大规模地形的快速漫游算法

虚拟战场仿真中,对大规模地形进行实时、逼真的绘制是个关键问题.在ROME算法的基础上采用了基于视点的连续LOD优化算法,通过对大规模地行进行预分块,采用投影三角形扫描算法实现快速裁剪,较大的减少了实际需要绘制的网格数据量,提高了运算速度,在实时性方面有了较大的改进.对于不同等级网格边界间的裂缝问题,采用ROME算法中的基于二元三角形的强制分割方法消除.实验结果表明,算法运行结果良好,在普通PC机上实现了动态的具有连续细节层次的大规模地形实时漫游.算法简单实用,地形渲染后的视觉效果良好.     

视点相关的真实感三维风景区模拟研究与实现

提出采用八叉树组织真实山体数据,根据视点距离采用不同的细节层次LOD表示山体各部分,并结合视觉剪裁和帧间相关性进一步减少绘制的山体,达到加速三维地形的绘制和漫游的目的.使用混合纹理映射、阴影渲染技术,并加入山谷中的水面模拟,生成具有真实感的大规模三维风景场景.实验结果表明该方法有效地减少了算法复杂度,易实现碰撞检测,大大提高了绘制效率,在PC机上能实时地生成高分辨率、具有真实感的场景,完全满足VR交互要求.     

基于面向对象八叉树的飞机虚拟维修场景渲染

为了加速大规模虚拟场景的渲染速度,采用基于面向对象八叉树的方法对场景进行渲染。该方法将面向对象技术与传统八叉树技术相结合,采用面向对象八叉树剖分虚拟场景,对场景进行管理;将物体结构树的最小零部件作为最小存储单元,采用叶节点保存对象信息,减小树的存储量和处理时间,降低算法的计算负担;在面向对象八叉树的基础上,采用模型遮挡裁剪算法对位于视域范围内的模型进行遮挡裁剪,减小实际渲染的物体数量,提高渲染速率。通过对飞机虚拟维修场景进行渲染实验,证明了该方法的有效性。     

大规模点模型的实时高质量绘制

提出了一个针对大规模点模型的实时高质量绘制算法．该算法采用距离相关的自适应绘制策略,在不损失绘制质量的前提下简化了计算．在预处理阶段,对点模型进行剖分,为每一分片分别建立层次结构,并序列化为线性二叉树保存．绘制时,首先根据每片的包围盒和法向等信息进行快速视域裁剪与背面剔除．对于可见的分片,依据它与视点的距离和视线方向,选择恰当的细节层次,直接取出线性二叉树中对应的点几何数据,自适应地选择最合适的绘制模式对其进行绘制．为了减少存储空间的消耗,该文提出了一种面向保留模式图形硬件加速的点模型压缩和解压缩算法,压缩比例达到8：1．作者在可编程图形硬件中实现了该文算法,在普通微机上实现了百万数量级点模型的实时高质量绘制。     

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.     

Multiresolution volume visualization with a texture-based octree

Although 3D texture-based volume rendering guarantees image quality almost interactively, it is difficult to maintain an interactive rate when the technique has to be exploited on large datasets. In this paper, we propose a new texture memory representation and a management policy that substitute the classical one-texel per voxel approach for a hierarchical approach. The hierarchical approach benefits nearly homogeneous regions and regions of lower interest. The proposed algorithm is based on a simple traversal of the octree representation of the volume data. Driven by a user-defined image quality, defined as a combination of data homogeneity and importance, a set of octree nodes (the cut) is selected to be rendered. The degree of accuracy applied for the representation of each one of the nodes of the cut in the texture memory is set independently according to the user-defined parameters. The variable resolution texture model obtained reduces the texture memory size and thus texture swapping, improving rendering speed.     

Anti-aliased and real-time rendering of scenes with light scattering effects

Recently, for real-time applications such as games, the rendering of scenes with light scattering effects in the presence of volumetric objects such as smoke, mist, etc., has gained much attention. Slice-based methods are well-known techniques for achieving fast rendering of these effects. However, for real-time applications, it is necessary to reduce the number of slice planes that are used. As a result, aliasing (striped patterns) can appear in the rendered images. In this paper, we propose a real-time rendering method for scenes containing volumetric objects that does not generate aliasing in the rendered images. When a scene consists of volumetric and polygonal objects, the proposed method also does not generate aliasing at the boundaries between the polygonal and the volumetric objects. Moreover, we are able to reduce aliasing at shadows inside a volumetric object that are cast by polygonal objects by interpolating the occlusion rates of light at several locations. The proposed method can be efficiently implemented on a GPU.     

