1、<p><b>　　本科畢業(yè)設(shè)計</b></p><p><b>　　英文文獻(xiàn)及翻譯</b></p><p>　學(xué)院</p><p>　班級</p><p>　學(xué)號</p><p>　姓名</p><p>　指 導(dǎo) 教 師</p>

2、<p>　職 稱</p><p>　　Concepts and Descriptions of 3D Models</p><p>　　In the following, the concepts, descriptions and research directions for newly- developed digital media, 3D models, are p

3、resented. Based on three aspect of technical requirements, the basic concepts and the commonly-used techniques for multimedia compression, multimedia watermarking, multimedia retrieval and multimedia perceptual hashing

4、are then summarized.</p><p><b>　　3D Models</b></p><p>　　A model is the abstract representation of an objective, including structures, attributes, variation laws and relationships amo

5、ng components.3D modelsare the fourth generation of multimedia following sound, images and videos. A 3D model represents a 3D object using a collection of points in the 3D space, connected by various geometric entities

6、 such as triangles, lines, curved surfaces, etc. A typical example is shown in Fig. 1.1. Being a collection of data (points and other information), 3</p><p>　　3D models can be roughly classified into tw

7、o categories: (1) Solid models. These models define the volume of the object they represent (like a rock). These are more realistic, but more difficult to build. Solid models are mostly used for non-visual simulations su

8、ch as medical and engineering simulations, and for CAD and specialized visual applications such as ray tracing and constructive solid geometry.(2) Shell/Boundary models. These models represent the surface, e.g., the bo

9、undary of the obj</p><p>　　rendered scene. There are two types of information in a 3D model, geometrical information and topological information. Geometrical information generally represents shape

10、s, locations and sizes in the Euclidean space, while topological information stands for the connectivity between different parts of the 3D model. The 3D model itself is invisible, but we can perform the renderin

11、g operation at different levels of detail based on simple wireframes or shading based on dif</p><p>　　3D Modeling Schemes </p><p>　　When we use computers to analyze and research objectiv

12、e things, it is essential to adopt suitable models to represent the actual objects or abstract phenomena. This process is called modeling. In 3D computer graphics, 3D modeling [6] is the process of developin

13、g a mathematical, wireframe representation of any 3D object (either inanimate or living) via specialized software. It can be displayed as a 2D image through a process called 3D rendering or used in a computer sim

14、ulatio</p><p>　　3D scanners and image acquisition systems are rapidly becoming more affordable and allow the building of highly accurate models of real 3D objects in a cost- and time-effective ma

15、nner. To construct 3D models for actual objects, we must first acquire related attributes of samples, such as geometrical shapes and surface textures. The data that record such information are called 3

16、D data, and 3D data acquisition is the process by which the 3D information is </p><p>　　(1) Methods based on direct design or measurement. They are often used in early architecture 3D

17、 modeling. They utilize engineering drawing to obtain the three views of each model.</p><p>　　(2) Image-based methods. They construct 3D models based on pictures. They first obtain geometrical and

18、texture information simultaneously by taking photos, and then construct 3D models based on obtained images. </p><p>　　(3) Mechanical-probe-based methods. They acquire the surface data by physical tou

19、ch between the probe and the object. They require that the object hold a certain hardness. </p><p>　　(4) Methods based on volume data restoration. They adopt a series of slicing images of the object to res

20、tore the 3D shape of the object. They are often used in medical departments with X-ray slicing images, CT images and MRT images.</p><p>　　(5) Region-scanning-based methods. They obtain the position of each

21、vertex in the space by estimating the distance between the measuring instrument and each point on the object surface. Two examples of the methods are optical triangulation and interferometry.</p><p>　

22、　The main problem in 3D modeling is to render 3D models based on 3D data. To achieve a better visual effect, we should guarantee it has smooth surfaces, without burrs and holes, and make 3D models

23、 embody a third dimension and sense of reality. At the same time, we should organize the data in a better manner to reduce the storage space and speed up the displaying. Current modeling techniques can b

