大三05計(jì)算機(jī)圖形學(xué)-88texture mapping

1、Texture Mapping紋理映射原著Ed AngelProfessor of Computer Science, Electrical and Computer Engineering, and Media ArtsUniversity of New Mexico編輯 武漢大學(xué)計(jì)算機(jī)學(xué)院圖形學(xué)課程組ObjectivesIntroduce Mapping Methods介紹各種映射方法Texture Mapping紋理映射Environment Mapping環(huán)境映射Bump Mapping凹凸映射Consider basic strategies考慮基本策略Forward vs back

2、ward mapping正向映射和逆向映射Point sampling vs area averaging點(diǎn)采樣與區(qū)域平均The Limits of Geometric Modeling幾何建模的局限Although graphics cards can render over 10 million polygons per second, that number is insufficient for many phenomena雖然圖形顯示卡可以每秒鐘顯示多達(dá)一千萬個多邊形，但這個速度仍不能滿足模擬許多自然現(xiàn)象和天然物質(zhì)的要求Clouds云Grass草Terrain地貌Skin毛發(fā)樹皮水波

3、與火焰Modeling an Orange橙子的模型Consider the problem of modeling an orange (the fruit)考慮如何建立橙子的模型Start with an orange-colored sphere用橙色的球表示橙子Too simple太簡單Replace sphere with a more complex shape用更復(fù)雜形狀代替球Does not capture surface characteristics (small dimples)沒有表示出表面的特性(微凹, dimples)Takes too many polygons

4、to model all the dimples為了模擬所有的微凹，需要相當(dāng)多的多邊Modeling an Orange橙子的模型(2)Take a picture of a real orange, scan it, and “paste” onto simple geometric model獲取真實(shí)橙子的照片，掃描后把結(jié)果“粘貼”到簡單的幾何模型上This process is known as texture mapping這個過程就是紋理映射Still might not be sufficient because resulting surface will be smooth可能

5、結(jié)果仍然不令人滿意，因?yàn)樗们媸枪饣腘eed to change local shape需要改變局部形狀Bump mapping凹凸映射Three Types of Mapping三種映射方法Texture Mapping紋理映射Uses images to fill inside of polygons利用圖像填充多邊形Environment (reflection) mapping環(huán)境映射(反射映射)Uses a picture of the environment for texture maps利用環(huán)境的圖像進(jìn)行紋理映射Allows simulation of highly spe

6、cular surfaces可以模擬高度鏡面曲面Bump mapping凹凸映射Emulates altering normal vectors during the rendering process在生成顯示結(jié)果的過程中可以改變法向量Texture Mapping紋理映射geometric model幾何模型texture mapped紋理映射后Environment Mapping環(huán)境映射Bump Mapping凹凸映射三種映射的共同點(diǎn)作為片元處理的一部分，改變單個片元的明暗值都與存儲為一維、二維或三維數(shù)字圖像的貼圖有關(guān)保持低的幾何復(fù)雜性，但能產(chǎn)生復(fù)雜幾何的視覺效果都可能產(chǎn)生走樣誤差Wh

7、ere does mapping take place?映射在什么地方進(jìn)行？Mapping techniques are implemented at the end of the rendering pipeline映射技術(shù)是在繪制流水線的最后階段實(shí)現(xiàn)的Very efficient because few polygons make it past the clipper非常有效，因?yàn)樵诮?jīng)過所有的操作后，減少了許多不必要的映射二維紋理紋理：以數(shù)組形式存儲在處理器內(nèi)存里的二維圖像，此時數(shù)組元素稱為紋素(texel)由成 應(yīng)用程序生成 或 照片掃描輸入把離散數(shù)組看成一個連續(xù)的二維矩形區(qū)域紋理圖案

8、T(s,t)，獨(dú)立變量s和t是紋理坐標(biāo)一般把紋理坐標(biāo)縮放到0,1區(qū)間Is it simple?很容易嗎？Although the idea is simple-map an image to a surface-there are 3 or 4 coordinate systems involved雖然映射的想法很簡單，即把圖像映射到曲面上，但由于這時要用到三、四個坐標(biāo)系，因此實(shí)現(xiàn)起來并不容易2D image3D surfaceCoordinate Systems坐標(biāo)系Parametric coordinates參數(shù)坐標(biāo)系May be used to model curves and surf

9、aces可以用來建立曲線、曲面 Texture coordinates紋理坐標(biāo)系Used to identify points in the image to be mapped用來識別要被映射的圖像上的點(diǎn) Object or World Coordinates世界坐標(biāo)系Conceptually, where the mapping takes place從概念上說，就是映射發(fā)生的地方 Window Coordinates屏幕坐標(biāo)系Where the final image is really produced最終圖像生成的地方Texture Mapping紋理映射框架parametric c

10、oordinatestexture coordinatesworld coordinateswindow coordinatesMapping Functions映射函數(shù)Basic problem is how to find the maps基本問題就是如何定義映射Consider mapping from texture coordinates to a point a surface考慮從紋理坐標(biāo)到曲面上一點(diǎn)的映射Appear to need three functions直觀地看，應(yīng)當(dāng)需要三個函數(shù)x = x(s,t)y = y(s,t)z = z(s,t)But we really w

11、ant to go the other way雖然從概念上講，最終必定用到上述的函數(shù)，但實(shí)際采用的是卻是間接的方法ts(x,y,z)Backward Mapping逆映射We really want to go backwards我們需要的是逆向操作Given a pixel, we want to know to which point on an object it corresponds給定一個像素，我們想知道它對應(yīng)于對象上的哪個點(diǎn)Given a point on an object, we want to know to which point in the texture it co

12、rresponds給定對象上的一個點(diǎn)，我們想知道它對應(yīng)于紋理中的哪個點(diǎn)Need a map of the form此時需要如下形式的映射s = s(x,y,z)t = t(x,y,z)Such functions are difficult to find in general這樣的函數(shù)一般是很難求出來的Two-part mapping兩步映射 One solution to the mapping problem is to first map the texture to a simple intermediate surface解決映射問題的另外一種方法,首先把紋理映射到一個簡單的中間曲

13、面上Example: map to cylinder例如：映射到圓柱上Cylindrical Mapping圓柱面映射parametric cylinder高為h，半徑為r圓柱的參數(shù)方程是x = r cos 2p uy = r sin 2puz = v/hmaps rectangle in u,v space to cylinder of radius r and height h in world coordinates映射函數(shù)s = ut = vmaps from texture space從紋理坐標(biāo)到圓柱面上沒有變形適合于構(gòu)造與無底的圓柱面拓樸同構(gòu)的曲面上的紋理Spherical Map

14、球面映射We can use a parametric sphere半徑為r的球的參數(shù)方程x = r cos 2puy = r sin 2pu cos 2pvz = r sin 2pu sin 2pvhave to decide where to put the distortion類似于地圖繪制中的映射，存在變形Spheres are used in environmental maps用在環(huán)境映射中Box Mapping立方體映射Easy to use with simple orthographic projection適合應(yīng)用于正交投影Also used in environment

15、maps也用在環(huán)境映射中Second Mapping兩步映射中的第二個映射Map from intermediate object to actual object從中間對象到實(shí)際對象的映射Normals from intermediate to actual從中間形狀的法向到實(shí)際對象Normals from actual to intermediate從實(shí)際對象的法向到中間形狀Vectors from center of intermediate從實(shí)際對象的中心開始的向量31其它類型的紋理立體紋理程序生成的紋理Procedural textureAliasing走樣Point sampling of the texture can lead to aliasing error