『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：
как войти в mostbet http://www.mostbet50122.icu
1 win 1 win
Worth recognising that the post handled a familiar topic without reaching for any of the obvious hot takes, and a stop at securemarketbuyingplace continued that fresh treatment, sites that find new angles on subjects others have exhausted are sites worth following carefully and this one has clearly developed that exploratory instinct through patient practice.
A clean read with no irritations, and a look at trustedcorporateconnections continued that frictionless quality, the absence of small irritations is something I notice only when present elsewhere and this site is one of the rare places where everything just works and lets me focus on the substance rather than fighting the format.
Honest take is that this was better than I expected when I clicked through, and a look at clicktoexploreideas reinforced that, the bar for online content has dropped so much that finding something thoughtful and well constructed feels almost noteworthy now which says more about the average than about this site itself.
mostbet magyarország mostbet magyarország
Glad I stumbled across this post, the explanations actually make sense without needing background knowledge to follow along, and after a stop at zenvaxo the same was true there, no assumptions about the reader just clear writing that anyone can understand from the first line right through to the end.
Well structured and easy to read, that combination is rarer than people think, and a stop at globalbusinessrelationshiphub confirmed the same standard runs across the rest of the site, definitely the kind of place I will be coming back to when this topic comes up in conversation later again over the weeks ahead.
1win app скачать 1win app скачать
мостбет app mostbet50122.icu
Solid value for anyone willing to read carefully, and a look at globalbusinessrelationshiphub extends that value across the rest of the site, this is the kind of place that rewards return visits rather than offering everything in a single splashy post and then leaving readers nothing to come back for later which is unfortunately common.
If patience for careful reading is rare these days finding sites that reward it is rarer still, and a stop at discovernewbusinesspaths extended that rare reward, the diminishing returns on shallow content reading have made me more selective about where to spend reading time and this site is meeting the higher selectivity bar consistently.
Came here from a search and stayed for the side links because they were that interesting, and a stop at globalenterprisebonds took me even further into the site, the kind of organic exploration that good content invites is something most sites kill through aggressive interlinking and pushy navigation choices rather than relying on quality.
A nicely understated post that does not shout for attention, and a look at businessrelationshipnetwork maintained the same quiet quality, understatement is a stylistic choice that distinguishes serious writing from attention seeking writing and this site has clearly committed to the understated approach as a core editorial value rather than just a phase.
Loved the writing voice here, friendly without being fake and confident without being arrogant, and a stop at securecommercialalliances carried the same tone forward, the kind of personality that makes a reader feel welcome rather than lectured at which is a balance plenty of writers struggle to find no matter how long they have been at it.
aviator truques mostbet http://www.mostbet24687.shop
Honestly this was the highlight of my reading queue today, and a look at globalenterprisebonds extended that across more pages I will return to, ranking what I read against what else I read each day is something I do informally and this site keeps moving up in those rankings the more I explore it.
Grateful for posts like this one, they remind me there are still places online run by people who care about quality, and a look at bondedcompass reflected the same standards, you can tell the difference between content made for readers and content made just for search engines today and this is the former.
Quality work here, the post reads cleanly and the points stay focused throughout, and a stop at explorelongtermopportunities kept the standard high, you can tell the writer cares about the final result rather than just hitting publish for the sake of having something new on the page to feed the search engines.
Well done, the writing is professional without being stiff, and the topic is treated with care, and a look at modernshoppingecosystem reflected that approach, the kind of site I would point a colleague to if they asked for a reliable starting point on this topic in the future without any hesitation at all.
Vague feelings of recognition kept surfacing as I read because the writing names things I have been thinking, and a look at globalshoppinginfrastructure produced more of those recognition moments, content that gives shape to private intuitions is content that makes me feel less alone in my own thinking and this site has that effect.
como apostar na mostbet como apostar na mostbet
Approaching this site through a casual link click and being surprised by what I found, and a look at clicktofindstrategicoptions extended the surprise, the rare experience of stumbling into excellent independent content rather than predictable mediocrity is one of the actual remaining pleasures of casual web browsing and this site provided it cleanly.
Worth saying that this is one of the better things I have read on the topic in months, and a stop at professionaltrustalliances reinforced that ranking, the topic is well covered by many sources but few do it with this level of care and the few that do deserve to be flagged so other readers can find them.