24、e mainly classified in three categories: geometric-model</p><p>　　Geometric-Modeling-Based Techniques </p><p>　　Geometric modeling is a branch of applied mathematics and computatio

25、nal geometry that studies methods and algorithms for the mathematical description of shapes. The shapes studied in geometric modeling are mostly 2D or 3D, although many of its tools and principles can be applied

26、to sets of any finite dimension. </p><p>　　Today most geometric modeling processes are done with computers and for computer-based applications. 2D models are important in computer typo

27、graphy and technical drawing. 3D models are central to CAD/CAM, and widely used in many applied technical fields such as civil and mechanical engineering, architecture, geology and medical image proces

28、sing. Geometric models are usually distinguished from procedural and object-oriented models, which define the </p><p>　　mathematical equations. Also, a fractal model yields a parametric o

29、r implicit model when its recursive definition is truncated to a finite depth. A geometric modeling technique involves the development from wireframe modeling through surface modeling to solid modeling,

30、 where the representation of geometric volume information becomes more and more accurate, and the range of “design” problems which we are able to solve is wider and wider. These three modeling tech</p><

31、p>　　(1) Wireframe modeling. A wireframe model is a visual presentation of a 3D or physical object used in 3D computer graphics. It is created by specifying each edge of the physical object where two mathemati

32、cally continuous smooth surfaces meet, or by connecting an object’s constituent vertices using straight lines or curves. The object is projected onto the computer screen by drawing lines at the location

33、 of each edge. Using a wireframe model allows visualizatio</p><p>　　(2) Surface modeling. Unlike wireframe models, surface models introduce the concept of “surfaces”. It is a mathematical technique

34、for representing solid-appearing objects. Surface modeling is a more complex method for representing objects than wireframe modeling, but not as sophisticated as solid modeling. Surface modeling is widely used in CAD for

35、 illustrations and architectural renderings. It is also used in 3D animation for games and other presentations. Although surface and solid </p><p>　　1) Polygonal modeling. In 3D computer gra

36、phics, polygonal modeling is an approach for modeling objects by representing or approximating their surfaces using polygons. Polygonal modeling is well suited to scan line rendering and is therefo

37、re the choice for real-time computer graphics. We will discuss this kind of model in detail in the next subsection. </p><p>　　2) NURBS modeling. Non-uniform rational B-spline (NURBS) is a mathema

38、tical model commonly used in computer graphics for generating and representing curves and surfaces which offers great flexibility and precision for handling both analytic and freeform shapes. The develop

39、ment of NURBS began in the 1950s by engineers who were in need of a mathematically precise representation of freeform surfaces like those used for ship hulls, aerospace exterior su</p><p&

40、gt;　　3) Subdivision surface modeling. Subdivision surface modeling, in the field of 3D computer graphics, is a method of representing a smooth surface via the specification of a coarser piecewise linear polyg

41、on mesh. The smooth surface can be calculated from the coarse mesh as the limit of a recursive process of subdividing each polygonal face into smaller faces that better approximate the smooth surf

42、ace. The subdivision surfaces are defined recursively. The</p><p>　　(3) Solid modeling. Solid modeling is the unambiguous representation of the solid parts of an object, which means models of soli

43、d objects suitable for computer processing. As we know, surface models are used extensively in automotive and consumer product design as well as entertainment animation, while wireframe models are ambiguous

44、 about solid volume. Primary uses of solid modeling are for CAD, engineering analysis, computer graphics and animation, rapid prototyping, medi</p><p>　　3D Scanner-Based Techniques</p><p>　

45、　A 3D scanner is a device that analyzes a real-world object or environment to collect data on its shape and possibly its appearance (e.g., color). The collected data can then be used to construct

46、 digital, 3D models useful for a wide variety of applications. These devices are used extensively by the entertainment industry in the production of movies and video games. Other common applications of t

47、his technology include industrial design, orthotics and prost</p><p>　　The purpose of a 3D scanner is usually to create a point cloud of geometric samples on the surface of the subject. These po

48、ints can then be used to extrapolate the shape of the subject (a process called reconstruction). If the color information is collected at each point, then the colors on the surface of the subject can also be determined.

