Global Illumination using Photon Ray Splatting

We present a novel framework for efficiently computing the indirect illumination in diffuse and moderately glossy scenes using density estimation techniques. Many existing global illumination approaches either quickly compute an overly approximate solution or perform an orders of magnitude slower computation to obtain high-quality results for the indirect illumination. The proposed method improves photon density estimation and leads to significantly better visual quality in particular for complex geometry, while only slightly increasing the computation time. We perform direct splatting of photon rays, which allows us to use simpler search data structures. Since our density estimation is carried out in ray space rather than on surfaces, as in the commonly used photon mapping algorithm, the results are more robust against geometrically incurred sources of bias. This holds also in combination with final gathering where photon mapping often overestimates the illumination near concave geometric features. In addition, we show that our photon splatting technique can be extended to handle moderately glossy surfaces and can be combined with traditional irradiance caching for sparse sampling and filtering in image space.     

Progressive Virtual Beam Lights

A recent technique that forms virtual ray lights (VRLs) from path segments in media, reduces the artifacts common to VPL approaches in participating media, however, distracting singularities still remain. We present Virtual Beam Lights (VBLs), a progressive many‐lights algorithm for rendering complex indirect transport paths in, from, and to media. VBLs are efficient and can handle heterogeneous media, anisotropic scattering, and moderately glossy surfaces, while provably converging to ground truth. We inflate ray lights into beam lights with finite thicknesses to eliminate the remaining singularities. Furthermore, we devise several practical schemes for importance sampling the various transport contributions between camera rays, light rays, and surface points. VBLs produce artifact‐free images faster than VRLs, especially when glossy surfaces and/or anisotropic phase functions are present. Lastly, we employ a progressive thickness reduction scheme for VBLs in order to render results that converge to ground truth.     

基于中介面加快光线跟踪计算

提出一种新的光线跟踪方法,以提高光线找到相交面片的效率.它在场景中生成一些面积较大的规整中介面片,然后为中介面上的每个点建立一个场,以记录到达该点的不同方向的光线将相交的面片.由此,光线跟踪时,一条光线可方便地找到相交的中介面,并通过查找中介面上所记录的内容,就能得到它所相交的面片.与已有方法相比,新方法不仅能很好加速主光线与阴影光线的计算,而且能很好地加速反射、折射等二次光线的计算.它能在GPU上方便地实现,并能有效地处理动态场景.     

Interactive Glossy Reflections using GPU‐based Ray Tracing with Adaptive LOD

We present an interactive GPU‐based algorithm for accurately rendering high‐quality, dynamic glossy reflection effects from both HDR environment maps and local scene objects. Our method uses hardware rasterization to produce primary pixels, and GPU‐based BRDF importance sampling [ [CK07] ] to quickly generate reflected rays. We utilize a fast GPU ray tracer proposed by Carr et al. [ [CHCH06] ] to compute reflection hits. Our main contribution is an adaptive level‐of‐detail (LOD) control algorithm that greatly improves ray tracing performance during reflection shading. Specifically, we use the solid angle represented by each reflected ray to adaptively pick the level of termination in the BVH traversal step during ray tracing. This leads to 2 ～ 3x speedup over an unmodified implementation of [ [CHCH06] ]. Based on the same solid angle measure, we derive a texture filtering formula to reduce reflection aliasing artifacts, taking advantage of hardware MIP mapping. This extends the filtering algorithm presented in [ [CK07] ] from environment mapping to local scene reflection. Using our algorithm, we demonstrate interactive rendering rates for several scenes featuring dynamic lighting and material changes, spatially varying BRDF parameters, and rigid‐body object movement.     

Quantizing Intersections Using Compact Voxels

Efficient intersection queries are important for ray tracing. However, building and maintaining the acceleration structures is demanding, especially for fully dynamic scenes. In this paper, we propose a quantized intersection framework based on compact voxels to quantize the intersection as an approximation. With high‐resolution voxels, the scene geometry can be well represented, which enables more accurate simulation of global illumination, such as detailed glossy reflections. In terms of memory usage in our graphics processing unit implementation, voxels are binarized and compactly encoded in a few 2D textures. We evaluate the rendering quality at various voxel resolutions. Empirically, high‐fidelity rendering can be achieved at the voxel resolution of 1 K³ or above, which produces images very similar to those of ray tracing. Moreover, we demonstrate the feasibility of our framework for various illumination effects with several applications, including first‐bounce indirect illumination, glossy refraction, path tracing, direct illumination, and ambient occlusion.     

