『部分工程文件点我下载』
『知乎转载链接』
『有点相关的Unreal Circle演讲』
Summary
因为本文涉及到的内容比较广，关于Niagara最基础的一些部分比如怎么新建、怎样添加使用模块就不详细说明了，没用过的同学可以先看一下Epic官方文档的介绍：
Niagara核心概念
和贾越同学的系列教学直播：
https://www.bilibili.com/video/av73602807
本次工程效果：
为了实现海浪拍到岩石上激起千层浪的感觉，我们可以看到有几个表现上的关键点
- 在岩石附近生成水花
- 水花激起的时机和位置与海水的流动匹配
- 出射速度受到海水运动和石头表面的撞击情况影响
- 激起后水花的体积感
海面的模拟 – Gerstner Waves
游戏里模拟海水的途径是一门艰深的课题，本文就不多涉及了。这里为了制作需要，介绍一下相对直观实用的Gerstner Waves。
在流体动力学中，Gerstner Waves是周期表面重力波的欧拉方程方程的精确解。它计算机图形技术之前很久就出现了，用来描述不可压缩流体的表面波动。
单一Gerstner Wave材质应用在一个平面上的效果如图：
数学时间
我们可以看到每一个点不止有Z轴方向的运动，也有XY轴的运动。并且这不是一个简单的sin波而是带有‘浪尖’的波形，这些都是Gerstner Wave的重要特征。
因为篇幅原因这里就不具体说公式了，本节末尾提供了我做好的材质节点可以直接下载取用。详细算法可以参考：
Chapter 1. Effective Water Simulation from Physical Models
https://developer.download.nvidia.com/books/HTML/gpugems/gpugems_ch01.html
简单说来，定义一个波形的属性有：
[高度, 前进方向, 波长, 陡峭度]
我们定义相位φ
φ = 方向 ⋅ 位置 / 波长 + 频率 * 时间
其中
频率 = sqrt(重力 * 2π / 波长)
那么以
z轴偏移 = 高度 * sin(φ)
xy轴偏移 = 陡峭度 * 高度 * 方向 * cos(φ)
的形式去移动海面的顶点，顶点会沿着椭圆绕圈（左图），一系列的顶点组合在一起就会形成具有浪尖的波形。
引擎内实现
在引擎里的实现可以用HLSL写在Custom节点里，你也可以选择用节点连。执行效率上，节点连出来的公式和HLSL代码没有区别。
材质函数 MF_GerstnerWave
核心部分：
float3 d = float3(sin(DirectionInDegrees * 0.0174533), cos(DirectionInDegrees * 0.0174533), 0); // 为了控制方便，输入并不是一个向量而是角度，这里转换一下
float w = 6.2831854 / L;
float theta = sqrt(6.2831854 * Gravity / L); // 由波长算出频率
float q = Stiffness / (Amplitude * w); // q是控制浪尖陡峭度的系数
float phase = dot((w * d).xy, WorldPosition.xy) – theta * Time; // 喂给cos和sin的相位
float3 offset = float3(q * Amplitude * d.xy * cos(phase), Amplitude * sin(phase)); // 照抄上面公式
这部分我放在了BlueprintUE.com里，点下面链接直接把节点复制粘贴到UE材质编辑器里就可以得到图上的节点：
https://blueprintue.com/blueprint/tt5p124n/
波形叠加
显然，一条波并不能说非常炫酷，Gerstner Wave的典型做法是把多条波叠加在一起。上面提到每条波可以控制的变量有：[高度, 前进方向, 波长, 陡峭度]
要合并多个波形，首先考虑的是波长，因为能组合的波长数量有限（8个已经很多了），我们无法完全参照真实世界的海洋数据，只能尽可能利用能付出的资源。由于波长相似的wave在叠加时看起来更有错综复杂的动态表现，我们可以在输入参数里首先给一个波长的中值LengthMedian，然后让不同wave的波长围绕这个中值变化。这样只要保留所有wave的波长与LenghMedian的比例，就可以通过改变LengthMedian来调整整个海洋的波浪大小：
在定义一组waves的结构FGerstnerWavesParameters中，美术可以在海洋的蓝图Actor上直接填写LengthMedian。
接下来，美术可以手动填写各个wave和LengthMedian的比例，也可以像我一样偷懒，填一个LengthMultiplierRange然后随机选取范围内的比例，在场景里拖动Seed直到随出一个好看的组合。
类似的方法同样适用于高度, 前进方向, 陡峭度。要注意的是，wave的高度和波长存在正相关关系，简单的做法是定义一个常数比例，让每个wave高度和和波长的比例保持一致。而在定义前进方向时会类似的给出一个DirectionRange定义前进方向的范围。
Niagara GPU粒子的生成
匹配水面位移
上面说了这么多，海面Shader内的位移数据其实并不能被外界直接读取，为了让Niagara能获取到海面的位移数据从而实现位置的同步，我们需要额外做一些工作：
方块：Niagara GPU particles
因为Gerstner Wave算法是确定性的，即给定同样的 [ 高度 | 前进方向 | 波长 | 陡峭度 ] 这样一组参数，不管在哪算出来的波形都是一样一样的。所以我们只要把与海面同样的参数传入Niagara System，就可以让Niagara粒子去‘追踪‘海面的粒子。（如上图）
