摘要: 本文概述了RenderMan点云的一些特性，如何渲染点云，提供RM以及GPU渲染的效果对比。  阅读全文

最近几个星期工作比较忙，博客没有更新。这个星期最大的两个成果一个是RSL到NVIDIA Cg的转换器的完工，还有一个就是能够将RenderMan分裂的的所有Grid以及Micropolygon弄出来，这样就可以实现Lightspeed中的Indirect Framebuffer，由此一来运动模糊、景深、乱续采样就都可以在GPU上完成了，一切的一切都在计划中，追赶上ILM与PIXAR的工具只是努力与时间的问题了。当第一阶段完工后将放上详尽文章，敬请期待。

　　从事先bake好ST的点云中构造切线。说起来极其简单，自适应的根据点的数目生成一个类似于棋盘格的加速结构，随后在每个网格内部进行搜索。示意图如下，

　　因为标记为Best的那个样本最接近于vec2(1,0)，也就是保证了一个方向上的连续性，计算结果为当前采样点（五角星表示）的dPds。相同的道理，我们也可以得到dPdt。当然，我们也可以在RSL中手工计算这两个分量。

　　在我的笔记本上T2250＋2G的机器上跑了3秒，在E6550＋2G的台式机上跑了1秒，速度还是可以的。生成的图如下，注意为了显示已经把向量数值缩放。

　　这样计算对ST分布有要求，就是不能颠倒物体方向，导致P不能在ST空间内连续，所以模型上肯定会有一块不会连续了。或者更加智能的，bake出ObjectID一样的玩意，这样就在物体的每个部分内独立计算切线了。

 

 

 

 

花开花落，阳春三月，随身背着相机在学校里游走，不断的寻找视角。知道自己拍的不好，总觉得自己拍的片有各式各样的缺陷，也许这就是大师与学徒的区别吧。用好手头的装备，出好片，锻炼Visual Effect TD的内在素养，这才是学习的正道。君不见好莱坞那么多视觉TD都能拍的一手好片，所以，这些都是融合到人家的日常生活中去的，这也是受过艺术训练的专业人员与普通劳工的区别。

用同事的适马105/2.8微距拍的玩具，不得不承认，除了德系装备，“日本光学”的素质还是比其他的镜头要高一些的

这是在亚马逊上找到的封面，人家都第四版本了，手头拿到的还是第二版本。我还是在林业大学，学的还是木材专业，此时只能用不是滋味来形容我的心情。
老师上课说啥，这是内部交流刊物，我当即表示嗤之以鼻，这些人的思想还停留在上古时代，根本不知道分享知识的意义与价值，还基本停留在科技官僚的时代。这里是下载地址，感兴趣的朋友可以没事研究一下，以后打家具弄装潢可就非常自信了，再也不会被那些居心不良的木材销售商家给忽悠了。

简单的尝试了下用Lafortune BRDF渲染纸张。为了公平起见统一把Ka设为0，Kd设为1.0，没有Specular项。使用3Delight的免费Licence和学习版maya 8.5，所以没有版权问题，但没有出阴影，所以看起来比较奇怪。效果对比如下：
左边是用内建matte材质，也就是Lambert，右边的就是Lafortune了，使用了2个lobe。右边的Fresnel效果更加的强烈。

过一阵子准备尝试使用高级BRDF模型，如这个赫赫有名的Lafortune BRDF，调整参数极其的不直观 —— 27个系数形成的矩阵表示了材质各向同性与各向异性的特性。康奈尔的站点上提供的BMRT的RIB，可惜现在BMRT早已经被NVIDIA和法院“和谐”掉了，想要下载很麻烦。这是使用免费的3Delight渲染出的图，修改过一些Attribute。下面准备尝试使用康奈尔人做实验测量出来的BRDF数据渲染。

K-Means算法是一种比较有意思的数据簇算法，原始的意义是将n个物体分为k个区域，并且k<n。当我看到这个算法第一个想法就是是不是和Voronoi图相关，事实上的确是的，两个簇边界上的点大体构成Voronoi图的剖分线段。这个算法广泛应用于分子生物学，以及CG —— 康奈尔的那帮家伙用这个算法加速间接照明部分的计算，很是强大。

This is a very simple blobby scene rendered by 3Delight with the free licence in my laptap PC.
I will dive into several CFD papers from Stanford University, UC Berkeley and ILM, try to use blobby to handle the fluid effect.

开源软件很好，可惜想编译的话，如果软件自己又依赖于其他平台或者是其他的库，就会很麻烦很麻烦，误工误时不说，心情还烦躁。PBRT是个好东西，可是编译起来需要先安装cygwin的相关组建，也就是编译器的编译器bison、Yacc、Flex相关的黑客玩意，很是麻烦。琢磨了一下编译好了PBRT，提供给大家下载。

I hate to recompile opensource software, spent lots of time on configuation hell. Furthermore, we can not learn anything usefull except being more familiar with our IDE.