49、 3D scanners are very analogous to cameras. Like cameras, they have a cone-like field of view, and they can only collect information about su</p><p>　　merged to create a complete model. This whole

50、 process, going from the single range map to the whole model, is usually known as the 3D scanning pipeline.</p><p>　　There are two types of 3D scanners, i.e., contact and non-contact scanners.

51、 Non-contact 3D scanners can be further classified into two main categories, active scanners and passive scanners. There are a variety of technologies that fall under </p><p>　　each of these categories.(1)Co

52、ntact. Contact 3D scanners probe the subject through physical touch. A coordinate measuring machine (CMM) is an example of a contact 3D scanner. It is used mostly in manufacturing and can be very precise. The

53、disadvantage of CMMs is that they require contact with the object being scanned. Thus, the scanning operation might modify or damage the object. This fact is very significant when scanning delicat

54、e or valuable objects such as</p><p>　　(2) Non-contact active. Active scanners emit some kind of radiation or light and detect its reflection in order to probe an object or environment. Possible type

55、s of emissions used include light, ultrasound or X-ray. For example, both time-of-flight and triangulation 3D laser scanners are active scanners that use laser lights to probe the subject or environment. Th

56、e advantage of time-of-flight range finders is that they are capable of operating over very long distances, in</p><p>　　(3) Non-contact passive. Passive scanners do not emit any radiation themsel

57、ves, but instead rely on detecting reflected ambient radiation. Most scanners of this type detect visible light because it is a readily available ambient radiation. Other types of radiation, such as inf

58、rared, could also be used. Passive methods can be very cheap, because in most cases they do not need particular hardware. For example, stereoscopic systems usually employ two video</p><p><

59、;b>　　3D的概念與描述</b></p><p>　　以下是呈現(xiàn)的是最新研發(fā)的電子媒體3D的概念，描述和研究方向。根據(jù)技術(shù)要求的三方面，基礎(chǔ)概念和普通應(yīng)用的多媒體壓縮，多媒體水影制作，多媒體檢索和多媒體概念分布進(jìn)行以下總結(jié)。</p><p><b>　　3D模型</b></p><p>　　3D是一個物體的抽象概念，它包括組

60、成部分之間的結(jié)構(gòu)，屬性，變化的準(zhǔn)則和關(guān)系。所謂3D是聲音多媒體，圖像多媒體，圖像多媒體。一個3D模型代表了一個3D空間的點(diǎn)的集合，是各種幾何圖形的相互交織，例如，三角形，直線，曲面等等。3D可以通過手工制作，通過算數(shù)法（程序模型），或者瀏覽法。3d模型被廣泛應(yīng)用在3d制圖中。實(shí)際上，他們3D制圖在很早之前就在個人筆記本電腦中使用，許多電腦游戲在真正使用圖像之前，就是用3D的預(yù)渲染的精靈圖片。今天3d應(yīng)用的領(lǐng)域很廣泛，現(xiàn)在醫(yī)藥公司用這種技

61、術(shù)來描述人的身體器官，電影公司用這種技術(shù)用到人物及物體的上，是制作出來的圖像很有動態(tài)感。游戲軟件開發(fā)公司應(yīng)用這種技術(shù)來評估電腦游戲?？茖W(xué)家用它來詳細(xì)描述化學(xué)成分。建筑業(yè)用它來展示模擬大樓的全貌。工程師用它來設(shè)計新工具及其他應(yīng)用。在最近的十年，地球科學(xué)也足見開始在構(gòu)建3D地理模型。3D模型大致可分為兩類（1）實(shí)體模型，這些模型規(guī)定了實(shí)體本身的體積(像巖石一樣)，他們更逼真一些，但是很難去構(gòu)建。實(shí)體模大部分用在非視覺模擬的例子上，例如醫(yī)藥和

