基于可见性预处理和细节简化的虚拟环境快速漫游算法

本文针对复杂多边形场早的漫游提出了一个新的预处理方法，并给出了相应的快速漫游算法。     

结构化骨架求取和内窥可见性计算

提出了包含距离和拓扑信息的结构化骨架提取方法和骨架指导的内窥可见性计算方法,并将其有机地应用于虚拟漫游系统中.结构化骨架提取算法结合了并行细化算法和距离变换算法,使得骨架能够有效地控制虚拟内窥镜的视点移动和漫游位置的跟踪,有利于准确地观察病变位置.内窥可见性算法利用人体器官的封闭管状特征,将其分割成许多网格单元,并在预处理中使用深度缓存计算单元间的可见性,同时对每个单元建立可见性树.虚拟漫游时,通过当前视点信息可进一步动态地删减所在单元的可见性树,从而得到实时可见性,最终实现实时漫游并保持漫游图像的质量.     

基于纹理数组的大规模地形绘制算法

将Shader Model 4.0引入的纹理数组技术同顶点纹理拾取技术、瓦片块四叉树算法和地形分块技术等相结合,提出了一种基于GPU的大规模地形绘制方法。将整个大规模地形数据分割成地形块,按照金字塔模型保存在CPU内存里,将地形中潜在的可见部分以纹理数组形式驻留在GPU Cache里;在CPU上发送瓦片块四叉树平面网格,利用存储在GPU Cache里的高程值生成相应的地形;GPU Cache随着视点运动而连续更新。实验证明该方法充分利用了现代GPU的特性,适合于大规模地形的漫游。     

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.     

基于时空连贯性和几何简化技术的复杂场景快速消隐绘制算法

提出了一个结合层次遮挡图像缓存的快速消隐绘制算法,本算法首先利用空间连贯性对场景实行快速保守的消隐,对可能可见的近景、中景使用几何绘制,对可能可见的远景实现了基于图像和几何混合的加速绘制,实验表明,由于充分利用了空间连贯性和图像简化技术,本算法效果良好,可适合各种复杂度场景的快速绘制。     

多视点成像的多边形模板法

该文提出一种对场景进行多视点成像的方法。该方法首先为场景中的多边形生成多边形模板，一个多边形模板，包括一条轮廓路径和一组纹条，而一个纹条是平行成像画的一个平面与多边形相交的直线段。由于纹条相对于不同视点的透视投影的变化是线性的，因此，绘制多边形时可以基于模板逐个纹条地处理，而不必按照传统的扫描转换方法逐个点地处理，绘制速度可以提高很多。同时，与视点无关的光照和纹理可以预先计算并保存在模板中，以便在成像时利用基于图像绘制的技术来生成高质量的图像。该方法中，视点可以放置在三维空间的任意位置，并且在场景漫游时可以根据视点位置自动地实现多分辨率绘制。     

RayCaching: Amortized Isosurface Rendering for Virtual Reality

Real-time virtual reality requires efficient rendering methods to deal with high- resolution stereoscopic displays and low latency head-tracking. Our proposed RayCaching method renders isosurfaces of large volume datasets by amortizing raycasting over several frames and caching primary rays as small bricks that can be efficiently rasterized. An occupancy map in form of a clipmap provides level of detail and ensures that only bricks corresponding to visible points on the isosurface are being cached and rendered. Hard shadows and ambient occlusion from secondary rays are also accumulated and stored in the cache. Our method supports real-time isosurface rendering with dynamic isovalue and allows stereoscopic visualization and exploration of large volume datasets at framerates suitable for virtual reality applications.     

Fast Tracing of Acoustic Beams and Paths Through Visibility Lookup

The beam tracing method can be used for the fast tracing of a large number of acoustic paths through a direct lookup of a special tree-like data structure (beam tree) that describes the iterated visibility information from one specific position. This structure describes the branching of bundles of rays (beams) as they encounter reflectors in their paths. For this reason, beam tracing is suitable for real-time acoustic rendering even when the receiver is moving. In this paper, we propose a novel technique that enables the fast tracing of a large number of acoustic beams through the iterative lookup of a special data structure that describes the global visibility between reflectors. The method enables the immediate generation of the beam tree corresponding to an arbitrary source location, which can then be used for path tracing through direct lookup. In practice, this technique generalizes the traditional beam-tracing method as it makes it suitable for real-time acoustic rendering not just when the receiver is moving but also when the source is moving. The method enables real-time modeling of acoustic propagation and real-time auralization in complex 2-D and 2-Dtimes1-D environments (e.g., vertical walls limited by horizontal floor and ceiling), which makes it suitable for applications of real-time virtual acoustics, immersive gaming, and advanced acoustic rendering. Some experimental results show the effectiveness of fast beam tracing with respect to the state of the art in acoustic beam tracing.     

