,快速绘制出物体表面材质.实验结果表明,使用该算法,用户可以通过调节BRDF的参数,实现物体表面材质的实时动态编辑,同时支持动态视点和动态环境光.     

基于光辐射预计算的场景实时绘制算法

光辐射传输预计算对于提高场景绘制效果很关键。对光辐射传输预计算进行了研究,提出了一种基于SH的光辐射传输预计算的算法,在低频环境光照条件下实时生成漫反射物体。将光源分布函数和光辐射传输函数分别用SH进行分解,则出射光强为两者生成的系数向量点积。绘制的场景中包含了阴影,相互反射等全局光照效果。实验结果表明算法逼近程度好,绘制质量高,具有一定的实用价值。     

辐射度场景中双向纹理函数表面的绘制

双向纹理函数(BTF)表面一般采用点采样数据来定义表面的光照属性,因而这类表面很难运用基于面片分割的辐射度方法进行绘制,提出一种将辐射度算法扩展到包括BTF表面场景的有效方法.对表面的BTF样本区域首先进行像素聚类,再在各个像素类内对视线采样方向做进一步自适应的聚类,在各个视线类内像素分别拟合一个低频光照甬数,并求它们在各个视线类内光照细节的高频光照函数.低频光照函数作为该表面区域的平均反射属性参与辐射度计算,生成场景的整体光照效果;然后利用计算的辐射度值和高频光照函数重建该表面区域的BTF材质细节.文中方法不仅取得了较高的压缩效率,而且在BTF材质表面产生了辉映等全局光照效果.最后利用硬件实现了视点快速改变时的场景绘制.     

基于Harr 小波的动态场景全频阴影绘制算法

针对现有的预计算辐射传递算法对三维场景限制严格、适合于低频光照环境等问题,提出了一种动态场景的全频阴影绘制算法.在预处理阶段使用球体对三维物体进行拟合,同时对光照函数和BRDF(bidirectional reflectance distribution function)函数进行Harr小波变换;在运行时阶段利用不同基函数的优势,在像素基空间进行多个球体可见性函数的快速合并,在小波基空间进行光照函数、BRDF函数和可见性函数的三乘积分,得到最终的光照值.使用CUDA(computed unified device architecture)实现了该算法,充分利用了图形硬件的最新功能.实验结果表明,阴影绘制质量有很大的提高,可以基本达到实时绘制.     

复杂环境光源下虚实融合光照实时计算

在互动电子游戏、增强现实等对实时计算要求很高的交互式图形应用中,大量使用复杂环境光源对虚拟物体进行照明,使其和真实场景的光照一致,虚实融合.提出了用Cook Torrance光照模型进行虚实场景的光照计算;利用球面调和基函数的方法,实时地计算高动态范围环境映射光照系数,得到高动态范围环境映射的二次多项式表达形式,在着色器计算该式得到漫反射分量;通过环境映射技术对镜面反射进行模拟,全部光照计算在GPU中完成.实验结果表明,该方法在动态变化的复杂环境光源下,完成对虚拟物体光照实时计算,绘制速度每秒30帧以上,绘制结果具有较强的真实感.     

采用重要性面片采样的实时全局光照

提出了一种实时全局光照的计算方法。该方法支持任意视点下动态光源的一次间接光照计算,并且物体表面材质可实时编辑,该算法预计算了各面片上的形状因子来解决遮挡问题,并记录形状因子较大的重要性面片作为间接光源。渲染时先从光源方向对场景记录了一个扩展的阴影图,包含了光源照射到的面片ID和其光通量,再根据采样好的间接光源来计算间接光照。使用CUDA,整个光照计算过程在GPU中完成,可以对静态场景进行实时渲染,并能达到逼真的渲染效果。     

Neural Free-Viewpoint Relighting for Glossy Indirect Illumination

Precomputed Radiance Transfer (PRT) remains an attractive solution for real-time rendering of complex light transport effects such as glossy global illumination. After precomputation, we can relight the scene with new environment maps while changing viewpoint in real-time. However, practical PRT methods are usually limited to low-frequency spherical harmonic lighting. All-frequency techniques using wavelets are promising but have so far had little practical impact. The curse of dimensionality and much higher data requirements have typically limited them to relighting with fixed view or only direct lighting with triple product integrals. In this paper, we demonstrate a hybrid neural-wavelet PRT solution to high-frequency indirect illumination, including glossy reflection, for relighting with changing view. Specifically, we seek to represent the light transport function in the Haar wavelet basis. For global illumination, we learn the wavelet transport using a small multi-layer perceptron (MLP) applied to a feature field as a function of spatial location and wavelet index, with reflected direction and material parameters being other MLP inputs. We optimize/learn the feature field (compactly represented by a tensor decomposition) and MLP parameters from multiple images of the scene under different lighting and viewing conditions. We demonstrate real-time (512 x 512 at 24 FPS, 800 x 600 at 13 FPS) precomputed rendering of challenging scenes involving view-dependent reflections and even caustics.     

Shell radiance texture functions

The appearance of an inhomogeneous translucent material depends substantially on its volumetric variations and their effects upon subsurface scattering. For efficient rendering that accounts for both surface mesostructures and volumetric variations of such materials, shell texture functions have precomputed irradiance within a volume with respect to incoming illumination, but even with this irradiance data a fair amount of runtime computation is still required. Rather than precompute volume irradiance, we introduce the shell radiance texture function (SRTF), which relates incoming illumination more directly to outgoing surface radiance by representing a set of subsurface transport components from which surface radiance can be calculated without ray marching or runtime evaluation of dipole diffusion. Using this precomputed SRTF information, inhomogeneous objects can be rendered in real time with distant local lighting or global lighting.     