If the program shows that missing some dll such as OpenEXR or the others, please download them manually. My environment is not the same as yours, so I can't be sure that you can running this little program as free as me.

Download Here

Enjoy !

声明：所有内容均搜索自互联网，如有违反相关法律，本人第一时间撤下所有有争议的内容

搜索到了VS2008 Team英文正式版本的BT种子文件，祝广大开发人员新春愉快幸福健康！

下载速度很快，我1M的ADSL花费了17个小时平均速度70KByte/s，主要是中途速度有些慢，到后面就一直保持极限速度。已经刻录为DVD保存！

推荐使用Ayu版本的迅雷绿色版本，点击这里下载

Google很容易就搜索到，虽然说Rank很高，可是高处不胜寒，连连叫嚣bullshit的人不少，算了，我还是不献丑了

我喜欢根据那些论文，提取出重要的内容，当然包括数学公式，再加上现有的实现法则，应该大部分都算是Introduction与Tutorial，为什么要骂我呢？真受不了。

中文的有价值的CG资料那么少，基本上都是建模软件使用方面的，根本没有多少资料会谈到最新的CG技术成果，现在人家都开始为EG08写论文了。我做好事为大部分人总结，却找来“这么多数学能说明什么问题，你也不是数学家，纯属胡扯”这样的评论。

原谅我，我只是一个二流大学的本科生，还不是CG专业、数学专业的，高数分数也不高全靠自学，我却要去琢磨Pat Hanrahan、Joeg Kreuger这样的人的论文，我容易么我？有本事你做点事情到Siggraph上发表给我看看去，那时候我承认你骂我的是完全正确合理的，否则，你也就一个废柴。

Bo Schwarzstein

Copyright

The referred graphs have been announced.

We’d better view some principle about BSSRDF and human skin. Repeat, this is not a math hell. BSSRDF is not used for completely remove the BRDFs (include Phong, Blinn, Cook-Torrance etc.) from the earth. It’s only for correctly representing translucent material in computer graphics. If you want to learn something deepter, you’d better ask your solid physics or applied optics teacher. They will explain to you that all the problem is about the reaction between light energy and atom at quantum level. Oh, it sounds terrible, not easy at all. So, we will show something much easier than you dive into papers all by your self.

In computer graphics, we want to “correctly” illuminate the appearances of objects in scene, whatever from psychology, like Phong shading, or physical, just like the Monte Carlo method. So we have used some classic BRDF model in computer graphics. In fact, the BRDF is a approximation of BSSRDF, it is assumed that light enters and leaves at the same point [A Practical Model for Subsurface Light Transport]. But if we investigate the translucent objects near us, such as grape, marble, milk, skin, we will know that every lighten points we saw on them were effected by nearby point even inner of them when the light was traveling through them. The origin formula is:

 

 

If we want to solve equation is quite stupid. In reality, artist likes to face to many linear parameters, modifies them by simple arithmetic, or just in a compositing software such as Shake, After Effect, not these mathematics devils. So we have to separate something important parts from the origin formula. Let’s continue now.

The radiant exitance is our final wanted result, because we can use it as diffuse radiance data directly.

E_p is irradiance at the location p, calculated like this:

It means that E_p is the sum of ambient and diffuse components multiply with K_dt surface color, also the radiance diffusely transmitted through the surface. A_p is the area of the location. From simple to difficult, the last part can be expressed like this:

 

Oh, we attract more unknown variable quantities, before we can solve this, we have to get something ready:

 

And more:

Solving Rd is just for get α'. Because:

But this function is not easily to invertible, otherwise we can directly get everything. But we first should see its graph:

It has a simple monotonic shape in the valid region α' is in [0,1], we can use a secant root finder to compute α'.

Now, do not feel complicated about so many symbol. The most important parameters which dominate skin appearance are sigma_s' and sigma_a, luckily, these two parameters can be measured from experiment. So this is the plot from [Analysis of Human Faces using a Measurement-Based Skin Refectance Model]:

This plot gives us a very valuable reference to our deciding approximate parameters for different human skins. The circle shape encodes male the other is female. Mark size encodes age, the bigger the older. Color encodes skin appearance, from black, dark brown, to brown, white to olive, until white, very white.