大型网格模型多分辨率的外存构建与交互绘制

结合多分辨率、网格排布和基于视点的绘制技术,提出一种外存多分辨率构建和绘制算法.采用适应性八叉树对模型的包围盒进行划分,自顶向下构建模型的多分辨率层次结构,较好地保持了原模型的细节分布;并对多分辨率结构中每个节点所包含的三角形片段进行网格排布优化,降低了缓存的平均失效率;在实时绘制时,采用基于视点的细节层次选择策略进行模型的细化;最后通过引入数据预取机制来隐藏磁盘I/O延时,进一步提高绘制性能.实验结果表明,该算法在绘制速度与细节保留上均优于同类MRMM算法.     

Discrete shading of three-dimensional objects from medial axis transform

In this paper, we present a discrete shading technique using medial axis transform (MAT) of 3D binary image data based on digital generalized octagonal distances. Our method is computationally attractive as it does not require the explicit computation of surface normals. We have compared our results with images rendered from voxel and octree representations while using analytical surface rendered images as bench marks. The quality of rendering by our method is certainly superior to those obtained from voxel and octree representations.     

一种点面混合的复杂三维模型加速绘制方法

在主流个人计算机硬件条件下，为加速百万以上三角面片构成的复杂稠密几何模型的绘制速度，综合基于几何的建模与绘制方法GBMR和基于点的建模与绘制方法PBMR的优点，提出了一种同时使用三角面片和点作为基本单元进行对象建模与绘制的点面混合方法．在预处理阶段，对模型表面进行网格分割，存储子块三角面片和顶点点云数据，同时对顶点点云按顶点重要度排序并序列化为线性结构．在实时绘制阶段，进行视相关的裁剪和背面剔除，不同子块按视点距离分别由三角形或点进行绘制．以上过程充分利用图形处理单元GPU，实现了基于GPU的点面混合的对象连续多分辨率绘制，有效地提高了复杂模型的绘制效率．     

虚拟环境建模及实时性改善方法

提出了虚拟现实软件实现方法的概念,利用交互方式的,比较,阐明了虚拟现实与仿真的区别,通过计算机三维实体造型和真实感环境泻染技术,实现了虚拟环境建模,通过融入增强现实的思想,把握虚实结合尺度,将基于图象的方法与基于建模的相结合,从而改善了眯拟环境下交互的实时性。     

一种基于松散八叉树的复杂场景可见性裁剪算法

针对传统八叉树方法的不足,在采用松散八叉树组织场景、利用八叉树空间划分优点的同时弥补其局限性.为提高遮挡查询效率,将子节点依视点排序,针对复杂场景采用双层裁剪技术以进一步提高性能.实验结果表明,文中算法对深度复杂度高、面片数量大的复杂场景具有较好的裁剪效率,能够很好地满足实时绘制的要求.     

基于参数空间的混合多分辨率绘制

根据点和多边形在表示和绘制物体上各自不同的特点,提出了一种有效绘制细节高度复杂物体的多分辨率方法.3D表面被映射到参数平面,经规则采样成为几何图像,P-Quadtrees是基于几何图像建立的四叉树多分辨率层次结构.通过对四叉树的遍历,面向视点的表面用较大多边形面片绘制,光照细节通过法向映射完成;轮廓部分通过视点相关的LOD(level of detail)控制进行细化,使用点来绘制物体复杂精细的轮廓.通过此方法,细节复杂模型的绘制不仅可以被硬件加速,而且无论在表面还是在轮廓部分都能获得很好的视觉效果.     

Integrating Occlusion Culling and Levels of Detail through Hardly-Visible Sets

Occlusion culling and level-of-detail rendering have become two powerful tools for accelerating the handling of very large models in real-time visualization applications. We present a framework that combines both techniques to improve rendering times. Classical occlusion culling algorithms compute potentially visible sets (PVS), which are supersets of the sets of visible polygons. The novelty of our approach is to estimate the degree of visibility of each object of the PVS using synthesized coarse occluders. This allows to arrange the objects of each PVS into several Hardly-Visible Sets (HVS) with similar occlusion degree. According to image accuracy and frame rate requirements, HVS provide a way to avoid sending to the graphics pipeline those objects whose pixel contribution is low due to partial occlusion. The image error can be bounded by the user at navigation time. On the other hand, as HVS offer a tighter estimation of the pixel contribution for each scene object, it can be used for a more convenient selection of the level-of-detail at which objects are rendered. In this paper, we describe the new framework technique, provide details of its implementation using a visibility octree as the chosen occlusion culling data structure and show some experimental results on the image quality.     