A PCA Decomposition for Real‐time BRDF Editing and Relighting with Global Illumination

We propose a novel rendering method which supports interactive BRDF editing as well as relighting on a 3D scene. For interactive BRDF editing, we linearize an analytic BRDF model with basis BRDFs obtained from a principal component analysis. For each basis BRDF, the radiance transfer is precomputed and stored in vector form. In rendering time, illumination of a point is computed by multiplying the radiance transfer vectors of the basis BRDFs by the incoming radiance from gather samples and then linearly combining the results weighted by user‐controlled parameters. To improve the level of accuracy, a set of sub‐area samples associated with a gather sample refines the glossy reflection of the geometric details without increasing the precomputation time. We demonstrate this program with a number of examples to verify the real‐time performance of relighting and BRDF editing on 3D scenes with complex lighting and geometry.     

Spherical piecewise constant basis functions for all-frequency precomputed radiance transfer

This paper presents a novel basis function, called spherical piecewise constant basis function (SPCBF), for precomputed radiance transfer. SPCBFs have several desirable properties: rotatability, ability to represent all-frequency signals, and support for efficient multiple product. By smartly partitioning the illumination sphere into a set of subregions, and associating each subregion with an SPCBF valued 1 inside the region and 0 elsewhere, we precompute the light coefficients using the resulting SPCBFs. Efficient rotation of the light representation in SPCBFs is achieved by rotating the domain of SPCBFs. We run-time approximate the BRDF and visibility coefficients using the set of SPCBFs for light, possibly rotated, through fast lookup of summed-area-table (SAT) and visibility distance table (VDT), respectively. SPCBFs enable new effects such as object rotation in all-frequency rendering of dynamic scenes and on-the-fly BRDF editing under rotating environment lighting. With graphics hardware acceleration, our method achieves real-time frame rates.     

Interactive cloth rendering of microcylinder appearance model under environment lighting

This paper proposes an interactive rendering method of cloth fabrics under environment lighting. The outgoing radiance from cloth fabrics in the microcylinder model is calculated by integrating the product of the distant environment lighting, the visibility function, the weighting function that includes shadowing/masking effects of threads, and the light scattering function of threads. The radiance calculation at each shading point of the cloth fabrics is simplified to a linear combination of triple product integrals of two circular Gaussians and the visibility function, multiplied by precomputed spherical Gaussian convolutions of the weighting function. We propose an efficient calculation method of the triple product of two circular Gaussians and the visibility function by using the gradient of signed distance function to the visibility boundary where the binary visibility changes in the angular domain of the hemisphere. Our GPU implementation enables interactive rendering of static cloth fabrics with dynamic viewpoints and lighting. In addition, interactive editing of parameters for the scattering function (e.g. thread's albedo) that controls the visual appearances of cloth fabrics can be achieved.     

Anisotropic Radiance-Cache Splatting for Efficiently Computing High-Quality Global Illumination with Lightcuts

Computing global illumination in complex scenes is even with todays computational power a demanding task. In this work we propose a novel irradiance caching scheme that combines the advantages of two state-of-the-art algorithms for high-quality global illumination rendering: lightcuts , an adaptive and hierarchical instant-radiosity based algorithm and the widely used (ir)radiance caching algorithm for sparse sampling and interpolation of (ir)radiance in object space. Our adaptive radiance caching algorithm is based on anisotropic cache splatting, which adapts the cache footprints not only to the magnitude of the illumination gradient computed with light-cuts but also to its orientation allowing larger interpolation errors along the direction of coherent illumination while reducing the error along the illumination gradient. Since lightcuts computes the direct and indirect lighting seamlessly, we use a two-layer radiance cache, to store and control the interpolation of direct and indirect lighting individually with different error criteria. In multiple iterations our method detects cache interpolation errors above the visibility threshold of a pixel and reduces the anisotropic cache footprints accordingly. We achieve significantly better image quality while also speeding up the computation costs by one to two orders of magnitude with respect to the well-known photon mapping with (ir)radiance caching procedure.     

Wavelet Point‐Based Global Illumination

Point‐Based Global Illumination (PBGI) is a popular rendering method in special effects and motion picture productions. This algorithm provides a diffuse global illumination solution by caching radiance in a mesh‐less hierarchical data structure during a preprocess, while solving for visibility over this cache, at rendering time and for each receiver, using microbuffers, which are localized depth and color buffers inspired from real time rendering environments. As a result, noise free ambient occlusion, indirect soft shadows and color bleeding effects are computed efficiently for high resolution image output and in a temporally coherent fashion. We propose an evolution of this method to address the case of non‐diffuse inter‐reflections and refractions. While the original PBGI algorithm models radiance using spherical harmonics, we propose to use wavelets parameterized on the direction space to better localize the radiance representation in the presence of highly directional reflectance. We also propose a new importance‐driven adaptive microbuffer model to capture accurately incoming radiance at a point. Furthermore, we evaluate outgoing radiance using a fast wavelet radiance product and contain the induced larger memory footprint by encoding hierarchically the wavelets in the PBGI tree. As a result, our algorithm can handle non‐lambertian BSDF in the light transport simulation, reproducing caustics and multiple reflections/refractions bounces with a similar quality to bidirectional path tracing in a large number of cases and for only a fraction of its computation time. Our approach is simple to implement and easy to integrate into any existing PBGI framework, with an intuitive control on the approximation error. We evaluate it on a collection of example scenes.