Interactive Depth-of-Field Rendering with Secondary Rays

This paper presents an efficient method to trace secondary rays in depth-of-field (DOF) rendering, which significantly enhances realism. Till now, the effects by secondary rays have been little addressed in real-time/interactive DOF rendering, because secondary rays have less coherence than primary rays, making them very difficult to handle. We propose novel measures to cluster secondary rays, and take a virtual viewpoint to construct a layered image-based representation for the objects that would be intersected by a cluster of secondary rays respectively. Therefore, we can exploit coherence of secondary rays in the clusters to speed up tracing secondary rays in DOF rendering. Results show that we can interactively achieve DOF rendering effects with reflections or refractions on a commodity graphics card.     

Decoupled Space and Time Sampling of Motion and Defocus Blur for Unified Rendering of Transparent and Opaque Objects

We propose a unified rendering approach that jointly handles motion and defocus blur for transparent and opaque objects at interactive frame rates. Our key idea is to create a sampled representation of all parts of the scene geometry that are potentially visible at any point in time for the duration of a frame in an initial rasterization step. We store the resulting temporally‐varying fragments (t‐fragments) in a bounding volume hierarchy which is rebuild every frame using a fast spatial median construction algorithm. This makes our approach suitable for interactive applications with dynamic scenes and animations. Next, we perform spatial sampling to determine all t‐fragments that intersect with a specific viewing ray at any point in time. Viewing rays are sampled according to the lens uv‐sampling for depth‐of‐field effects. In a final temporal sampling step, we evaluate the predetermined viewing ray/t‐fragment intersections for one or multiple points in time. This allows us to incorporate all standard shading effects including transparency. We describe the overall framework, present our GPU implementation, and evaluate our rendering approach with respect to scalability, quality, and performance.     

A hybrid ray tracer for rendering polygon and volume data

Volume rendering, a technique for visualizing sampled functions of three spatial dimensions by computing 2-D projections of a colored semitransparent volume, is extended to handle polygonally defined objects. A hybrid ray-tracing algorithm, whereby rays are simultaneously cast through a set of polygons and a volume data array, is used. Samples of each are drawn at equally spaced intervals along the rays, and the resulting colors and opacities are composited together in depth-sorted order. To avoid aliasing of polygonal edges at modest computational expense, a form of selective supersampling is used. To avoid errors in visibility at polygon-volume intersections, special treatment is given to volume samples lying immediately in front of and behind polygons. The cost, image quality, and versatility of the algorithm are evaluated using data from 3-D medical imaging applications     

Two Methods for Fast Ray‐Cast Ambient Occlusion

Ambient occlusion has proven to be a useful tool for producing realistic images, both in offline rendering and interactive applications. In production rendering, ambient occlusion is typically computed by casting a large number of short shadow rays from each visible point, yielding unparalleled quality but long rendering times. Interactive applications typically use screen‐space approximations which are fast but suffer from systematic errors due to missing information behind the nearest depth layer. In this paper, we present two efficient methods for calculating ambient occlusion so that the results match those produced by a ray tracer. The first method is targeted for rasterization‐based engines, and it leverages the GPU graphics pipeline for finding occlusion relations between scene triangles and the visible points. The second method is a drop‐in replacement for ambient occlusion computation in offline renderers, allowing the querying of ambient occlusion for any point in the scene. Both methods are based on the principle of simultaneously computing the result of all shadow rays for a single receiver point.     

一种基于交互式多维传递函数的纹理映射体绘制算法

本文设计了一种基于空间信息的交互式多维传递函数的纹理映射体绘制算法。该算法不仅可以根据体数据的强度而且还利用体素的空间位置来设定绘制的颜色和阻光度。通过采用一种独特的空间投影变换,根据用户需求,将体数据划分为不同区域,并分别定义各自的传递函数。该特点使得本文的算法可以有效地对体数据进行交互式分析。在算法实现中,利用了通用图形硬件的可编程特性,在普通PC上可以达到理想的绘制质量和交互速度。     

Spherical Visibility Sampling

Many 3D scenes (e.g. generated from CAD data) are composed of a multitude of objects that are nested in each other. A showroom, for instance, may contain multiple cars and every car has a gearbox with many gearwheels located inside. Because the objects occlude each other, only few are visible from outside. We present a new technique, Spherical Visibility Sampling (SVS), for real‐time 3D rendering of such – possibly highly complex – scenes. SVS exploits the occlusion and annotates hierarchically structured objects with directional visibility information in a preprocessing step. For different directions, the directional visibility encodes which objects of a scene's region are visible from the outside of the regions' enclosing bounding sphere. Since there is no need to store a separate view space subdivision as in most techniques based on preprocessed visibility, a small memory footprint is achieved. Using the directional visibility information for an interactive walkthrough, the potentially visible objects can be retrieved very efficiently without the need for further visibility tests. Our evaluation shows that using SVS allows to preprocess complex 3D scenes fast and to visualize them in real time (e.g. a Power Plant model and five animated Boeing 777 models with billions of triangles). Because SVS does not require hardware support for occlusion culling during rendering, it is even applicable for rendering large scenes on mobile devices.     

