Synthesis and rendering of bidirectional texture functions on arbitrary surfaces

The bidirectional texture function (BTF) is a 6D function that describes the appearance of a real-world surface as a function of lighting and viewing directions. The BTF can model the fine-scale shadows, occlusions, and specularities caused by surface mesostructures. In this paper, we present algorithms for efficient synthesis of BTFs on arbitrary surfaces and for hardware-accelerated rendering. For both synthesis and rendering, a main challenge is handling the large amount of data in a BTF sample. To addresses this challenge, we approximate the BTF sample by a small number of 4D point appearance functions (PAFs) multiplied by 2D geometry maps. The geometry maps and PAFs lead to efficient synthesis and fast rendering of BTFs on arbitrary surfaces. For synthesis, a surface BTF can be generated by applying a texton-based sysnthesis algorithm to a small set of 2D geometry maps while leaving the companion 4D PAFs untouched. As for rendering, a surface BTF synthesized using geometry maps is well-suited for leveraging the programmable vertex and pixel shaders on the graphics hardware. We present a real-time BTF rendering algorithm that runs at the speed of about 30 frames/second on a mid-level PC with an ATI Radeon 8500 graphics card. We demonstrate the effectiveness of our synthesis and rendering algorithms using both real and synthetic BTF samples.     

Table-based Alpha Compression

In this paper we investigate low-bitrate compression of scalar textures such as alpha maps, down to one or two bits per pixel. We present two new techniques for 4 × 4 blocks, based on the idea from ETC to use index tables. We demonstrate that although the visual quality of the alpha maps is greatly reduced at these low bit rates, the quality of the final rendered images appears to be sufficient for a wide range of applications, thus allowing bandwidth savings of up to 75%. The 2 bpp version improves PSNR with over 2 dB compared to BTC at the same bit rate. The 1 bpp version is, to the best of our knowledge, the first public 1 bpp texture compression algorithm, which makes comparison hard. However, compared to just DXT5-compressing a subsampled texture, our 1 bpp technique improves PSNR with over 2 dB. Finally, we show that some aspects of the presented algorithms are also useful for the more common bit rate of four bits per pixel, achieving PSNR scores around 1 dB better than DXT5, over a set of test images.     

A Survey of Haptic Rendering Techniques

Computer Graphics technologies have developed considerably over the past decades. Realistic virtual environments can be produced incorporating complex geometry for graphical objects and utilising hardware acceleration for per pixel effects. To enhance these environments, in terms of the immersive experience perceived by users, the human's sense of touch, or haptic system, can be exploited. To this end haptic feedback devices capable of exerting forces on the user are incorporated. The process of determining a reaction force for a given position of the haptic device is known as haptic rendering. For over a decade users have been able to interact with a virtual environment with a haptic device. This paper focuses on the haptic rendering algorithms which have been developed to compute forces as users manipulate the haptic device in the virtual environment.     

Light Space Cascaded Shadow Maps Algorithm for Real Time Rendering

Owing to its generality and efficiency.Cascaded Shadow Maps(CSMs) has an important role in real-time shadow rendering in large scale and complex virtual environments.However,CSMs suffers from redundant rendering problem—objects are rendered undesirably to different shadow map textures when view direction and light direction are not perpendicular.In this paper,we present a light space cascaded shadow maps algorithm.The algorithm splits a scene into non-intersecting layers in light space,and generates one shadow map for each layer through irregular frustum clipping and scene organization,ensuring that any shadow sample point never appears in multiple shadow maps.A succinct shadow determination method is given to choose the optimal shadow map when rendering scenes.We also combine the algorithm with stable cascaded shadow maps and soft shadow algorithm to avoid shadow flicking and produce soft shadows.The results show that the algorithm effectively improves the efficiency and shadow quality of CSMs by avoiding redundant rendering. and can produce high-quality shadow rendering in large scale dynamic environments with real-time performance.     