62、工程模仿。像計算機(jī)輔助設(shè)計（cad）和專門需要視覺的應(yīng)用上，例如追蹤射線，建構(gòu)實(shí)體幾何</p><p>　　由于物體的外形很大程度上取決于物體的外部形狀，邊界模型在電腦制圖中就很普遍。盡管這些物體是非流線的，但是2d平面仍能很好的解釋物體在制圖中的應(yīng)用。因?yàn)檫@些平面是無限的，所以數(shù)字估算值還是需要清晰嚴(yán)密。盡管一點(diǎn)帶面的想法在近幾年比較流行，多邊網(wǎng)狀模型的仍然是目前最普遍的例子。對于經(jīng)過許多拓?fù)渥兓淖冃伪砻?，?/p>

63、平集合仍是這些變形表面的有用表現(xiàn)形式。他轉(zhuǎn)型的過程中表示的對象(比如中間點(diǎn)坐標(biāo)的球和其周長點(diǎn)成一個多邊形表示一個球體,叫做異型。這一步應(yīng)用在多邊網(wǎng)狀模型的渲染中，對象是分解從抽象的表征(“原語”)如球體,錐等等，所謂的網(wǎng)格，就是互相連接的三角形。網(wǎng)格的三角形(例如,廣場)很受歡迎，他們已被證明是容易實(shí)施使用掃描的。多邊形變現(xiàn)形式是沒有使用所有的渲染技術(shù)的，在這些情況下，異性步驟是不包括從抽象變現(xiàn)形式到呈現(xiàn)場景的轉(zhuǎn)型。</p>

64、<p>　　在一個3D模型中包括兩種信息，幾何信息和拓?fù)湫畔?。幾何信息通常代表形? 位置和大小的歐氏空間，而拓?fù)湫畔⒄臼窃诓煌糠值?d模型中起連接作用的。3D模型本身是看不見的,但我們可以執(zhí)行呈現(xiàn)操作在不同級別的細(xì)節(jié)基于簡單的線框圖,或者陰影基于不同的方法。在此，渲染的過程就是從一個模擬的計算機(jī)程序中生成一個圖像。3D模型就是在一個嚴(yán)格的定義語言過數(shù)據(jù)結(jié)構(gòu)中對一個3d對象進(jìn)行描述，她可能包括幾何，觀點(diǎn)，紋理，照明和著色

65、信息。生成的圖像是一個數(shù)字圖像或光標(biāo)圖形圖像。它可以被定義為“藝術(shù)家的渲染”的一個場景。渲染也用來形容這個過程的計算效果在一段視頻編輯文件生成最終的視頻輸出。</p><p>　　著色是一個在紙上描繪出黑色的水平線通過運(yùn)用媒體更多的入口或者在較黑的地方著上較黑的顏色，或者在較淡顏色的地方著上較淡的顏色。在計算機(jī)圖形學(xué)中,著色是指改變顏色的過程根據(jù)其角燈和它的距離,從燈光到創(chuàng)建一個逼真的效果。陰影渲染是在此過程中執(zhí)

66、行的。然而，大量的3D模型都覆蓋著紋理,我們稱這個過程為紋理映射。它是一個方法來添加細(xì)節(jié),表面紋理、或顏色計算機(jī)生成的圖形或3D模型。將其應(yīng)用到3D繪圖首創(chuàng)的埃德溫·卡特莫爾博士在他1974年的博士論文。一個紋理是應(yīng)用(映射)到表面的形狀或多邊形。這個過程是類似的應(yīng)用一個簡單的白色圖案紙箱子。從而產(chǎn)生的像素在屏幕上的計算出的texel(紋理像素)是由材質(zhì)過濾。最快的方法是使用最近鄰插值技術(shù),而雙線性插值和trilinear插值

67、之間mipmaps是兩種常用的替代方案可以減少別名或鋸齒。在事件的紋理坐標(biāo)之外,質(zhì)地,要么是加強(qiáng)或包裝。</p><p><b>　　3D模型建模</b></p><p>　　當(dāng)我們使用電腦來分析和研究客觀事物,采用合適的模型來表示實(shí)際的對象或抽象的現(xiàn)象是很重要的。這過程稱為建模。在3D電腦圖像,3D建模[6]程序開發(fā)一個數(shù)學(xué)、線框圖表示任意3D對象(要么是無生命的或

