运用层次纹理映射的基于图像绘制算法

提出一种基于图像绘制(image-basedrendering,简称IBR)的算法,以发挥图形卡的纹理映射功能,并能表现物体表面的三维凹凸细节.首先将深度图像的像素按其相关深度分为多层纹理图像,然后利用纹理映射的硬件支持,将这些层次纹理图像依次投影到与视点相关的成像面上,以得到所需目标图像.为了避免目标图像上出现空洞,在生成时,将纹素在深度层次上进行扩展.由于层次纹理图像需要较大的存储空间,并且在装入纹理缓冲时要花费大量时间,为此还提出一种基于压缩层次纹理图像的目标图像生成算法.实验表明新算法是有效的,尤其适于处理与整个物体大小相比深度层次不太多的三维景物,如有表面凹凸纹理(浮雕、门窗等)的建筑物表面等.     

虚平面映射:深度图像内部视点的绘制技术

首先推导与归纳了图像三维变换中像素深度场的变换规律,同时提出了基于深度场和极线原则的像素可见性别方法,根据上述理论和方法,提出一种基于深度图像的建模与绘制(image-based modeling and rendering,简称IBMR)技术,称为虚平面映射.该技术可以基于图像空间内任意视点对场景进行绘制.绘制时,先在场景中根据视线建立若干虚拟平面,将源深度图像中的像素转换到虚平面上,然后通过对虚平面上像素的中间变换,将虚平面转换成平面纹理,再利用虚平面的相互拼接,将视点的成像以平面纹理映射的方式完成.新方法还能在深度图像内侧,基于当前视点快速获得该视点的全景图,从而实现视点的实时漫游.新方法视点运动空间大、存储需求小,且可以发挥图形硬件的纹理映射功能,并能表现物体表面的三维凹凸细节和成像视差效果,克服了此前类似算法的局限和不足.     

LAMP: 3D layered, adaptive-resolution, and multi-perspective panorama—a new scene representation

A compact visual representation, called the 3D layered, adaptive-resolution, and multi-perspective panorama (LAMP), is proposed for representing large-scale 3D scenes with large variations of depths and obvious occlusions. Two kinds of 3D LAMP representations are proposed: the relief-like LAMP and the image-based LAMP. Both types of LAMPs concisely represent almost all the information from a long image sequence. Methods to construct LAMP representations from video sequences with dominant translation are provided. The relief-like LAMP is basically a single extended multi-perspective panoramic view image. Each pixel has a pair of texture and depth values, but each pixel may also have multiple pairs of texture-depth values to represent occlusion in layers, in addition to adaptive resolution changing with depth. The image-based LAMP, on the other hand, consists of a set of multi-perspective layers, each of which has a pair of 2D texture and depth maps, but with adaptive time-sampling scales depending on depths of scene points. Several examples of 3D LAMP construction for real image sequences are given. The 3D LAMP is a concise and powerful representation for image-based rendering.     

A Geometrical Based Method for Highly Complex Structured Textures Generation

Conventional 2D or 3D texturing methods do not permit an efficient simulation of highly complex structured textures like fire, fur, cotton, etc. More recent techniques, using specific kinds of 3D textures, such as hypertextures or texels based on volume rendering algorithms, are more interesting, for the simulation of such special types of textures. Unfortunately, these techniques remain still restricted because either they need a functional modelling of the object, as it is the case of hypertextures, or they are strictly limited to one specific kind of texture, as it is the case of texels. In this paper we present a new approach for applying a wide range of very different types of highly complex structured textures (fur, fire, water drops, cotton, fume, …) on every kind of objects. This method is particularly based on the geometrical information given by a geometrical model (as the polyhedral or CSG modelling). Like hypertextures or texels, our method uses the volume density rendering, but it isfree of the serious above mentioned restrictions of these methods. In addition, it allows an easy and very intuitive control of the global geometrical shape of generated textures. Its manipulation is simple evenfor a novice user.     