基于矢量量化的高效BTC图像编码算法

作为一种有损图像编码技术,块截短编码算法（BTC）的计算量较少,速度快,有较好的信道容错力,重建图像质量较高。然而,标准BTC算法的主要缺点是其压缩比特率比其他基于块图像编码的算法（如变换编码和矢量量化）高。为了降低比特率,提出了几种有效的BTC算法,还提出了一种简单的查表算法对每块的BTC量化数据编码,另外还引入了矢量量化技术以减少对位平面编码的比特数。为了减少由改进算法引入的额外失真,在每种提出的算法中,采用最优阈值而不用平均值作为量化阈值。     

Fast vector quantization for efficient rendering of compressed point-clouds

We present a point-cloud compression algorithm that allows fast parallel decompression on the GPU suitable for interactive applications. The algorithm is based on vector quantization of an atlas of height-fields that have been sampled over primitive shapes which approximate the geometry. We introduce novel vector quantization acceleration techniques to facilitate fast compression as well. We achieve bitrates of less than four bits per normal-equipped point. Our method enables hole-free level-of-detail point rendering. We also show that using only up to two bits per point, high-quality renderings can still be obtained if normals are estimated in image-space. Even lower bitrates are obtained for storage on disk if arithmetic coding is used.     

An FPGA stereo matching unit based on fuzzy logic

Stereo matching is one of the most used algorithms in real-time image processing applications such as positioning systems for mobile robots, three-dimensional building mapping and recognition, detection and three-dimensional reconstruction of objects. In order to improve the performance, stereo matching algorithms often have been implemented in dedicated hardware such as FPGA or GPU devices. In this paper an FPGA stereo matching unit based on fuzzy logic is described. The proposed algorithm consists of three stages. First, three similarity parameters inherent to each pixel contained in the input stereo pair are computed. Then, the similarity parameters are sent to a fuzzy inference system which determines a fuzzy-similarity value. Finally, the disparity value is defined as the index which maximizes the fuzzy-similarity values (zero up to d_max). Dense disparity maps are computed at a rate of 76 frames per second for input stereo pairs of 1280 × 1024 pixel resolution and a maximum expected disparity equal to 15. The developed FPGA architecture provides reduction of the hardware resource demand compared to other FPGA-based stereo matching algorithms: near to 72.35% for logic units and near to 32.24% for bits of memory. In addition, the developed FPGA architecture increases the processing speed: near to 34.90% pixels per second and outperforms the accuracy of most of real-time stereo matching algorithms in the state of the art.     

开放式虚拟打印系统设计与实现

在研究虚拟打印系统体系的基础上,建立了适用于应用系统间集成的开放式接口模型。采用Render Plug-ins方式实现截获打印系统中的打印任务,并对打印结果图像文件进行比例缩放、压缩处理,在此基础上详细论述虚拟打印系统的对外接口的构成,建立了基于VBA和JNI技术的系统接口模型。     

Pre‐convolved Radiance Caching

The incident indirect light over a range of image pixels is often coherent. Two common approaches to exploit this inter‐pixel coherence to improve rendering performance are Irradiance Caching and Radiance Caching. Both compute incident indirect light only for a small subset of pixels (the cache), and later interpolate between pixels. Irradiance Caching uses scalar values that can be interpolated efficiently, but cannot account for shading variations caused by normal and reflectance variation between cache items. Radiance Caching maintains directional information, e.g., to allow highlights between cache items, but at the cost of storing and evaluating a Spherical Harmonics (SH) function per pixel. The arithmetic and bandwidth cost for this evaluation is linear in the number of coefficients and can be substantial. In this paper, we propose a method to replace it by an efficient per‐cache item pre‐filtering based on MIP maps — such as previously done for environment maps — leading to a single constant‐time lookup per pixel. Additionally, per‐cache item geometry statistics stored in distance‐MIP maps are used to improve the quality of each pixel's lookup. Our approximate interactive global illumination approach is an order of magnitude faster than Radiance Caching with Phong BRDFs and can be combined with Monte Carlo‐raytracing, Point‐based Global Illumination or Instant Radiosity.     