68、生活)通過特殊的軟件。它可以顯示為一個2D圖像通過這一過程被稱為3D渲染或用于計算機(jī)模擬物理現(xiàn)象。這個模型也可被身體使用了3D印刷設(shè)備。模型可以自動或手動創(chuàng)建。手動建模制備過程中幾何數(shù)據(jù)為3D電腦圖像類似于塑料藝術(shù)如雕刻。3D建模中起到了重要作用架構(gòu),醫(yī)學(xué)成像、文物保護(hù)、3D動畫3D游戲,電影的技術(shù)的令人眼花繚亂的制作,等等。3D掃描儀和圖像采集系統(tǒng)正在迅速成為更多負(fù)擔(dān),讓建筑的高度精確的模型的實(shí)時3D對象成本和省時有效的方式。 構(gòu)建3

69、D模型對實(shí)際對象,我們必須首先獲得相關(guān)的屬性的樣品, 如幾何形狀和表面紋理。數(shù)據(jù)的記錄,這些信息被稱為 D數(shù)據(jù),和3D數(shù)據(jù)采集的過程是3D信息中獲得樣品和有組織的代表與樣品一致結(jié)構(gòu)。這個方法獲得3D信息從樣品分為以下五類:</p><p>　?。?）方法基于直接設(shè)計或測量。它們常常用于早期的架構(gòu)的3D建模。他們利用工程繪圖來獲得每個模型的三個視圖。</p><p>　?。?）基于圖像的方法

70、。他們構(gòu)造3D模型基于圖像。他們首先獲得幾何和紋理信息同時被拍照,然后構(gòu)造3D模型基于獲得圖像。</p><p>　?。?）Mechanical-probe-based方法。 他們獲得表面數(shù)據(jù)身體上的接觸在探測器和對象之間。他們要求對象持有一個特定的硬度。</p><p>　?。?）方法基于體數(shù)據(jù)恢復(fù)。他們采取了一系列切片物體的圖像恢復(fù)3D對象的形狀。它們常常用于醫(yī)療部門與x射線切片圖像,

71、CT圖像和捷運(yùn)圖像。</p><p>　　（5）Region-scanning-based方法。 他們獲得這個職位的每個頂點(diǎn)在通過估算的空間之間的距離,測量儀器和每個點(diǎn)在物體表面上。兩個例子是光學(xué)三角法的方法和干涉。</p><p>　　3D建模主要問題是渲染3D模型基于三維數(shù)據(jù)。達(dá)到一個更好的視覺效果,我們應(yīng)該保證有光滑的表面,無毛刺和漏洞,讓3D模型體現(xiàn)了第三個維度和對現(xiàn)實(shí)的感覺。同時

72、,我們要組織數(shù)據(jù)以更好的方式減少存儲空間和加快顯示。目前的模型技術(shù)主要可分為三類:幾何建模,3D瀏覽建模和圖像,如下。</p><p><b>　　幾何建模技術(shù)</b></p><p>　　幾何模型是應(yīng)用數(shù)學(xué)的一個分支和計算幾何形狀的研究方法和算法的數(shù)學(xué)描述形狀。研究了形狀幾何建模大多是2D和3D,雖然它的許多工具和原理可以應(yīng)用到任何有限集維度。今天,大部分的幾何建模

73、過程是通過電腦和基于計算機(jī)的應(yīng)用程序。2D模型中重要的電腦排版和技術(shù)圖紙。3D模型是CAD / CAM的中心,廣泛應(yīng)用于許多應(yīng)用技術(shù)等領(lǐng)域的民事和機(jī)械工程,體系結(jié)構(gòu)地質(zhì)和醫(yī)學(xué)圖像處理。幾何模型通常有別于過程和面向?qū)ο蟮哪Ｐ?它定義通過一個不透明的形狀隱式算法,生成它的外觀。他們也與數(shù)位影像和體積模型代表了嗎作為子集的形狀微細(xì)的常規(guī)分區(qū)空間不足,與分形模型給一個無限遞歸定義的形狀。然而,這些區(qū)分例如,一個數(shù)字圖像可以被解釋為一組各種顏色的