iCheat: A Representation for Artistic Control of Indirect Cinematic Lighting

Thanks to an increase in rendering efficiency, indirect illumination has recently begun to be integrated in cinematic lighting design, an application where physical accuracy is less important than careful control of scene appearance. This paper presents a comprehensive, efficient, and intuitive representation for artistic control of indirect illumination. We encode user's adjustments to indirect lighting as scale and offset coefficients of the transfer operator. We take advantage of the nature of indirect illumination and of the edits themselves to efficiently sample and compress them. A major benefit of this sampled representation, compared to encoding adjustments as procedural shaders, is the renderer‐independence. This allowed us to easily implement several tools to produce our final images: an interactive relighting engine to view adjustments, a painting interface to define them, and a final renderer to render high quality results. We demonstrate edits to scenes with diffuse and glossy surfaces and animation.     

一种面向视景仿真的快速环境贴图方法

环境贴图是绘制物体表面漫反射和镜面反射效果的一种有效方法.为了把环境贴图应用于视景仿真中,实现场景对象的真实感绘制,首先从分析球面调和函数入手,提出了漫反射环境纹理图的快速计算方法;然后在研究镜面反射模型时,提出采用箱式滤波器代替Phong余弦函数滤波的方法,从而简化了镜面反射环境纹理图的滤波计算;最后在实现阶段,采用立方体环境纹理图表示场景光照环境,并对纹理图进行分级细化从而提高了绘制效率.实验表明,该方法在增强对象真实感的同时,其运算速度也能满足交互系统的需求,非常适合视景仿真应用.     

Ray tracing-based interactive diffuse indirect illumination

Despite great efforts in recent years to accelerate global illumination computation, the real-time ray tracing of fully dynamic scenes to support photorealistic indirect illumination effects has yet to be achieved in computer graphics. In this paper, we propose an extended ray tracing model that can be readily implemented on a GPU to facilitate the interactive generation of diffuse indirect illumination, the quality of which is comparable to that generated by the traditional, time-consuming photon mapping method and final gathering. Our method employs three types of (multilevel) grids to represent the indirect light in a scene using a form that facilitates the efficient estimation of the reflected radiance caused by diffuse interreflection. This method includes the mathematical tool of spherical harmonics and a rendering scheme that performs the final gathering step with a minimal cost during ray tracing, which guarantees the interactive frame rates. We evaluated our technique using several dynamic scenes with nontrivial complexity, which demonstrated its effectiveness.     

Adaptive Sampling of Area Light Sources in Ray Tracing Including Diffuse Interreflection

Ray tracing algorithms that sample both the light received directly from light sources and the light received indirectly by diffuse reflection from other patches, can accurately render the global illumination in a scene and can display complex scenes with accurate shadowing. A drawback of these algorithms, however, is the high cost for sampling the direct light which is done by shadow ray testing. Although several strategies are available to reduce the number of shadow rays, still a large number of rays will be needed, in particular to sample large area light sources. An adaptive sampling strategy is proposed that reduces the number of shadow rays by using statistical information from the sampling process and by applying information from a radiosity preprocessing. A further reduction in shadow rays is obtained by applying shadow pattern coherence, i.e. reusing the adaptive sampling pattern for neighboring sampling points.     

环境贴图在VR系统中的应用研究

为了把环境贴图应用于VR系统中,实现场景对象的真实感绘制,首先从分析球面调和函数入手,提出了漫反射环境纹理图的快速计算方法。在研究镜面反射模型时,提出采用箱式滤波器代替Phong余弦函数滤波的方法,从而简化了镜面反射环境纹理图的滤波计算。在实现阶段,采用立方体环境纹理图表示场景光照环境,并对纹理图进行分级细化从而提高了绘制效率。实验表明,该方法实现了物体漫反射和镜面反射的快速绘制,非常适合虚拟现实应用。     

Ray Differentials and Multiresolution Geometry Caching for Distribution Ray Tracing in Complex Scenes