Ray-tracing triangular trimmed free-form surfaces

This paper presents a new approach to rendering triangular algebraic free-form surfaces. A hierarchical subdivision of the surface with associated tight bounding volumes provides for quick identification of the surface regions likely to be hit by a ray. For each leaf of the hierarchy, an approximation to the corresponding surface region is stored. The approximation is used to compute a good starting point for the iteration, which ensures rapid convergence. Trimming curves are described by a tree of trimming primitives, such as squares, circles, polygons, and free-form curves, combined with Boolean operations. For trimmed surfaces, an irregular adaptive subdivision is constructed to quickly eliminate all parts outside the trimming curve from consideration during rendering. Cost heuristics are introduced to optimize the rendering time further     

Layered occlusion map for soft shadow generation

This paper presents a high-quality high-performance algorithm to compute plausible soft shadows for complex dynamic scenes. Given a rectangular light source, the scene is rendered from the viewpoint placed at the center of the light source, and discretized into a layered depth map. For each scene point sampled in the depth map, the occlusion degree is computed, and stored in a layered occlusion map. When the scene is rendered from the camera’s viewpoint, the occlusion degree of a scene point is computed by filtering the layered occlusion map. The proposed algorithm produces soft shadows the quality of which is quite close to that of the ground truth reference. As it runs very fast, a scene with a million polygons can be rendered in real-time. The proposed method does not require pre-processing and is easy to implement in contemporary graphic hardware.     

虚拟环境漫游系统绘制优化技术的集成框架

In the walkthrough of virtual environments, the number of polygons consisting the virtualenvironments often far exceeds the rendering capacity of graphics pipeline with interactive frame rates.One solution is to reduce the number of primitives before they are sent into the graphics pipeline by ex-ploiting various optimal rendering techniques. Since no single such technique can achieve satisfactoryframe rate, frameworks for integrating several optimal rendering techniques are needed to solve theproblem. In this paper ,some frameworks and the data structures needed by them are discussed in detail.And one kind of proposed framework suitable for PC is presented 1in this paper.     

Ray-Casted BlockMaps for Large Urban Models Visualization

We introduce a GPU-friendly technique that efficiently exploits the highly structured nature of urban environments to ensure rendering quality and interactive performance of city exploration tasks. Central to our approach is a novel discrete representation, called BlockMap, for the efficient encoding and rendering of a small set of textured buildings far from the viewer. A BlockMap compactly represents a set of textured vertical prisms with a bounded on-screen footprint. BlockMaps are stored into small fixed size texture chunks and efficiently rendered through GPU raycasting. Blockmaps can be seamlessly integrated into hierarchical data structures for interactive rendering of large textured urban models. We illustrate an efficient output-sensitive framework in which a visibility-aware traversal of the hierarchy renders components close to the viewer with textured polygons and employs BlockMaps for far away geometry. Our approach provides a bounded size far distance representation of cities, naturally scales with the improving shader technology, and outperforms current state of the art approaches. Its efficiency and generality is demonstrated with the interactive exploration of a large textured model of the city of Paris on a commodity graphics platform.     

Boundary‐Aware Extinction Mapping

We introduce Boundary‐Aware Extinction Maps for interactive rendering of massive heterogeneous volumetric datasets. Our approach is based on the projection of the extinction along light rays into a boundary‐aware function space, focusing on the most relevant sections of the light paths. This technique also provides an alternative representation of the set of participating media, allowing scattering simulation methods to be applied on arbitrary volume representations. Combined with a simple out‐of‐core rendering framework, Boundary‐Aware Extinction Maps are valuable tools for interactive applications as well as production previsualization and rendering.     

Sparse GPU Voxelization of Yarn‐Level Cloth

Most popular methods in cloth rendering rely on volumetric data in order to model complex optical phenomena such as sub‐surface scattering. These approaches are able to produce very realistic illumination results, but their volumetric representations are costly to compute and render, forfeiting any interactive feedback. In this paper, we introduce a method based on the Graphics Processing Unit (GPU) for voxelization and visualization, suitable for both interactive and offline rendering. Recent features in the OpenGL model, like the ability to dynamically address arbitrary buffers and allocate bindless textures, are combined into our pipeline to interactively voxelize millions of polygons into a set of large three‐dimensional (3D) textures (>10⁹ elements), generating a volume with sub‐voxel accuracy, which is suitable even for high‐density woven cloth such as linen.