【双语阅读】计算机图像生成技术.

　　电影《美国上尉》、《创战记》和《本杰明?巴顿奇事》表明计算机图像生成技术能以惊人的逼真手法使演员看上去比实际更年轻、老态或懦弱。 这种效果至少在那些被改造过的演员与观众保持合适距离时能够实现。 皮肤和面料纹理重建的困难在于它们被放大观看时，效果并不理想。

　　Science and Technology

　　Computer-generated imagery

　　Fabricating fabric

　　How to generate more realistic images of clothes

　　FILMS like "Captain America", "Tron Legacy" and "The Curious Case of Benjamin Button" have shown that it is possible to use computer-generated imagery (CGI) to make actors look younger, older or wimpier than they actually are, in a surprisingly realistic manner. At least, it is possible if those altered actors are kept at a suitable distance from the viewer. The difficulty of recreating the textures of both skin and fabric means the fect is less convincing when seen close up.

　　The reason is that, whereas it is possible to simulate realistically the forces which make virtual skin and fabric hang, bend, flap and stretch, recreating the subtle ways they rlect light has so far proved extremely tricky. The shimmer and sheen of both fabric and skin depend on the geometry of their internal structures—the exact arrangement of threads or protein fibres. This is hard to model accurately. Steve Marschner and his colleagues at Cornell University have, though, come up with a way to get round that problem. Instead of modelling, they are copying. They are using computerised tomography (CT) to analyse the structures of fabrics at high resolution and then plugging the results into CGI. That, allied to the laws of optics and some heavy-duty computer power, seems to do the trick.

　　Computerised tomography is most familiar as a medical technique for examining people&aposs insides. Like classical radiology it uses X-rays. But because the image is constructed inside a computer using shots taken from many different directions, rather than being a single exposure recorded on photographic film, CT can capture fine detail and record soft tissues that are invisible to classical radiology.

　　Dr Marschner and his colleagues used a benchtop version of CT, developed for looking at the structure of materials rather than at human bodies, for their experiment. Employing doses of X-rays many times stronger than those used to study people, they obtained high-resolution information about small pieces of fabric. Computerised tomography allows the three-dimensional structure of the fibres in such scraps to be recorded, with all their kinks and imperfections. A number of small pieces can then be patched together into an entire garment inside a computer, in the same way that a handful of actors are turned into a CGI crowd. But because the internal structure of each bit of the garment matches that of a real piece of cloth, the way light will play on it can be calculated far more realistically than if it were just a computer model of what the interior of cloth is thought to look like.

　　Demonstrating the results of their technique at the SIGGRAPH computer-graphics conference in Vancouver this week, Dr Marschner and his colleagues showed realistic renderings of felt, gaberdine, silk and velvet. Moreover, their renderings remain realistic even when viewed close up. Sadly, skin is still beyond them. The high intensity of the X-rays involved would be too damaging for use on a living human being, and a corpse would probably not produce the right results. But once the rendering technique has been speeded up (at the moment it is still a bit slow and clunky), the swish of a virtual cloak or the doffing of a computerised hat should look far more realistic than it does now.

　　In the meantime, according to Dr Marschner&aposs colleague Kavita Bala, the technology might have an application in online retailing. At the moment, people buying clothes over the internet have only standard photographs to help them choose their purchases. Using CT-based computer graphics might, paradoxically, give a better idea of what the material an item of clothing is made from is really like than can be garnered from a boring, old photograph of the original.

　　【中文对照翻译】

　　科技

　　计算机图像生成技术

　　虚拟面料

　　如何生成更逼真的衣物图像

　　电影《美国上尉》、《创战记》和《本杰明?巴顿奇事》表明计算机图像生成技术能以惊人的逼真手法使演员看上去比实际更年轻、老态或懦弱。 这种效果至少在那些被改造过的演员与观众保持合适距离时能够实现。 皮肤和面料纹理重建的困难在于它们被放大观看时，效果并不理想。

　　原因在于尽管计算机能够真实地模拟使虚拟皮肤及面料产生悬挂、弯曲、飘动和拉伸的外力，但迄今为止想要找到构建反射光的精妙方法仍是极为困难的事情。 皮肤和面料的微光及光泽都取决于它们内部的几何结构——织线或蛋白纤维的精确排列。 这种模型很难精确地建立。 不过康奈尔大学的史蒂夫?马施纳及其同事们发现了一个解决此问题的方法。 他们不是建立模型而是采用拷贝技术。 他们利用电脑断层扫描术(简称CT)以高分辨率分析面料的结构，然后将结果输入到计算机影像生成系统。 结合光学定律和强大的计算机处理能力，这种方法似乎能够达到理想效果。

　　"电脑断层扫描术"是最广为人知的医学技术，主要用于人体内部检查。 与传统放射学相同，它也使用X射线。 但是因为计算机构建内部图像时采集的是多角度拍摄数据，而不是靠摄影胶片上的单一曝光影像，所以电脑断层扫描术能够捕捉细节并记录下传统放射学无法看到的软组织。