Shot	Reduced Scattering Coeff.	Absorption Coeff.	Addtion
	0.74 0.88 1.01	0.032 0.17 0.48	Built-in skin1
	1.09 1.59 1.79	0.013 0.070 0.145	Built-in skin2
	1.4 1.18 1.3	0.07 0.23 0.46	Table Data
	2.18 1.67 1.9	0.025 0.18 0.4	Table Data

In Pixar RenderMan Pro Server there is a tool names ptfilter. We can use this command line tool to do sub-surface diffusion in a point cloud. Then we can make a brick file and use the illumination data in surface shader by texture3d() OP. We will touch 4 parameters if we want to get correct SSS effect manually. They are :

Unit Length, important physical-based parameter, depend on size of your 3D model, if you miss it and your scece unit is not mm, you will get wrong result.

Diffuse Mean Free Path, we use "DMFP" for short. This is a also physical-based parameter. This parameter is a attribute of translucency material surface. This parameter will be convert to Mean Free Path (MFP), means the average distance at which light scatters. This parameters will multiply with unit length above, so it’s physical preparation of material, even it’s spatially-varying.

Scattering Coefficient, just the σs', main parameters in controlling SSS effect, should be fixed before modify others.

Absorption Coefficient, the σa, as the same as σs'.

Albedo, for simple, it’s the diffuse color, to represent different degree of light scattering.

We have to know that this tool is based on dipole diffusion approximation algorithm in [A Practical Model for Subsurface Light Transport]. So it can't represent the effect such as refraction of light at the boundary casing internal caustics, translucent inter-scattering. For that we can use more advance method such as photon diffusion, but it have not been supplied public tool.

The next stage is about choosing which BRDF model used to fit the data. In [Analysis of Human Faces using a Measurement-Based Skin Reflectance Model], they have discussed enough about the BRDF model. The Torrance-Sparrow BRDF is physically-based, so it is quite suitable to represent the bump skin surface. There is nobody's face is perfect flat. This BRDF must be used spatially-varying, not uniform. So we have to reconstruct all the needed render data.

Refer to [Analysis of Human Faces using a Measurement-Based Skin Reflectance Model], artists should divide the whole face texture map into different part so that each region will be painted with correct BRDF parameters. And each region owns its distribution.

First, we should make sure what kind of human skin we want to simulate, male or female, how old, black or white. From these conditions we have had the bound of scattering parameter σa and absorption parameter σs'. General speaking, dark brown and black skin have higher absorption and lower scattering coefficients than white skin. It's natural that the absorption coefficient σa is highest for the blue and green channels, this is caused by blood in deep level of skin. From the function graph we know σa decreases with higher scattering coefficient σs'.

Then we can along the path showed by Christophe Hery in [Implementing a Skin BSSRDF]. When we have fixed the σs' , then we modify the σa, even the diffuse mean free path. But these parameters are quite unfriendly to artist. The origin data is measured in laboratory, is not intuitive. The less key parameters the artist will control, the more easier to handle. The index of refraction is fixed to 1.3 about many materials, such as skin, ketchup, skimmilk. We suggest that all of physical parameter such as DMPF, σs' and σa should be fixed by Technical Director (TD), then artist can convert approximate parameters into textures, combined with shading program, here are the channels baked by RenderMan shaders. Even TD manually write little C++ program to calculate these numbers by Pixar RenderMan SDK after discussing with artist or some specialist about material.

The perfect effect must be handed by artist. SSS is not the everything, a compellent cinematic effect comes from heavy compositing, high quality texture maps, accurate displacement, good lighting design, the SSS is just a good way to illuminate translucent materials. Good luck.

　　表哥买了台HP 3706，预装了鸡肋Vista Home Basic，只有C盘和D盘，其中D盘容量为9G是恢复分区，C盘达110G。谁也没有一个疯子会把东西都放到C盘里面，那样日后数据整理，故障恢复都极其不方便。所以么需要重新分区，琢磨了几次，发现了最好的分区方法。不过需要注意的是，DOS版本的PQ仿佛没有用，最新的番茄花园附带的PQ 8.05仿佛不认HP的分区，显示为全部占用，检测显示严重错误。如果有重要数据请备份到外部存储介质，如USB移动硬盘什么的。这种分区方法会把C盘恢复到出厂状态。

　　HP的恢复程序启动的是一个精简的Vista，而且没有锁定硬盘，随便你如何分都可以，把C盘删除也可以。开机直接按F11进入HP的恢复程序，出现第一个对话框后不要按“下一步”，直接按键盘Alt和D键调出命令行，使用DISKPART这个微软提供的命令行分区工具分区，但是要注意的是C盘不能小于30G，否则从恢复分区拷贝的数据不够放,3706出厂时C盘占用大概11G。

　　这样就可以分区了，而且保留了HP的恢复功能。不知道为什么这个破HP不允许装XP，驱动不完善BUG一堆，真让人生气。