毛发的结构造型方法

人物的绘制是当今计算机图形学中的一项重要任务,特别吸引人的是头发的生成技术.头发生成技术的难点在于其精细的形状和庞大的数量.文章针对头发提出了弯曲纹元和三维纹理体的概念,并给出了它们的数学表达式以及对不同发型的描述方法.弯曲纹元和三维纹理体是一般三维纹理的结构.带三维纹理的景物是由弯曲纹元和三维纹理体构成的,三维纹理体可再分成多个弯曲纹元,因而指明了三维纹理空间的结构,并解决了三维纹理曲面的造型问题.文章通过不同例子表明了结构造型方法在描述头发中的应用价值.     

基于图像的三维光照效果的动态重现

传统图像纹理可以给虚拟物体增加丰富的细节,但由于纹理是在特定光照条件下获取的照片,当虚拟光照条件变化时,纹理细节缺少随之动态变化的真实感。凹凸纹理通过扰动法向量可以实现细节的基本阴影变化,但生成一张真实照片的凹凸纹理是困难的。文中介绍了BRDF和BTF的概念和理论,提出了一种基于图像的适用于漫反射物体表面细节动态重现的方法,可以使映射后的纹理在不同虚拟光照条件下呈现动态变化的效果。     

纹理元提取与纹理合成的自由参数估计

该文分析了常见的两类纹理:随机性纹理与结构性纹理的特性,针对其不同的统计特征,采用两种方法提取纹理元。对随机性纹理采用变尺度窗口特征跟踪的方法提取纹理元;对结构性纹理,利用其具备较为明显的边界特性,采用基于图像分割的方法提取纹理元。为避免图像噪声和自然边界不连续造成的提取误差,使用Bayes分类进行二次精细分割加以修正。实验证明,该文提出的方法对两类纹理元有较好的提取效果,同时可以用来估计纹理合成时的自由参数。     

Compression-Based 3D Texture Mapping for Real-Time Rendering

While 2D texture mapping is one of the most effective of the rendering techniques that make 3D objects appear visually interesting, it often suffers from visual artifacts produced when 2D image patterns are wrapped onto the surfaces of objects with arbitrary shapes. On the other hand, 3D texture mapping generates highly natural visual effects in which objects appear carved from lumps of materials rather than laminated with thin sheets as in 2D texture mapping. Storing 3D texture images in a table for fast mapping computations, instead of evaluating procedures on the fly, however, has been considered impractical due to the extremely high memory requirement. In this paper, we present a new effective method for 3D texture mapping designed for real-time rendering of polygonal models. Our scheme attempts to resolve the potential texture memory problem by compressing 3D textures using a wavelet-based encoding method. The experimental results on various nontrivial 3D textures and polygonal models show that high compression rates are achieved with few visual artifacts in the rendered images and a small impact on rendering time. The simplicity of our compression-based scheme will make it easy to implement practical 3D texture mapping in software/hardware rendering systems including real-time 3D graphics APIs such as OpenGL and Direct3D.     

纹理映射中的平面校正技术研究

为了快速实时地进行由平面组成的结构景物的3D建模问题,文中介绍了一种在进行图像3D重构时纹理映射中的平面校正方法。介绍了三角形模型在图像处理、图形绘制、虚拟现实等技术中的重要作用。从射影几何的角度出发,给出了从两幅视图进行景物三维重构的分层重构方法。在已知欧氏重构即摄像机内参数的基础上,介绍一种基于标定的平面射影失真矫正方法。通过此方法,将矫正过的纹理映射到欧式点重构结构中,得到景物的3D模型。经实验验证,这种方法在处理由平面组成的景物的3D重构中是实时有效的。     

Method of Direct Texture Synthesis on Arbitrary Surfaces

A direct texture synthesis method on arbitrary surfaces is proposed in this paper. The idea is to recursively map triangles on surface to texture space until the surface is completely mapped. First, the surface is simplified and a tangential vector field is created over the simplified mesh. Then, mapping process searches for the most optimal texture coordinates in texture sample for each triangle, and the textures of neighboring triangles are blended on the mesh. All synthesized texture triangles are compressed to an atlas. Finally, the simplified mesh is subdivided to approach the initial surface. The algorithm has several advantages over former methods: it synthesizes texture on surface without local parameterization; it does not need partitioning surface to patches; and it does not need a particular texture sample. The results demonstrate that the new algorithm is applicable to a wide variety of texture samples and any triangulated surfaces.     