A flexible multi-volume shader framework for arbitrarily intersecting multi-resolution datasets

We present a powerful framework for 3D-texture-based rendering of multiple arbitrarily intersecting volumetric datasets. Each volume is represented by a multi-resolution octree-based structure and we use out-of-core techniques to support extremely large volumes. Users define a set of convex polyhedral volume lenses, which may be associated with one or more volumetric datasets. The volumes or the lenses can be interactively moved around while the region inside each lens is rendered using interactively defined multi-volume shaders. Our rendering pipeline splits each lens into multiple convex regions such that each region is homogenous and contains a fixed number of volumes. Each such region is further split by the brick boundaries of the associated octree representations. The resulting puzzle of lens fragments is sorted in front-to-back or back-to-front order using a combination of a view-dependent octree traversal and a GPU-based depth peeling technique. Our current implementation uses slice-based volume rendering and allows interactive roaming through multiple intersecting multi-gigabyte volumes.     

Fast walkthroughs with image caches and ray casting

We present an output-sensitive rendering algorithm for accelerating walkthroughs of large, densely occluded virtual environments using a multi-stage Image-Based Rendering Pipeline. In the first stage, objects within a certain distance are rendered using the traditional graphics pipeline, whereas the remaining scene is rendered by a pixel-based approach using an Image Cache, horizon estimation to avoid calculating sky pixels, and finally, ray casting. The time complexity of this approach does not depend on the total number of primitives in the scene. We have measured speedups of up to one order of magnitude.     

Node Partitioning in an Octree Display Pipeline

The node partitioning stage of a graphics pipeline to transform and display octree encoded objects is described. Its function is to accept a sequence of octree node codes, from which the hidden nodes have been removed, and to partition it into sets of nodes which are suitable for a multiprocessor rendering stage. Each set is capable of being processed and displayed independently.     

Octree data structures and perspective imagery

The fundamental elements of perspective image generation from octree encoded graphics databases are examined. Procedures are presented for the encoding of spatially enumerated objects into an octree which is stored as a doubly-linked list. A perspective image rendering procedure is developed, based on raycasting, and involves a bidirectional traversal of the octree together with an adaptive level-of-detail control mechanism. The concepts and algorithms are verified by the synthesis of perspective terrain imagery.     

Multilevel representation and transmission of real objects with progressive octree particles

We present a multilevel representation scheme adapted to storage, progressive transmission, and rendering of dense data sampled on the surface of real objects. Geometry and object attributes, such as color and normal, are encoded in terms of surface particles associated to a hierarchical space partitioning based on an octree. Appropriate ordering of surface particles results in a compact multilevel representation without increasing the size of the uniresolution model corresponding to the highest level of detail. This compact representation can progressively be decoded by the viewer and transformed by a fast direct triangulation technique into a sequence of triangle meshes with increasing levels of detail. The representation requires approximately 5 bits per particle (2.5 bits per triangle) to encode the basic geometrical structure. The vertex positions can then be refined by means of additional precision bits, resulting in 5 to 9 bits per triangle for representing a 12-bit quantized geometry. The proposed representation scheme is demonstrated with the surface data of various real objects.     

Interactive fragment tracing

One of the main challenges in real-time rendering is to enable more and more effects that were previously available in offline rendering only. An important effect among these is physically correct reflections of arbitrary objects in curved reflectors like windshields. In this paper we propose fragment tracing on the GPU as a solution to interactively realizing this effect for large scenes as employed in industrial applications. For each rasterized fragment, a ray is traced through an octree representing the original geometry and surface material. By introducing a GPU implementation of an octree traversal, for the first time hierarchical data structures can efficiently be used on the GPU. As a result, the approach allows both handling of large geometries such as those employed in virtual prototyping and accurate rendering. Several examples show the generality and achievable rendering quality of our method.