74、方塊,和幾何圖形(如圈被定義為隱式的數(shù)學(xué)方程。同樣,一個分形模型得到的參數(shù)或隱式的模型時,其遞歸定義是截斷為一個有限的深度。一個幾何建模技術(shù)是指從線框圖建模順利發(fā)展曲面造型,造型厚實(shí),那里的表示幾何量信息變得越來越精確,所提供的“設(shè)計”的問題我們能夠解決的是廣泛的。這三個建模技術(shù)能夠說明如下。</p><p>　　（1）線框模型。線框圖模型是一種視覺呈現(xiàn)3D或物理對象用于3D電腦圖像。它是由指定每個邊緣的物理對象

75、,其中兩個連續(xù)光滑表面的數(shù)學(xué)滿足,或者通過連接物體的組成頂點(diǎn)使用直線或曲線。對象會被投射到上的電腦畫線條在每個邊的位置。使用一個線框模型允許可視化底層設(shè)計結(jié)構(gòu)的三維模型。傳統(tǒng)的2D視圖和圖紙能是由適當(dāng)?shù)男D(zhuǎn)對象和選擇隱藏的行通過減少飛機(jī)切除。因?yàn)榫€框圖呈現(xiàn)相對簡單和快速計算,通常的情況下,有一個很高的屏幕的幀速率是必需的(為舉例來說,當(dāng)處理一個特別復(fù)雜的3D模型,或者在實(shí)時系統(tǒng)模型表面現(xiàn)象)。更大的圖形細(xì)節(jié),是理想的,表面紋理可以添加自

76、動的完成后的最初的渲染的線框圖。這允許設(shè)計者快速瀏覽更改或旋轉(zhuǎn)對象所需的新視圖不長延遲引起更多現(xiàn)實(shí)的呈現(xiàn)。線框圖的格式也適合和廣泛應(yīng)用于編程工具路徑直接數(shù)控(DNC)機(jī)床。</p><p>　?。?）表面建模。與線框模型、表面模型的介紹“面”的概念。這是一個數(shù)學(xué)技術(shù)solid-appearing代表對象表面建模是一種更復(fù)雜的方法用于表示對象比線框圖建模,但不一樣復(fù)雜的實(shí)體建模。曲面造型廣泛應(yīng)用于CAD插圖和建筑效

77、果圖。它也可以用于在3D動畫游戲和其他講話。盡管表面和固體型號出現(xiàn)相同的屏幕上,他們有很大的不同。表面模型不能被切開作為實(shí)體模型。此外,在曲面造型,對象可以幾何不正確的,然而,在實(shí)體建模,它必須是正確的。典型表面建模技術(shù)可描述如下。</p><p>　　1）多邊形建模。在3D電腦圖像,多邊形建模是一個方法通過近似建模對象代表或它們的表面使用多邊形。多邊形建模非常適合用來掃描行呈現(xiàn)和是因此,選擇用于實(shí)時計算機(jī)圖形學(xué)

78、。我們將討論這種模型下一小節(jié)中詳細(xì)介紹。</p><p>　　2）NURBS建模。 非均勻有理b樣條(NURBS)是一個常用的數(shù)學(xué)模型用于生成和計算機(jī)圖形學(xué)代表的曲線和面提供極大的靈活性和精度同時處理分析和自由的形狀。NURBS的發(fā)展開始于1950年代由工程師需要一個數(shù)學(xué)精度自由表面的表示像那些用于船體、航空外觀表面和汽車機(jī)構(gòu), 每當(dāng)從技術(shù)上講需要，它們可以被精確地復(fù)制。這一發(fā)展的先驅(qū)是皮埃爾·貝茲曾是