References:
Lollybet Casino Bonus ohne Umsatzbedingungen https://utmagazine.ru/r?url=https://schreinerei-leonhardt.de/lollybet-online-casino-slots-roulette-blackjack-mehr
Bookmark earned and the bookmark feels like a permanent addition rather than a maybe, and a look at globalretailcommercehub confirmed that permanent status, the difference between durable bookmarks and ephemeral ones is something I have learned to feel quickly and this site triggered the durable feeling almost immediately during my first read here.
mostbet lucky jet tippek mostbet lucky jet tippek
References:
Lollybet Casino Login http://toolbarqueries.google.co.uk
A piece that exhibited the kind of patience that good writing requires, and a look at trusteddealstore continued that patient quality, hurried writing is easy to spot and this site reads as having been written without time pressure which produces a different feel than the rushed content that dominates much of the modern blog space.
Now setting this aside as a model of how to write thoughtfully on the topic, and a stop at smartbuyingcorner extended that model status, content that becomes a reference for how a kind of writing should be done is content with influence beyond its own readership and this site is reaching that level for me clearly today.
The clarity here is something I really appreciate, especially compared to sites that pile on jargon for no reason, and a look at businessgrowthpartnerships was the same, simple direct sentences that actually deliver information instead of dancing around the point for paragraphs at a time which wastes reader patience.
Reading this confirmed a hunch I had been carrying about the topic without having articulated it, and a stop at valuebasedshoppingonline extended the confirmation, content that gives shape to fuzzy intuitions is doing the rare work of making private thoughts public and this site is providing that articulating service consistently for me lately.
Now thinking the topic is more interesting than I had given it credit for, and a stop at clicktofindclarity continued that elevated interest, content that revives my curiosity about subjects I had set aside is doing genuine work in the structure of my interests and this site is providing that revivifying effect today actually.
Started a draft response in my head and ended without publishing it because the post said it well enough, and a look at onlineconsumerbuying produced the same effect, content that satisfies my urge to add to it by being complete enough on its own is rare and represents a particular kind of editorial completeness here.
Reading this in a quiet hour and finding it suited the quiet, and a stop at clicktoadvanceknowledge extended the quiet reading mood, content that matches its own optimal reading conditions rather than fighting them is content that has been thoughtfully calibrated and this site reads as having a particular reading mood in mind throughout.
chicken road demo lucky jet http://www.chicken-road79220.icu
Honestly impressed, did not expect to find this level of care on the topic, and a stop at everydaypurchaseplatform 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.
Now thinking about how to apply some of this to a project I have been planning, and a look at clickforgrowthinsights added more material for the planning, content that connects to my actual creative work rather than just being interesting in the abstract is the kind that earns priority placement in my reading rotation consistently going forward.
Thanks for keeping things clear and to the point, that is honestly hard to find online these days, and after reading through globaldigitalshoppingmarket 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 magas RTP nyerőgépek mostbet magas RTP nyerőgépek
Felt the writer was being honest with the reader which is rare enough that I want to acknowledge it, and a look at professionalbusinessbonding continued that honest feel, content built on actual knowledge rather than aggregated summaries is something I value highly and rarely come across in regular searches on the open internet these days.
mostbet mines szorzó http://mostbet36442.shop
chicken road italia slots chicken-road79220.icu
Now feeling slightly more committed to my own careful reading practices having read this, and a stop at clicktoexplorefutures reinforced that commitment, content that models the kind of attention it deserves is content that calibrates the reader and this site has clearly raised my own bar for what to bring to good writing today.
Started this morning and finished at lunch with a small sense of having spent the time well, and a look at learnfromexpertinsights extended that satisfaction into the afternoon, content that fits naturally into the rhythm of a working day rather than demanding a dedicated reading block is increasingly the kind I prefer.
1win официальный сайт https://1win32946.icu/
The conclusions felt earned rather than tacked on at the end like an afterthought, and a look at bluecrestbond kept that careful structure going, you can tell when a writer has thought about the shape of their post versus just letting it ramble out and hoping for the best at the end which most do.
Granted I am giving this site more credit than I usually give new finds, and a look at airycargo continued earning that credit, the calibration of how much trust to extend after limited exposure is something I do carefully and this site has earned more trust on shorter exposure than most due to consistent quality across.
Did not expect much when I clicked through but ended up reading the whole thing carefully, and a stop at globalcommercialalliances kept that engagement going, sometimes the unassuming sites turn out to deliver more than the flashy ones which is something I have learned to look out for over time online lately and across topics.
Reading this site over the past week has changed how I evaluate content in this space, and a look at explorebusinessopportunities extended that recalibration, the standards I bring to reading on the topic have shifted upward as a direct result of regular exposure to this kind of work and that shift will outlast any single reading session.