Texture-based Dither Matrices

Continuous tone images must be halftoned to be displayed on binary output devices such as printers. Halftoning algorithms at low resolutions of the output hardware introduce textures into the resulting display. In this work we control halftoning texture by generating a threshold matrix from an image-based texture. We demonstrate that processing textures by the adaptive histogram equalization algorithm approximates pixel distribution properties of traditional dither screens. Ordered dithering with the resulting threshold matrix enables us to define texture in the halftoned image. We control the appearance of this texture by a combination of the ordered dither algorithm with an error diffusion process. We present applications of texture-based dither screens to both photorealistic and artistic rendering. In the case of photorealistic tone reproduction our technique preserves textures and edges of the original image. The ability to define an arbitrary texture enables us to introduce a variety of artistic effects, including embossing of images with textures and text, and approximation of the appearance of of conventional illustration media. We evaluate the resulting halftoning using multi-scale edge distortion measures. Our quantitative evaluation closely corresponds to the visual observations.     

Adaptive scale filtering: a general method for obtaining shape fromtexture

Introduces adaptive scale filtering, a general method for deriving shape from texture under perspective projection without recourse to prior segmentation of the image into geometric texture elements (texels), and without thresholding of filtered images. If texels on a given surface can be identified in an image then the orientation of that surface can be obtained. However, there is no general characterization of texels for arbitrary textures. Furthermore, even if the size and shape of texels on the surface is invariant with regard to position, perspective projection ensures that the size and shape of the corresponding image texels vary by orders of magnitude. Commencing with an initial set F_O of identical image filters, adaptive scale filtering iteratively derives a set F_N which contains a unique filter for each image position. Each element of F_N is tuned to the three-dimensional structure of the surface; that is, all image filters in F_N back-project to an identical shape and size on the surface. Thus image texels of various sizes, but associated with a single spatial scale on the surface, can be identified in different parts of the image. When combined with conventional shape from texture methods, edges derived using F_N provide accurate estimates of surface orientation. Results for planar surfaces are presented     

基于图象的三维模型建模绘制系统

基于图象绘制技术近几年来在图形学领域引起了极大的重视，它不仅弥补了传统的基于几何绘制技术中存在的不足，而且能给出了更加逼真的图象显示，在构建一种基于图象建模系统的基础上重点讨论了图象变形，图象重构，以及最后采用传统纹理映射技术进行复杂三维模型建模的主要原理与方法。     

Modeling, animating, and rendering complex scenes using volumetrictextures

Complex repetitive scenes containing forests, foliage, grass, hair, or fur, are challenging for common modeling and rendering tools. The amount of data, the tediousness of modeling and animation tasks, and the cost of realistic rendering have caused such kind of scene to see only limited use even in high-end productions. The author describes how the use of volumetric textures is well suited to such scenes. These primitives can greatly simplify modeling and animation tasks. More importantly, they can be very efficiently rendered using ray tracing with few aliasing artifacts. The main idea, initially introduced by Kajiya and Kay (1989), is to represent a pattern of 3D geometry in a reference volume, that is tiled over an underlying surface much like a regular 2D texture. In our contribution, the mapping is independent of the mesh subdivision, the pattern can contain any kind of shape, and it is prefiltered at different scales as for MIP-mapping. Although the model encoding is volumetric, the rendering method differs greatly from traditional volume rendering. A volumetric texture only exists in the neighborhood of a surface, and the repeated instances (called texels) of the reference volume are spatially deformed. Furthermore, each voxel of the reference volume contains a key feature which controls the reflectance function that represents aggregate intravoxel geometry. This allows for ray tracing of highly complex scenes with very few aliasing artifacts, using a single ray per pixel (for the part of the scene using the volumetric texture representation). The major technical considerations of our method lie in the ray-path determination and in the specification of the reflectance function     

一种融合纹理的三维图像重建快速实现方法

为了得到完整的三维模型,介绍了一种融合纹理的三维图像重建快速实现方法。通过对不同视角的深度图像的手动粗配准、ICP算法精配准以及全局配准得到这些深度图像的旋转平移矩阵。通过vrippack,三维重建出完整的三维图像,用TextureStitcher对得到的三维图像进行纹理映射,从而实现融合纹理的三维图像的快速重建。文中在论述配准算法主要思想和实现步骤的同时,也用实验验证了方法的可行性与通用性。     