When rendering only directly visible objects, ray tracing a few levels of specular reflection from large, low‐curvaturesurfaces, and ray tracing shadows from point‐like light sources, the accessed geometry is coherentand a geometry cache performs well. But in many other cases, the accessed geometry is incoherent and a standardgeometry cache performs poorly: ray tracing of specular reflection from highly curved surfaces, tracing rays thatare many reflection levels deep, and distribution ray tracing for wide glossy reflection, global illumination, widesoft shadows, and ambient occlusion. Fortunately, less geometric accuracy is necessary in the incoherent cases.This observation can be formalized by looking at the ray differentials for different types of scattering: coherentrays have small differentials, while incoherent rays have large differentials. We utilize this observation to obtainefficient multiresolution caching of geometry and textures (including displacement maps) for classic and distributionray tracing in complex scenes. We use an existing multiresolution caching scheme (originally developed forscanline rendering) for textures and displacement maps, and introduce a multiresolution geometry caching schemefor tessellated surfaces. The multiresolution geometry caching scheme makes it possible to efficiently render scenesthat, if fully tessellated, would use 100 times more memory than the geometry cache size.     

Pre‐computed Gathering of Multi‐Bounce Glossy Reflections

Recent work in interactive global illumination addresses diffuse and moderately glossy indirect lighting effects, but high‐frequency effects such as multi‐bounce reflections on highly glossy surfaces are often ignored. Accurately simulating such effects is important to convey the realistic appearance of materials such as chrome and shiny metal. In this paper, we present an efficient method for visualizing multi‐bounce glossy reflections at interactive rates under environment lighting. Our main contribution is a pre‐computation–based method which efficiently gathers subsequent highly glossy reflection passes modelled with a non‐linear transfer function representation based on the von Mises–Fisher distribution. We show that our gathering method is superior to scattered sampling. To exploit the sparsity of the pre‐computed data, we apply perfect spatial hashing. As a result, we are able to visualize multi‐bounce glossy reflections at interactive rates at a low pre‐computation cost.     

Constrained Convex Space Partition for Ray Tracing in Architectural Environments

This paper explores constrained convex space partition (CCSP) as a new acceleration structure for ray tracing. A CCSP is a graph, representing a space partition made up of empty convex volumes. The scene geometry is located on the boundary of the convex volumes. Therefore, each empty volume is bounded with two kinds of faces: occlusive ones (belonging to the scene geometry), and non‐occlusive ones. Given a ray, ray casting is performed by traversing the CCSP one volume at a time, until it hits the scene geometry. In this paper, this idea is applied to architectural scenes. We show that CCSP allows to cast several hundreds of millions of rays per second, even if they are not spatially coherent. Experiments are performed for large furnished buildings made up of hundreds of millions of polygons and containing thousands of light sources.     

Efficient,complete radiosity ray tracing using a shadow-coherence method

Most two-pass rendering methods calculate a radiosity shading for each patch or element in a scene in the first pass. This shading contains two components: one for the light received directly from the main light sources and one representing the intensity of the light received indirectly by means of diffuse and specular interreflection between patches. However, it is very difficult to achieve accurate representation of the distribution of this radiosity shading over the patch, particularly where clearly visible shadow boundaries exist. A better approach is to store only the indirect reflection component in the form of radiosity shading, and to calculate the direct reflection component during the second pass by casting shadow rays. This approach normally requires that many shadow rays must be cast. However, the number of rays for shadow testing can be kept low by selecting only those light sources that substantially contribute to the shading of a patch and applying an adaptive image refinement technique in combination with a shadow coherence method.     

Realistic computer graphics and free form surfaces

An image generation approach in computer graphics leading to images of considerable optical quality is ray tracing, deduced from geometric optics. Crucial for the efficiency of ray tracing is to find quickly an intersection point closest to a ray's origin. This requires to restrict the candidate patches of a given scene as well as to find the intersections of a ray with a patch. We survey approaches to solve these two problems, and present a new method, based on space sweep and patch subdivision. Its advantages are subdivision adapted to the distribution of rays and consideration of ray coherence. This implies that useless subdivisions are avoided.     

GPU Ray Tracing using Irregular Grids

We present a spatial index structure to accelerate ray tracing on GPUs. It is a flat, non‐hierarchical spatial subdivision of the scene into axis aligned cells of varying size. In order to construct it, we first nest an octree into each cell of a uniform grid. We then apply two optimization passes to increase ray traversal performance: First, we reduce the expected cost for ray traversal by merging cells together. This adapts the structure to complex primitive distributions, solving the “teapot in a stadium” problem. Second, we decouple the cell boundaries used during traversal for rays entering and exiting a given cell. This allows us to extend the exiting boundaries over adjacent cells that are either empty or do not contain additional primitives. Now, exiting rays can skip empty space and avoid repeating intersection tests. Finally, we demonstrate that in addition to the fast ray traversal performance, the structure can be rebuilt efficiently in parallel, allowing for ray tracing dynamic scenes.     