传参
Normal难度：
上面说到，海面的波形是由好几个不同的Gerstner Wave组成，这里我用了8个，那么需要匹配海面的形状，其实在生成浪花时我们不用考虑所有8个waves，只考虑最大的一或两级wave，视觉上就很难看出不完全吻合的细节差别了。
Hard难度:
这一块可选阅读，可以跳过，对下面内容的理解没有影响。
可是如果我觉得这样做身体不适，非想传所有数据达到完美同步呢？ 也不是没有办法。4个参数 x 8组 = 一共32个float变量，手动命名并且在蓝图里拉面条，再在Niagara模块上一个一个拖上去也不是不行.. 如果你想做得灵巧些，我发掘了一个输入数组的hack：
因为Niagara还在beta阶段，一些功能还在持续改进中，这个hack可能以后也不需要。我介绍一下的另一个原因是觉得对增加对Niagara的理解有所帮助。
开始我想把数据通过DrawToRenderTarget写到一张贴图上，让Niagara读，但因为操作太不友好并且不支持CPU粒子放弃了。后来发现Niagara里的Curve型变量是同时支持CPU和GPU的，读取也很方便。我在蓝图里把需要的变量写入到一个Curve asset里，Niagara内reimport更新就可以，很方便。
不过Curve key的格式是有讲究的，Niagara CPU sim在读curve时，直接就读对应位置的key value。但GPU sim上会首先把curve编码成一个1×128像素的Lookup table，然后再读对应位置的数据，这就导致如果key的time位置不是100%对到LUT的像素位置上，encode后会出现偏差，这个偏差在我们现在的应用中是无法容许的。
好在encode的逻辑非常直接，只是取第一个key和最后一个key的time看范围，normalize到[0.0, 1.0]之间：
所以要达到key和LUT的像素对齐，最简单的方式是让time = [0, 1, 2… 127]，这样不需要额外操作，在GPU sim上读取即可。
最后一个key的time是127.0
这样我们就实现了可以在一个curve里记录128个float值。剩下要做的就是再BP里根据一定规则把8个wave的一系列参数编码，再从Niagara模块里通过相同规则解码。这里4个参数*8个wave，规则就是很简单的按照一定间隔记录了32个值。
按照一定间隔，在整数Time上记录了每一组parameters
在Niagara module中通过类似方式解压
定义GPU粒子的碰撞行为
Distance Field Collision
系统提供的collision模块非常健全，包括CPU trace碰撞，GPU depth / distance field碰撞等分支可以在下拉菜单中选择使用。我们这里使用GPU distance field碰撞，depth碰撞比较适合比较粗犷的效果，比如下雨下雪，稍微有点问题也看不出来，但如果水浪使用depth碰撞，岩石背面屏幕看不到的地方就会出错。
我们想有一些比较细腻的控制逻辑，比如这里水花撞到石头上，如果用默认的collision碰撞，反弹是朝着下图绿色的反射向量方向飞出去的，而流体撞到刚体的行为并不是这样完美的反弹，我们更想让它偏向下图紫色的角度。
下面三个gif分别展示不同反射角度的视觉：
定义这样的碰撞行为需要自己写模块，幸运的是这种逻辑都可以用Niagara的节点实现。引擎提供的Collision模块本身是一个宝藏，里面有各种碰撞的实现方式和模块编辑的best practice。在Collision – CollisionQueryAndResponse模块中我们探索一下可以发现：
点进去最底层的模块是：
即给出world position获得global distance field的数值和gradient，和材质里的DistanceToNearestSurface / DistanceFieldGradient结果是一样的，都是GPU内的query。
知道了这些，我们就可以很方便的自己写一个简单的collision判断：
并且在后面接上上面说到的反射角和切线角的逻辑–如果发现水浪粒子进行了第一次和岩石碰撞，那么就沿着我们想要的角度给一个初速度：
距离优化
同时因为有了distance field信息，可以在particle spawn时判断水浪粒子是否在岩石附近，如果不在就直接删除。
Smear
水花的材质想做好也是一项涉及很广的任务，我这里就是用了一个比较简单的云的贴图，稍微加工了下。值得一提的是通过particle的速度可以在材质里实现速度拉伸效果，对表达浪花飞溅的夸张形态非常有帮助：
实现方式也很简单，因为粒子sprite是朝向camera的，我们只要以local position和移动速度的点积缩放sprite即可：
Now planning to write about the topic myself eventually using this post as a reference, and a look at nextlevelpurchasehub would also serve in that future piece, content that becomes raw material for my own writing rather than just informing my reading is content with multiplicative value and this site is generating that multiplicative effect.