79、一工程師在雷諾,和保羅?德曾在雪鐵龍,無論是在法國。貝塞爾幾乎平行工作Casteljau,既不了解其他的工作。但因?yàn)锽ezier結(jié)果發(fā)表他的工作,一般的計算機(jī)圖形用戶今天認(rèn)識到——這為花鍵控制點(diǎn)躺了曲線本身- - -貝塞爾曲線插值,而Casteljau的名字是唯一已知的和用于他開發(fā)的算法來評價參數(shù)的表面。在1960年代,它越來越清晰的表明,NURBSs是花鍵的泛化貝塞爾曲線,它可以被視為制服,NURBSs。起初,非均勻有理b樣條只有用在

80、專有的CAD軟件包的汽車公司。后來他們成了標(biāo)準(zhǔn)的計算機(jī)圖形包。1985年,第一個交互式NURBSmodeler為個人電腦、叫Macsurf(后來Maxsurf)發(fā)明的形成設(shè)計系統(tǒng),一個小型創(chuàng)業(yè)公司,總部在澳大利亞。M</p><p>　　3）細(xì)分曲面建模。細(xì)分曲面建模,田野里的3 D電腦圖像,是一個平滑的表面的表達(dá)方式通過規(guī)范的一個較粗分段線性多邊形網(wǎng)格。表面光滑,可以從粗網(wǎng)格計算的一個遞歸程序的限制的每個多邊形

81、的臉分成更小的細(xì)分的面孔,更好的近似的光滑的表面。細(xì)分表面是遞歸地定義。這個過程開始與一個給定多邊形網(wǎng)格。一個優(yōu)化方案被應(yīng)用于這個網(wǎng)格。這一過程需要網(wǎng)狀和細(xì)分,創(chuàng)造了新的頂點(diǎn)新面孔。這個位置的新網(wǎng)格頂點(diǎn)計算基于舊的頂點(diǎn)的位置附近。在一些改進(jìn)方案的位置老頂點(diǎn)也可能改變(可能是基于新的頂點(diǎn)的位置)。這個過程可以產(chǎn)生一個網(wǎng)格密度比原來的,包含更多多邊形面。這個結(jié)果網(wǎng)格可以通過相同的細(xì)化計劃再次。細(xì)分曲面的限制是表面獲得的迭代過程應(yīng)用無限多倍。

82、在實(shí)際使用中,然而,該算法只適用于有限的次數(shù)。</p><p>　?。?）實(shí)體建模。實(shí)體建模是明確的表示固體部分的一個對象,這意味著模型適合計算機(jī)的固體物體處理。我們知道,表面模型被廣泛地用于汽車和消費(fèi)者產(chǎn)品設(shè)計以及娛樂的動畫,而線框圖模型是固體體積模棱兩可。主要使用實(shí)體建模為CAD、工程分析、計算機(jī)圖形和動畫、快速原型,醫(yī)學(xué)檢測,產(chǎn)品可視化和可視化的科學(xué)研究。</p><p><b

83、>　　掃描技術(shù)</b></p><p>　　一個3D掃描是一種設(shè)備,分析一個真實(shí)世界的對象或環(huán)境收集了形狀甚至它的外觀(例如,顏色)。收集到的數(shù)據(jù)可以用來構(gòu)建數(shù)字,3D模型用于各種各樣的應(yīng)用程序。這些設(shè)備都被廣泛使用的娛樂產(chǎn)業(yè)生產(chǎn)的電影和視頻游戲。其他常見的應(yīng)用技術(shù)包括工業(yè)設(shè)計、矯正器和假肢,逆向工程和原型設(shè)計、質(zhì)量控制、檢查的文物?？梢允褂迷S多不同的技術(shù)來構(gòu)建這些3D掃描設(shè)備,每個未來有自己的