Real‐Time Indirect Illumination and Soft Shadows in Dynamic Scenes Using Spherical Lights

We present a method for rendering approximate soft shadows and diffuse indirect illumination in dynamic scenes. The proposed method approximates the original scene geometry with a set of tightly fitting spheres. In previous work, such spheres have been used to dynamically evaluate the visibility function to render soft shadows. In this paper, each sphere also acts as a low‐frequency secondary light source, thereby providing diffuse one‐bounce indirect illumination. The method is completely dynamic and proceeds in two passes: In a first pass, the light intensity distribution on each sphere is updated based on sample points on the corresponding object surface and converted into the spherical harmonics basis. In a second pass, this radiance information and the visibility are accumulated to shade final image pixels. The sphere approximation allows us to compute visibility and diffuse reflections of an object at interactive frame rates of over 20 fps for moderately complex scenes.     

Non‐Linear Beam Tracing on a GPU

Beam tracing combines the flexibility of ray tracing and the speed of polygon rasterization. However, beam tracing so far only handles linear transformations; thus, it is only applicable to linear effects such as planar mirror reflections but not to non‐linear effects such as curved mirror reflection, refraction, caustics and shadows. In this paper, we introduce non‐linear beam tracing to render these non‐linear effects. Non‐linear beam tracing is highly challenging because commodity graphics hardware supports only linear vertex transformation and triangle rasterization. We overcome this difficulty by designing a non‐linear graphics pipeline and implementing it on top of a commodity GPU. This allows beams to be non‐linear where rays within the same beam do not have to be parallel or intersect at a single point. Using these non‐linear beams, real‐time GPU applications can render secondary rays via polygon streaming similar to how they render primary rays. A major strength of this methodology is that it naturally supports fully dynamic scenes without the need to pre‐store a scene database. Utilizing our approach, non‐linear ray tracing effects can be rendered in real‐time on a commodity GPU under a unified framework.     

Ray geometry in non-pinhole cameras: a survey

A pinhole camera collects rays passing through a common 3D point and its image resembles what would be seen by human eyes. In contrast, a non-pinhole (multi-perspective) camera combines rays collected by different viewpoints. Despite their incongruity of view, their images are able to preserve spatial coherence and can depict, within a single context, details of a scene that are simultaneously inaccessible from a single view, yet easily interpretable by a viewer. In this paper, we thoroughly discuss the design, modeling, and implementation of a broad class of non-pinhole cameras and their applications in computer graphics and vision. These include mathematical (conceptual) camera models such as the General Linear Cameras and real non-pinhole cameras such as catadioptric cameras and projectors. A unique component of this paper is a ray geometry analysis that uniformly models these non-pinhole cameras as manifolds of rays and ray constraints. We also model the thin lens as a ray transform and study how ray geometry is changed by the thin lens for studying distortions and defocusing. We hope to provide mathematical fundamentals to satisfy computer vision researchers as well as tools and algorithms to aid computer graphics and optical engineering researchers.     

Ray Accelerator: Efficient and Flexible Ray Tracing on a Heterogeneous Architecture

We present a hybrid ray tracing system, where the work is divided between the CPU cores and the GPU in an integrated chip, and communication occurs via shared memory. Rays are organized in large packets that can be distributed among the two units as needed. Testing visibility between rays and the scene is mostly performed using an optimized kernel on the GPU, but the CPU can help as necessary. The CPU cores typically handle most or all shading, which makes it easy to support complex appearances. For efficiency, the CPU cores shade whole batches of rays by sorting them on material and shading each material using a vectorized kernel. In addition, we introduce a method to support light paths with arbitrary recursion, such as multiple recursive Whitted‐style ray tracing and adaptive sampling where the result of a ray is examined before sending the next, while still batching up rays for the benefit of GPU‐accelerated traversal and vectorized shading. This allows our system to achieve high rendering performance while maintaining the flexibility to accommodate different rendering algorithms.