GPU‐Based Spherical Light Field Rendering with Per‐Fragment Depth Correction

Image‐based rendering techniques are a powerful alternative to traditional polygon‐based computer graphics. This paper presents a novel light field rendering technique which performs per‐pixel depth correction of rays for high‐quality reconstruction. Our technique stores combined RGB and depth values in a parabolic 2D texture for every light field sample acquired at discrete positions on a uniform spherical setup. Image synthesis is implemented on the GPU as a fragment program which extracts the correct image information from adjacent cameras for each fragment by applying per‐pixel depth correction of rays. We show that the presented image‐based rendering technique provides a significant improvement compared to previous approaches. We explain two different rendering implementations which make use of a uniform parametrisation to minimise disparity problems and ensure full six degrees of freedom for virtual view synthesis. While one rendering algorithm implements an iterative refinement approach for rendering light fields with per pixel depth correction, the other approach employs a raycaster, which provides superior rendering quality at moderate frame rates. GPU based per‐fragment depth correction of rays, used in both implementations, helps reducing ghosting artifacts to a non‐noticeable amount and provides a rendering technique that performs without exhaustive pre‐processing for 3D object reconstruction and without real‐time ray‐object intersection calculations at rendering time.     

Perception‐driven Accelerated Rendering

Advances in computer graphics enable us to create digital images of astonishing complexity and realism. However, processing resources are still a limiting factor. Hence, many costly but desirable aspects of realism are often not accounted for, including global illumination, accurate depth of field and motion blur, spectral effects, etc. especially in real‐time rendering. At the same time, there is a strong trend towards more pixels per display due to larger displays, higher pixel densities or larger fields of view. Further observable trends in current display technology include more bits per pixel (high dynamic range, wider color gamut/fidelity), increasing refresh rates (better motion depiction), and an increasing number of displayed views per pixel (stereo, multi‐view, all the way to holographic or lightfield displays). These developments cause significant unsolved technical challenges due to aspects such as limited compute power and bandwidth. Fortunately, the human visual system has certain limitations, which mean that providing the highest possible visual quality is not always necessary. In this report, we present the key research and models that exploit the limitations of perception to tackle visual quality and workload alike. Moreover, we present the open problems and promising future research targeting the question of how we can minimize the effort to compute and display only the necessary pixels while still offering a user full visual experience.     

三维数据域可视化体绘制中的色彩合成新方法

在科学视算的研究中，目前各种直接绘制三维数据域的方法，都是运用合成算子线性递推地合成各个象素的色彩。本文提出一种新方法，将色彩合成由线性递推的纵向合成变为二维面（虚拟面）上简单的分布合成。新方法在合成色彩时省去了线性递推方法中所需的乘法运算。更有意义的是，当此方法与三维数据域体绘制的投影成象方法结合时，利用已形成的虚拟面，可以方便地生成大小不同的可视图，而不必重复进行投影操作。本文同时证明了，在一定精度内，放大了的图象反映了在相应精度下具有插值效果的可视信息。     

Towards Perceptual Simplification of Models with Arbitrary Materials

Real‐time rendering of models with high polygon count is still an important issue in interactive computer graphics. A common way to improve rendering performance is to generate different levels of detail of a model. These are mostly computed using polygonal simplification techniques, which aim to reduce the number of polygons without significant loss of visual fidelity. Most existing algorithms use geometric error bounds, which are well‐suited for silhouette preservation. They ignore the fact that a much more aggressive simplification is possible in low‐contrast areas inside the model. The main contribution of this paper is an efficient simplification algorithm based on the human visual system. The key idea is to move the domain of error computation from image‐space to vertex‐space to avoid a costly per‐pixel comparison. This way the error estimation of a simplification operation can be accelerated significantly. To account for the human vision, we introduce a perceptually based metric depending on the contrast and spatial frequency of the model at a single vertex. Finally, we validate our approach with a user study.     

分布式虚拟集群渲染系统研究

针对三维动画领域的渲染瓶颈和分布式渲染的资源搜索问题,提出了一种基于双层体系结构的分布式虚拟集群渲染系统,并详细介绍了系统的各个功能模块.通过对渲染资源的统一描述,定义了虚拟集群渲染能力的计算方法,给出了分布式渲染资源的搜索算法,并验证了算法的有效性.     

A memory-efficient unified early z-test

The Unified Early Z-Test (U-EZT) is proposed to examine the visibility of pixels during tile-based rasterization in a mobile 3D graphics processor. U-EZT combines the advantages of the Z-max and Z-min EZT algorithms: the Z-max algorithm is improved by the independently updatable z-max tiles and the use of mask bits; and the Z-min algorithm is improved by reusing the mask bits from the z-max test to update the z-min tiles after tile rasterizing. As a result, storage requirements are reduced to 3 bits per pixel, and simulations suggest that U-EZT requires 20 percent to 57 percent less memory bandwidth than previous EZT algorithms.     