三维模型表面特征的水墨效果渲染算法

提出一种对三维模型实现水墨效果渲染的两步二维纹理映射方法.该方法首先通过模型顶点的视线向量和法线向量的点乘积计算其夹角的余弦值,作为第一步的纹理坐标进行映射;再通过欧拉公式计算径向曲率及其方向导数值,作为第二步映射的纹理坐标.为达到实时的交互浏览,在第二步映射时还可采用基于视点无关的曲率特征的映射策略(如平均曲率、高斯曲率、主曲率等).在快速生成水墨画效果的同时也显示出更多的特征细节.     

面向手持式3维扫描设备的本征纹理重建

目的 针对目前手持式3维扫描设备生成的模型纹理分辨率不够,且部分区域存在高光、阴影及明暗变化等问题,提出一种基于多幅实拍照片的纹理重建方法。方法 首先使用基于特征匹配的方法将照片图像与几何模型进行配准;其次根据重建纹理大小,采用特殊编码方式的位置纹理建立照片像素到纹理像素直接且精确的对应关系;然后根据多幅使用闪光灯作为光源拍摄的照片,通过位置纹理建立联立方程,求解漫反射分量;最后采用改进的基于混合权重的融合方法对求解的漫反射分量进行纹理融合。结果 使用本文方法对3个实验模型进行本征纹理重建,与3维扫描设备生成纹理和直接用照片生成纹理相比,该方法操作简单、使用方便,可获得高度清晰的,不含高光和明暗效果的本征纹理图像。结论 实验结果表明,重建纹理质量在分辨率、色彩还原性及一致性方面明显优于原有纹理,且该方法具有很高的精确性和鲁棒性,可满足高质量的纹理重建需求。     

基于体平滑的体绘制纹理伪边界消除算法

直接体绘制能够清楚的显示三维数据场的内部信息,是科学可视化中非常重要的一类方法。其中,基于二维纹理映射的三维数据直接体绘制方法具有绘制速度快、可交互性强的优点。其基本思路是将三维数据体在时间或深度方向形成一组水平纹理切片,通过这些切片的纹理贴图,实现三维数据体的体绘制,在交互性和资源消耗之间取得了较好的平衡。本文针对基于二维纹理映射的直接体绘制方法中在透明与不透明边界产生阶梯状条纹的伪边界问题,提出了一种基于体平滑的算法。该算法通过在透明数据与不透明数据的边界进行体平滑,使采样过程中缺失的数据表现到抽样的切片上,从而在最终图像生成阶段淡化甚至消除阶梯状条纹伪边界。实验结果表明,相对于传统二维纹理映射方法,本算法实现体绘制效果平滑,提高了绘制效果。     

Floating Textures

We present a novel multi‐view, projective texture mapping technique. While previous multi‐view texturing approaches lead to blurring and ghosting artefacts if 3D geometry and/or camera calibration are imprecise, we propose a texturing algorithm that warps (“floats”) projected textures during run‐time to preserve crisp, detailed texture appearance. Our GPU implementation achieves interactive to real‐time frame rates. The method is very generally applicable and can be used in combination with many image‐based rendering methods or projective texturing applications. By using Floating Textures in conjunction with, e.g., visual hull rendering, light field rendering, or free‐viewpoint video, improved rendering results are obtained from fewer input images, less accurately calibrated cameras, and coarser 3D geometry proxies.     

复用计算的虚拟肝脏体纹理合成与映射方法

虚拟手术中器官纹理能够给用户直观反映,加强临场感和操作感。研究在虚拟肝脏手术中纹理的表现技术,先合成体纹理空间,把二维情况下基于复用计算的纹理合成技术进行改进,扩展至三维空间中进行体纹理块分布并着色;同时计算每个三角面片内部点集并着色,以此增加纹理的真实感;切割肝脏体能看到内部纹理。实验结果表明,该方法能够生成具有高度真实感以及同样本体纹理相似的三维纹理,且合成速度能够满足虚拟手术要求。