84、局限性、優(yōu)勢和成本。人們該記得許多限制類型的對象,可以數(shù)字化仍然存在:例如,光學(xué)技術(shù)遇到許多困難與發(fā)光,鏡像或透明的對象然而,有很多方法來掃描閃亮對象,如覆蓋一層薄薄的白色粉末,這將幫助更多的光光子反射回掃描儀。激光掃描儀可以發(fā)送數(shù)萬億美元對一個對象的可見光子,而只接受一個小比例的那些光子通過光學(xué)回來,他們使用。一個對象的反射率的基礎(chǔ)在對象的顏色或地球反照率。一個白色的表面將反映出很多光和一個黑色的表面將只反映出了少量的光。透明對象(如

85、玻璃只會折射光線,從而給出虛假的3D信息。對于大多數(shù)情況,一個掃描從許多不同的方向通常會被要求以獲取所有各方的主題。要把這些掃描到一個共同的參照系統(tǒng),這個過程通常稱為校準(zhǔn)或登記,然后被合并到創(chuàng)建一個完整的模</p><p>　?。?）聯(lián)系。3D掃描探針接觸主體通過身體上的接觸。一個坐標(biāo)測量機(jī)(CMM)是一種典型的接觸3D掃描儀。這是通常在制造業(yè)和可以非常精確。不利的能力成熟度模型是他們需要接觸被掃描的物體。因此,

86、掃描操作可能會修改或損害對象。這一事實(shí)是非常當(dāng)掃描顯著清淡或貴重物品如歷史文物。其他的缺點(diǎn)是它們的能力成熟度模型相比,相對緩慢其他掃描方法。身體移動手臂,探測器安裝在可能非常慢和最快的能力成熟度模型可以只作用于幾百赫茲。相比之下,一個光學(xué)系統(tǒng)像一個激光掃描儀可以操作從10到500千赫。其他的例子是hand-driven觸摸探測器用來數(shù)字化粘土模型計算機(jī)動畫產(chǎn)業(yè)。</p><p>　?。?）非接觸式活躍。活躍的掃描

87、儀發(fā)出某種輻射或光線并探測它的反射為了探索一個對象或環(huán)境?？赡艿念愋褪褂玫呐欧虐ü狻⒊暫蛒射線。例如,兩個飛行時間和三角的三維激光掃描儀是積極使用激光掃描儀燈探測主題或環(huán)境。飛行時間范圍的優(yōu)勢撿到的是他們能夠操作在一個相當(dāng)長的距離,在順序公里。這些掃描器掃描大量結(jié)構(gòu)從而適合像建筑或地理特征。飛行的缺點(diǎn)測距儀是他們的準(zhǔn)確性。由于這個較高的光的速度、時間往返時間困難的準(zhǔn)確性和測量的距離也相對較低,在以毫米為單位的。三角測距儀則完全相反。

88、他們有一個有限的范圍內(nèi)一些米,但他們的準(zhǔn)確性是相對較高的。這個三角測距儀的精度以數(shù)以萬計的微米。</p><p>　?。?）非接觸式被動。被動掃描儀不發(fā)出任何輻射本身,而是依靠反映環(huán)境輻射檢測。大多數(shù)這種類型的掃描儀檢測可見光,因?yàn)樗且粋€現(xiàn)成的環(huán)境輻射。其他類型的輻射,例如紅外線,還可以用。被動方法可以很便宜,因?yàn)樵诖蠖鄶?shù)情況下,他們不需要特別的硬件。例如,立體系統(tǒng)通常使用兩個攝像頭,微微分開,查看相同的場景。

89、通過分析的細(xì)微差異圖像之間被每個相機(jī),就可以確定距離圖像中的每個點(diǎn)。這種方法基于人類立體視覺。在相反,光度系統(tǒng)通常使用一個相機(jī),但把多個圖像在不同光照條件的。這些技術(shù)嘗試調(diào)轉(zhuǎn)圖像形成模型以恢復(fù)表面取向各像素。另外,silhouette-based 3D掃描儀使用提綱的產(chǎn)生,一個序列在一個3D對象的照片與可視背景。這些剪影是擠壓交叉形成視覺船體的近似該對象。然而,某些類型的凹陷對象(如內(nèi)部的碗)不能使用這些技術(shù)。</p>&