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3S原理及应用2

(2007-04-23 13:36:49)
分类: 专业知识
 简单的"三维"半透明

For 3D objects, you need something a bit different.三维物体,你需要一些有点不同. Lets start with the straight-forward solution.让给先从简单的解决办法. Say for example, you have light enter a point on a surface, you then have the light continue to go through the object until it hits a back face, i.e., it leaves the object.譬如说,你轻点就进入了表面然后根据你继续办理,直到碰到一回面对象,即离开物体. You calculate a rough distance between those two points, the deeper the distance the less light works its way to the other side, and then you add that extra light to the other side of your surface.粗略计算,你们这两点之间的距离,距离越深,其作品结合的方式,对另一方,然后你补充说,鉴于额外的彼岸你表面. Grant Adam wrote a very interesting script that does something very similar, and can produce some nice results.亚当补助,并写了一首非常有趣的剧本很相似,可以产生一些很好的效果.

图片点击可在新窗口打开查看

Here's the max file max file , using max4.2, to make this image using Grant's Script.这里的马克斯马克斯档案文件,用max4.2,使这个图像给予的脚本.

A few disadvantages of this technique:这种技术的缺点数:

  • It assumes the substance is homogenous (ie, the same material all the way through), which means skin which has many layers of different materials doesn't work well.它的实质是假设均相(即同一物质的写法)即皮肤有许多层不同材料不做好.
  • It doesn't do back scattering, ie, light beams coming back towards the light source.不这样散射,即返往光束光源. As seen in the skin diagram above, a lot of light goes through the skin, say to the flesh layer, then gets sent back out the skin towards the light, but at a slightly different angle and position than it went in.从以上的图皮肤,很多指示灯就会透过皮肤,说肉层,然后送回得到了皮肤对轻工、但略有不同的角度和立场较来到进来
  • It's view-independent.它认为无关. Real translucency changes its look depending on where the camera is viewing the object from (view-dependance).真正半透明改变那里看视镜头是从观赏对象(视图依赖). This technique, since it's baking in the light into the surface, is view-independent, no matter where you look at it from, you'll be seeing the translucency "frozen in time" if you will.这种技术,因为它的光进入地表的烘烤,是针对独立、不管你从那里,您可以看到半透明的"冻结"如果你的意志.
The technique seem to be good for materials like wax, see below for some photos of a wax candle for comparison.技术似乎是好材料,如蜡、见下部分的蜡烛照片进行对比.

Full-Blown Volume Rendering全面爆发容积

For complete realism, what you actually want is a full volume render of your surface.完全写实,其实你想要的是一个完整的表面体积使你. When dealing with, for example, polygons, we're dealing with a surface, not a volume.在处理,例如,多边形,就要对付水面,不是数量. If you've ever taken a sphere and deleted some faces, you've noticed the sphere is in fact hollow.如果您删除一些领域迈出了有史以来脸上,你看到的其实是空心球. A volume means at any point inside that sphere, there is still sphere, not empty space.一卷意味着该领域内的任何时候,仍有空间,不留白. The two main problems with this approach though involve speed and the data set.这种方法主要有两个问题,虽然涉及速度和数据集. First, calculating a full volume gets very slow because of all the info you have.首先,计算量得到充分的信息很慢,因为你的一切. And then there's the question of how do you generate that information?再来是您的问题产生的资料? Say you have a human muscle you want to render volumetrically, you now need some way to specify information about every point inside your surface.你说你想使人类肌肉容积、你现在需要一些具体的资料,每点内你表面. Do you want to start modeling every atom that makes up your object?你想从模拟到每个原子弥补你对象? Because of these factors, volume rendering is still somewhat impractical for mimicking say the way light reacts to human skin (in the entertainment industry anyways, volume rendering is being heavily researched in the medical industry as it relates to the human body).由于这些因素,容积还是有点不切实际地说模仿人体皮肤反应轻(娱乐业anyways,容积正在大量的医学研究,因为它涉及到业人体). This desire to simplify is a lot like how the artistic wants to avoid worrying about the chemical composition of your material.这个愿望是简化了不少喜欢艺术怎么想避免你担心化学成分的物质. "Can't you just give me a button that makes it look cool?""你不能只给我一个按钮,轻松看酷"?