Reading this gave me a small mental break from the heavier reading I had been doing, and a stop at discovernewdirections extended that lighter feel, content that provides relief without becoming trivial is harder to produce than people realise and this site has clearly figured out how to be light without being shallow at all.
mostbet estatísticas http://mostbet24687.shop
Worth a slow read rather than the fast scan I usually default to, and a look at clicktoexpandknowledgebase earned the same slower pace from me, content that resets my reading speed downward is content with substance worth absorbing and this site has produced that effect on me multiple times now over the last week here.
Refreshing to read something where the words actually mean something instead of filling space, and a stop at flexibledigitalshopping kept that going, the writing here trusts the reader to follow along without endless repetition or constant reminders of what was already said earlier in the post which I appreciate.
1вин как зайти http://1win32946.icu
plinko chicken road http://chicken-road79220.icu/
Reading this in three sittings because the day was fragmented, and the piece survived the fragmentation, and a stop at exploreprofessionaldevelopment held up under similar reading conditions, content engineered for continuous attention is fragile in modern conditions and this site reads as durable across the realistic ways people consume content today.
If I had to defend the time I spend reading independent blogs this site would feature in the defence, and a look at learnfromexpertinsights reinforced that defensive utility, the ongoing case for non algorithmic reading is one I make to myself periodically and sites like this one provide the actual evidence that supports the case clearly.
Thanks for keeping things clear and to the point, that is honestly hard to find online these days, and after reading through modernbuyingstore the message stayed consistent which makes me trust the information being shared more than I usually do on similar pages that cover this same kind of topic.
mostbet ограничения аккаунта mostbet50122.icu
1win куда вводить промокод http://1win52998.icu/
Found this via a link from another piece I was reading and the click was worth it, and a stop at corporatecollaborationnetwork extended the value across more material, the open web still rewards clicking through citations when the underlying writers care about each other work and this site clearly belongs to that network.
1win служба поддержки Ош https://www.1win32946.icu
chicken road app non disponibile http://chicken-road79220.icu
1win демо plinko 1win32946.icu
link mirror chicken road link mirror chicken road
Skipped past the first paragraph thinking it was setup and had to come back when the rest referenced it, and a stop at reliabledealshoppingplace similarly rewarded careful reading from the start, content where every paragraph carries weight is content I now know to read from the beginning rather than skipping ahead.
Worth a quiet moment of recognition for the consistency I have noticed across multiple posts, and a stop at explorefuturepossibilities continued that consistent quality, sites that maintain quality across many pieces rather than peaking on one viral post are sites with real editorial discipline and this one has clearly developed that discipline carefully.
Reading this on a long flight and finding it the best thing I read across hours of trying, and a stop at clicktofindstrategicoptions kept the streak going, when content beats long flight reading you know it has substance because flight reading is a hard test of a piece given the alternatives available everywhere.
mostbet plinko gyakorlás mostbet plinko gyakorlás
73mate
Reading this brought back the satisfaction I used to get from blogs ten years ago, and a stop at balancedtrust kept that nostalgic quality alive, sites that capture what was good about an earlier era of internet writing are increasingly precious and this one is doing that without feeling like a deliberate throwback at all.
