本人水平有限，若有错误也请指正~
上面说到pathtracing（pt）的一些优点和缺点，优点即其实现很简单，这就是大概为什么当今市面上流行的很多渲染器如今都相继采用pathtracing算法为核心进行实现，但是pathtracing的最大缺点就是收敛速度很慢，其原因就在于全局光照的那个积分式要求在半球面上连续采样，这就需要每次发射n条采样光线，n的数量直接决定了最终图像的质量（对于pt来讲它的图像质量取决于噪点数量的多少，噪点多的话有点像用老式胶片拍出来的照片那样），一般n越大图像过渡越光滑。前人学者们也早已发现了pt的这个收敛慢的缺点，于是有了多种方法来降噪，具体来讲这些降噪手段有以下几种：
1）改进pt算法。如后续比较经典的bidirectional pathtracing（bdpt），metropolis pathtracing
2）保持pt算法不变，但对其内部的采样机制进行改进。比如：
a. 根据BRDF函数进行重要性采样；
b. 采样时所用到的均匀分布，一般都是用c语言自带的rand函数或者c++ random库里面的uniform distribution来完成。但是这种采样方式对于pt算法本身来讲，并不利于最终图像的收敛，
现在的很多渲染器内部都是利用一种叫“低差异序列（Low Discrepancy Sequence）”的采样方式（详见https://zhuanlan.zhihu.com/p/20197323?columnSlug=graphics）。
c. pt算法的一大优势就是屏幕每个像素的采样都是独立的，每次迭代也是互相独立的，这就可以利用并行算法来对算法进行加速。CPU端有OpenMP，而且在诸多流行编译器内都直接内置集成了，
GPU端，N卡有CUDA，A卡可以利用OpenCL，而且渲染速度与GPU数量成线性增长关系，这就有点像暴力解算的意思，GPU端实现的最大区别就是不能递归（虽然N卡早已支持递归，但是
在计算规模很大的情况下每个线程的栈就很浅，再加上本身pt算法的采样光线有可能采样到很深的深度，所以并不是很敢用），递归主要发生在光线与场景求交的过程中，由于对场景采用的
是基于层级的划分（Bounding Volume Hierarchy）本身就有递归的意思。不能递归这点只要自己构造合理数据结构，把递归所用的那些信息尽量在构造BVH的时候自然写入结构内部即可，
可以比较容易的将递归降级为循环。
（ps：也难怪现在很多步进式渲染器都采用pathtracing...实现简单，开发简单，而且要提速的话直接借助并行算法，只不过多配几个GPU就行了，这就节省了一堆人力，成本就低...）
而最近浏览网页也发现了，很多人和企业也倾向于利用CPU渲染，具体请参见http://stackoverflow.com/questions/38029698/why-do-we-use-cpus-for-ray-tracing-instead-of-gpus
里面有个人的回答也是不错：
I‘m one of the rendering software architects at a large VFX and animated feature studio with a proprietary renderer (not Pixar, though I was once the rendering software architect there as well, long, long ago).
Almost all high-quality rendering for film (at all the big studios, with all the major renderers) is CPU only. There are a bunch of reasons why this is the case. In no particular order, some of the really compelling ones to give you the flavor of the issues:
- GPUs only go fast when everything is in memory. The biggest GPU cards have, what, 12GB or so, and it has to hold everything. Well, we routinely render scenes with 30GB of geometry and that reference 1TB or more of texture. Can‘t load that into GPU memory, it‘s literally two orders of magnitude too big. So GPUs are simply unable to deal with our biggest (or even average) scenes. (With CPU renderers, we can page stuff from disk whenever we need. GPUs aren‘t good at that.)
- Don‘t believe the hype, ray tracing with GPUs is not an obvious win over CPU. GPUs are great at highly coherent work (doing the same things to lots of data at once). Ray tracing is very incoherent (each ray can go a different direction, intersect different objects, shade different materials, access different textures), and so this access pattern degrades GPU performance very severely. It‘s only very recently that GPU ray tracing could match the best CPU-based ray tracing code, and even though it has surpassed it, it‘s not by much, not enough to throw out all the old code and start fresh with buggy fragile code for GPUs. And the biggest, most expensive scenes are the ones where GPUs are only marginally faster. Being lots faster on the easy scenes is not really important to us.
- If you have 50 or 100 man years of production-hardened code in your CPU-based renderer, you just don‘t throw it out and start over in order to get a 2x speedup. Software engineering effort, stability, and so on, is more important and a bigger cost factor.
- Similarly, if your studio has an investment in a data center holding 20,000 CPU cores, all in the smallest, most power and heat-efficient form factor you can, that‘s also a sunk cost investment you don‘t just throw away. Replacing them with new machines containing top of the line GPUs vastly increases the cost of your render farm, and they are bigger and produce more heat, so it literally might not fit in your building.
- Amdahl‘s Law: The actual "rendering" per se is only one stage in generating the scenes, and GPUs don‘t help with it. Let‘s say that it takes 1 hour to fully generate and export the scene to the renderer, and 9 hours to "render", and out of that 9 hours, an hour is reading texture, volumes, and other data from disk. So out of the total 10 hours of how the user experiences rendering (push button until final image is ready), 8 hours is potentially sped up with GPUs. So, even if GPU was 10x as fast as CPU for that part, you go from 10 hours to 1+1+0.8 = nearly 3 hours. So 10x GPU speedup only translates to 3x actual gain. If GPU was 1,000,000x faster than CPU for ray tracing, you still have 1+1+tiny, which is only a 5x speedup.
But what‘s different about games? Why are GPUs good for games but not film?
First of all, when you make a game, remember that it‘s got to render in real time -- that means your most important constraint is the 60Hz (or whatever) frame rate, and you sacrifice quality or features where necessary to achieve that. In contrast, with film, the unbreakable constraint is making the director and VFX supervisor happy with the quality and look he or she wants, and how long it takes you to get that is (to a degree) secondary.
Also, with a game, you render frame after frame after frame, live in front of every user. But with film, you effectively are rendering ONCE, and what‘s delivered to theaters is a movie file -- so moviegoers will never know or care if it took you 10 hours per frame, but they will notice if it doesn‘t look good. So again, there is less of a penalty placed on those renders taking a long time, as long as they look fabulous.
With a game, you don‘t really know what frames you are going to render, since the player may wander all around the world, view from just about anywhere. You can‘t and shouldn‘t try to make it all perfect, you just want it to be good enough all the time. But for a film, the shots are all hand-crafted! A tremendous amount of human time goes into composing, animating, lighting, and compositing every shot, and then you only need to render it once. Think about the economics -- once 10 days of calendar (and salary) has gone into lighting and compositing the shot just right, the advantage of rendering it in an hour (or even a minute) versus overnight, is pretty small, and not worth any sacrifice of quality or achievable complexity of the image.