VLOD: high-fidelity walkthrough of large virtual environments

We present visibility computation and data organization algorithms that enable high-fidelity walkthroughs of large 3D geometric data sets. A novel feature of our walkthrough system is that it performs work proportional only to the required detail in visible geometry at the rendering time. To accomplish this, we use a precomputation phase that efficiently generates per cell vLOD: the geometry visible from a view-region at the right level of detail. We encode changes between neighboring cells' vLODs, which are not required to be memory resident. At the rendering time, we incrementally construct the vLOD for the current view-cell and render it. We have a small CPU and memory requirement for rendering and are able to display models with tens of millions of polygons at interactive frame rates with less than one pixel screen-space deviation and accurate visibility.     

虚拟装配中基于导纳控制的力觉渲染技术

传统的力触觉渲染多采用阻抗控制,不能很好地满足虚拟装配的应用要求,相比之下导纳控制模式更适用这一领域.为此提出一种基于导纳控制的双线程力觉渲染构架,并给出相应的力觉渲染算法.首先建立用于导纳控制的动力学模型,并讨论了碰撞和约束这2个状态下的力觉渲染;为了使用力觉交互接口进行虚拟装配中的小间隙装配,提出物理约束与几何约束结合的力觉渲染方法;最后针对物理计算和力反馈循环2个线程刷新频率不匹配的问题,利用二次拉格朗日多项式进行数值插值,实现了力觉交互接口的平稳输出.通过力反馈设备与自主开发的虚拟装配原型系统VAPP的连接与应用,验证了所提出的算法满足虚拟装配系统中力觉交互的应用要求.     

Real-time stereo matching using memory-efficient Belief Propagation for high-definition 3D telepresence systems

New generations of telecommunications systems will include high-definition 3D video that provides a telepresence feeling. These systems require high-quality depth maps to be generated in a very short time (very low latency, typically about 40 ms). Classical Belief Propagation algorithms (BP) generate high-quality depth maps but they require huge memory bandwidths that limit low-latency implementations of stereo-vision systems with high-definition images.This paper proposes a real-time (latency inferior to 40 ms) high-definition (1280 × 720) stereo matching algorithm using Belief Propagation with good immersive feeling (80 disparity levels). There are two main contributions. The first is an improved BP algorithm with pixel classification that outperforms classical BP while reducing the number of memory accesses. The second is an adaptive message compression technique with a low performance penalty that greatly reduces the memory traffic. The combination of these techniques outperforms classical BP by about 6.0% while reducing the memory traffic by more than 90%.     

High-capacity image hiding scheme based on vector quantization

A novel grayscale image hiding scheme that is capable of hiding multiple secret images into a host image of the same size is proposed in this paper. The secret images to be hidden are first compressed by vector quantization with additional index compression process. Then, the compressed secret images are encrypted and embedded into the least-significant bits of the host pixels. To provide good image quality of the stego-image, the modulus function and the image property are employed to hide the secret bits into the host pixels and determine the number of hidden bits in each host pixel, respectively. According to the results, the proposed scheme provides a higher hiding capacity and a higher degree of security than that of the virtual image cryptosystem.     

Foveated Real‐Time Ray Tracing for Head‐Mounted Displays

Head‐mounted displays with dense pixel arrays used for virtual reality applications require high frame rates and low latency rendering. This forms a challenging use case for any rendering approach. In addition to its ability of generating realistic images, ray tracing offers a number of distinct advantages, but has been held back mainly by its performance. In this paper, we present an approach that significantly improves image generation performance of ray tracing. This is done by combining foveated rendering based on eye tracking with reprojection rendering using previous frames in order to drastically reduce the number of new image samples per frame. To reproject samples a coarse geometry is reconstructed from a G‐Buffer. Possible errors introduced by this reprojection as well as parts that are critical to the perception are scheduled for resampling. Additionally, a coarse color buffer is used to provide an initial image, refined smoothly by more samples were needed. Evaluations and user tests show that our method achieves real‐time frame rates, while visual differences compared to fully rendered images are hardly perceivable. As a result, we can ray trace non‐trivial static scenes for the Oculus DK2 HMD at 1182 × 1464 per eye within the the VSync limits without perceived visual differences.