Kubelka-Munk Pigment Model贝尔卡--芒克颜料模式

Fortunately there are other ways to approximate translucent surfaces.幸好有其他方法可以近似半透明的表面. If you start doing some research through various papers, the first one you're likely to run into is the Kubelka-Munk Pigment Model.如果你开始做一些研究,通过各种文件第一个你可能碰到的是贝尔卡--芒克颜料模式. This was developed in the 30s to describe light bouncing off wall paint.这是描述三十年代研制轻型蹦墙涂料. The idea is, how much of what pigments do you need so that when you paint a wall with x thickness of paint you get the desired color.想法是,多少颜料您需要什么,让你当墙涂料油漆厚度×预期会给你颜色. Research was done, and some useful models were created.做过研究,并形成了一些有益的模式. For more information, check out Andrew Glassner's "Principles of Digital Image Synthesis" .更多资料,检查家glassner的"数字图像合成原理".

The method has several problems though, such as it assumes the substance is homogenous all the way through.虽然若干问题的方法,例如它假定所有的物质均匀地通过. It does not take into account layers of different materials or different densities in a single surface.它没有考虑到不同层次不同密度的单一材料或表面.

Hanrahan-Krueger Multiple Layer Model汉-克鲁格多层次模型

This updated model allows you to solve a similar equation for multiple layers.这个模型允许你更新解决类似方程式多层. Much like human skin, it does calculations as your light ray travels from one layer to another layer to another layer.犹如人体皮肤、它为您计算游记光线从一层到一层一层地. Layers are described in terms of their refraction, absorption and scattering coefficients, the depth of the layer, and a Henyey-Greenstien phase function.形容自己是在层层折射、吸收和散射系数、深度的层一henyey-greenstien相功能. Again, check out Glassner's book for more information.再看看glassner书更多资料.

Henrik Jensen's BSSRDF亨里克琴bssrdf

The most advanced and accurate research so far in this area is a now well known paper by Henrik Wann Jensen, inventor of the photon map.最先进和准确迄今在这一领域的研究,现在是一个著名的文件亨里克万琴、发明的光子图. This paper is entitled "A Practical Model For Sub-Surface Light Transport".本文的标题是"一个实际模型分表面轻运输".

His experimentation started with using a laser pointer to send a beam of light through a glass of milk (an experiment I recreate below).他开始试验用激光教鞭派手电光透过一杯牛奶(我再造一个实验段). He noticed how a beam would enter the milk and scatter reasonably uniformly through the material.他看到了如何进入牛奶和散射光束会通过合理的物质均匀.

After a lot of research with colleagues, he has now proposed a new method for simulating sub-surface scattering.经过大量的研究,与同事,他已经提出了一个新方法模拟亚表面散射. The problem with many previous single scattering methods is they don't work well with highly scattering materials (multi-scattering) such as milk and human skin.以往单一的问题,有许多方法是不分散工作与高散射物质(多散射),如牛奶、人体皮肤. These materials tend to scatter light in an isotropic fashion (i.e., uniformly in all directions).这些材料往往各向同性散射光的时装(即清一色四望). Methods for simulating this already exist using diffusion theory.使用这种模拟方法已有扩散理论. The answer Henrik found was to combine both methods, add a single scattering term to a diffusion term, to get the full effect.亨里克找到答案是两种方法相结合,加上长期单一的散射弥散任期得到全面落实. His results were physically correct while retaining reasonable speed.其结果是在保留肢体正确合理的速度.

His proposed BSSRDF (Bidirectional Surface Scattering Reflectance Distribution Function) made use of a generalization of the Hanrahan-Krueger Model for the direct component (due to single scattering), and a diffusion approximation (due to multiple scattering).他建议bssrdf(双向表面散射反射分布函数)运用了概括,对汉-克鲁格模型直接组成(由于单一散射)和扩散逼近(因多重散射). For example, human skin's diffusion term is very high, whereas it's single scattering term is very small.例如,人体皮肤的扩散来说是非常高,而它的单散射而言是非常小. A substance such as marble or wax will have a much larger contribution from the single scattering effect, and consequently less contribution from the diffusion component (in Henrik's experiments, he calculates that the two components are approximately equal to each other for marble).大理石等物质或蜡将有更大的贡献,从单一的散射效果,因而较少从传播内容(亨里克的实验他的如意算盘是大约相等的两部分互相大理石). See the leaf section below for a discussion of observing the two different types of scattering.叶科见下文讨论了两种不同类型的散射观测.

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