　　马施纳博士及其同事们采用了台式版本的CT进行实验，这种CT是被研制用来观察材料结构而非人体的。 使用比人体研究高数倍的X射线剂量，他们获得了小块面料的高分辨率信息。 "电脑断层扫描术"能够记录零碎面料的三维结构和所有的扭结和缺陷。 然后计算机将大量细碎面料拼凑成一件完整的衣服，采用同样方法，可将少数演员转换成电脑合成人像。 但是因为这件服装每片面料的内部结构都与真实布料匹配，因此与仅是看上去象布料的计算机衣服模型相比，光线在其上的表现效果能够更加逼真。

　　本周，马施纳博士及其同事们将他们的技术成果在温哥华举行的计算机图形会议上展示，并向人们演示了毛毡、华达呢、丝绸和天鹅绒的逼真效果图。 此外，他们的效果图即使被放大观看依然很逼真。 但可惜的是，他们仍然无法让皮肤实现这样的效果。 因为高强度的X射线会对人体造成很大损伤，而采用尸体实验可能得不到正确的结果。 但是一旦绘图技术突飞猛进(目前技术仍有些缓慢和粗陋)，虚拟斗篷的抖动或电脑生成帽子的脱落动作会比现在看上去更为逼真。

　　同时，马施纳博士的同事卡维塔?巴拉表示这项技术可能会在网上零售业得到应用。 目前，人们在网上购买衣物仅能通过浏览标准图片帮助选购。 相反，基于电脑断层扫描术的计算机图形技术可能提供了一个更好的办法，人们能够看到一件衣服所使用材料的真实质地，而无须将令人生厌，老旧的原物照片收集来观看。

　　电影《美国上尉》、《创战记》和《本杰明?巴顿奇事》表明计算机图像生成技术能以惊人的逼真手法使演员看上去比实际更年轻、老态或懦弱。 这种效果至少在那些被改造过的演员与观众保持合适距离时能够实现。 皮肤和面料纹理重建的困难在于它们被放大观看时，效果并不理想。

　　Science and Technology

　　Computer-generated imagery

　　Fabricating fabric

　　How to generate more realistic images of clothes

　　FILMS like "Captain America", "Tron Legacy" and "The Curious Case of Benjamin Button" have shown that it is possible to use computer-generated imagery (CGI) to make actors look younger, older or wimpier than they actually are, in a surprisingly realistic manner. At least, it is possible if those altered actors are kept at a suitable distance from the viewer. The difficulty of recreating the textures of both skin and fabric means the fect is less convincing when seen close up.

　　The reason is that, whereas it is possible to simulate realistically the forces which make virtual skin and fabric hang, bend, flap and stretch, recreating the subtle ways they rlect light has so far proved extremely tricky. The shimmer and sheen of both fabric and skin depend on the geometry of their internal structures—the exact arrangement of threads or protein fibres. This is hard to model accurately. Steve Marschner and his colleagues at Cornell University have, though, come up with a way to get round that problem. Instead of modelling, they are copying. They are using computerised tomography (CT) to analyse the structures of fabrics at high resolution and then plugging the results into CGI. That, allied to the laws of optics and some heavy-duty computer power, seems to do the trick.

　　Computerised tomography is most familiar as a medical technique for examining people&aposs insides. Like classical radiology it uses X-rays. But because the image is constructed inside a computer using shots taken from many different directions, rather than being a single exposure recorded on photographic film, CT can capture fine detail and record soft tissues that are invisible to classical radiology.

　　Dr Marschner and his colleagues used a benchtop version of CT, developed for looking at the structure of materials rather than at human bodies, for their experiment. Employing doses of X-rays many times stronger than those used to study people, they obtained high-resolution information about small pieces of fabric. Computerised tomography allows the three-dimensional structure of the fibres in such scraps to be recorded, with all their kinks and imperfections. A number of small pieces can then be patched together into an entire garment inside a computer, in the same way that a handful of actors are turned into a CGI crowd. But because the internal structure of each bit of the garment matches that of a real piece of cloth, the way light will play on it can be calculated far more realistically than if it were just a computer model of what the interior of cloth is thought to look like.

　　Demonstrating the results of their technique at the SIGGRAPH computer-graphics conference in Vancouver this week, Dr Marschner and his colleagues showed realistic renderings of felt, gaberdine, silk and velvet. Moreover, their renderings remain realistic even when viewed close up. Sadly, skin is still beyond them. The high intensity of the X-rays involved would be too damaging for use on a living human being, and a corpse would probably not produce the right results. But once the rendering technique has been speeded up (at the moment it is still a bit slow and clunky), the swish of a virtual cloak or the doffing of a computerised hat should look far more realistic than it does now.

　　In the meantime, according to Dr Marschner&aposs colleague Kavita Bala, the technology might have an application in online retailing. At the moment, people buying clothes over the internet have only standard photographs to help them choose their purchases. Using CT-based computer graphics might, paradoxically, give a better idea of what the material an item of clothing is made from is really like than can be garnered from a boring, old photograph of the original.

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