In the middle of an otherwise scattered day this post landed as a moment of focus, and a stop at digitalbuyingexperience extended that focused feeling across more pages, content that anchors a fragmented day rather than contributing to the fragmentation is content with real centring effect and this site is providing that anchoring function for me.
Considered as a whole this site has developed a coherent point of view that comes through in individual pieces, and a look at easydigitalretail continued displaying that coherence, sites with a unified perspective rather than a grab bag of takes are sites with editorial maturity and this one has clearly developed that maturity through years of work.
Took a quick scan first and then went back to read properly because the post deserved it, and a stop at trustedmarketrelationship kept me reading carefully too, the kind of writing that earns a slower second pass rather than getting skimmed and forgotten is something I value highly when I happen to find it.
Useful reading material, the kind I can hand off to someone newer to the topic without worrying about confusing them, and a quick look at growwithinformedchoices confirmed the same beginner friendly tone runs throughout the site which is great for sharing with people just starting their learning journey on this particular topic.
Honestly impressed, did not expect to find this level of care on the topic, and a stop at discoveractionableideas cemented the impression, you can tell within the first few paragraphs whether a site is going to be worth the time and this one delivered on that early promise nicely throughout the rest of what I read.
mostbet értesítések mostbet36442.shop
Without overstating it this is a quietly excellent post, and a look at securestrategicbonds extended that quiet excellence, content that earns superlatives without demanding them through marketing language is content that has truly earned them through the substance and this site has clearly produced work in that earned excellence category today.
Really appreciate that the writer did not stretch the post to hit some target word count, the points end when they are made, and a stop at urbanretailshoppingzone reflected the same discipline, brevity is generosity in disguise and this site has clearly figured that out far better than most blog operations have.
Better than the average post on this subject by some distance, and a look at learnandscaleintelligently reinforced that, you can tell within the first paragraph that the writer here actually cares about the topic rather than just covering it for the sake of having something to publish that week or that day.
chicken road assistenza pagamenti chicken road assistenza pagamenti
chicken road slots app https://chicken-road79220.icu/
1win лимит на элсом http://1win32946.icu/
chicken road prelievo con paypal https://chicken-road79220.icu/
1win как вывести через мбанк http://1win32946.icu/
1win aviator apk http://1win32946.icu/
Started forming counter examples to test the claims and the post handled most of them implicitly, and a look at collaborativegrowthnetwork continued that anticipatory style, writers who think two steps ahead of the critical reader save themselves from a lot of follow up work and this writer has clearly internalised that habit consistently.
Started forming counter examples to test the claims and the post handled most of them implicitly, and a look at createbetteroutcomes continued that anticipatory style, writers who think two steps ahead of the critical reader save themselves from a lot of follow up work and this writer has clearly internalised that habit consistently.
Just want to record that this site is entering my regular reading list, and a look at buildyourstrategicfuture confirmed it deserves the spot, my regular reading list is short and well curated and adding to it requires meeting a fairly high quality bar that this site has clearly cleared without much effort apparently.
Came in tired from a long day and the writing held my attention anyway, and a stop at clicktolearnstrategically kept that going, content that can engage a fatigued reader is doing something right because most online reading happens in suboptimal conditions like that one and quality content adapts to it without complaint.
Split any two windows side by side on Mac with a keyboard shortcut. Rectangle sets it up in three minutes.
Even from a single post the editorial care is clear, and a stop at discovernewbusinesspaths extended that care across more pages, the kind of attention to quality that shows up in every paragraph is what separates serious sites from the rest and this one has clearly invested in that paragraph level attention across what I have read.
mostbet apk android https://www.mostbet55614.shop
Honestly impressed by the consistency of voice across what I have read so far, and a quick visit to fastbuyingoutlet continued that consistent feel, when a site reads like one careful person rather than a committee the experience is more rewarding for the reader who notices these subtle editorial details over time.
Genuine pleasure to read, and that is not something I say often after a casual click through, and a quick visit to enterprisevaluealliances kept the same feeling going across the rest of the site, finding writing that actually feels good to spend time with rather than just functional is increasingly rare on the open web.
chicken road modalità demo https://chicken-road79220.icu
1win ставки на Dota 2 http://1win32946.icu/
Polished and informative without feeling overproduced, that is the sweet spot, and a look at sustainablebusinesspartnerships hit it again, you can tell when a site has been built with care versus thrown together for the sake of having something to put online and this is clearly the former approach taken by the team.