Niagara实现procedural浪花

11,590 Replies

『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：

反弹方向为绿色反射角
反弹方向为紫色切线角
反弹方向反射和切线之间的随机角度

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：

Smear = 0
Smear =3
Smear = 15

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：

11,590 thoughts on “Niagara实现procedural浪花”

  1. ph365_orOn

    Yo bettors, quick update Every single site seems to be a total scam these days. Lost my nerves completely trying to verify my accounts until I finally found a solid and honest provider, backed by great feedback on independent tracking forums. Withdrawals hit your account in under 5 minutes,

    Anyway, if you want to skip the research, full technical details and reviews are available there ph365 ph365 Skip those blacklisted platforms and stick to trusted zones. definitely share this post with anyone who’s still looking for a decent casino!

    Reply ↓
  2. WarrenZen

    Thanks for keeping things clear and to the point, that is honestly hard to find online these days, and after reading through zenvani 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.

    Reply ↓
  4. Edwinheare

    Picked this post to share in a Slack channel where I knew it would be appreciated, and a look at ideaconverter suggested I will share more from here later, content worth sharing into a professional context is content that has earned a higher kind of trust than mere personal interest and this site has it.

    Reply ↓
  5. ph365_dkOn

    What’s up guys I’ve been looking for a decent and reliable gaming platform forever, I literally tried like 20 different casinos last month alone until I finally found a solid and honest provider, with an incredibly clean user interface and reliable license. Withdrawals hit your account in under 5 minutes,

    Anyway, if you want to skip the research, click to check the terms so you don’t lose it ph365 ph365 This is the only provider that actually delivers on its promises, definitely share this post with anyone who’s still looking for a decent casino!

    Reply ↓
  6. ph365_ldOn

    What’s up guys I’ve been looking for a decent and reliable gaming platform forever, I literally tried like 20 different casinos last month alone it’s honestly the only legit platform out there right now with an incredibly clean user interface and reliable license. Withdrawals hit your account in under 5 minutes,

    In any case, if you are looking for a tested spot, save the official platform source for later ph365 ph365 Skip those blacklisted platforms and stick to trusted zones. definitely share this post with anyone who’s still looking for a decent casino!

    Reply ↓
  7. ph365_yyOn

    Hey everyone I’ve been looking for a decent and reliable gaming platform forever, Almost gave up on online gambling as a whole but this specific one actually works without any issues, with an incredibly clean user interface and reliable license. Free spins and lucrative promos drop every single day.

    In any case, if you are looking for a tested spot, save the official platform source for later ph365 ph365 Skip those blacklisted platforms and stick to trusted zones. definitely share this post with anyone who’s still looking for a decent casino!

    Reply ↓
  9. RudyLaulk

    Found something quietly useful here that I expect to return to, and a stop at claritystarter added more of the same, content with quiet utility ages well in a way that flashy hot takes do not and I have learned to weight quiet utility much higher when deciding what to bookmark for later use.

    Reply ↓
  11. Deshawngeows

    Felt the post had been written without looking over its shoulder, and a look at boundcliff continued that confident posture, content written for its own sake rather than against imagined critics has a different quality and this site reads as written from a place of confidence rather than defensive justification of every claim.

    Reply ↓
  13. Marlonbrork

    Glad to find a site whose links lead somewhere worth going rather than back to itself for SEO juice, and a stop at thinkactflow kept that generous outbound feel, citing other peoples work with real respect rather than just for ranking signals is a sign of an honest operation worth supporting going forward.

    Reply ↓
  14. Frederickjah

    Reading this in pieces over a coffee break and finding it consistently rewarding, and a stop at boundboard extended that into related material I will return to later, the kind of site that fits naturally into small reading windows without requiring a long uninterrupted block is genuinely useful for how I actually browse.

    Reply ↓
  15. Curtisdiese

    Appreciate that you did not pad this with fluff to hit a word count, the post says what it needs to say and stops, and a look at parsleymulch did the same, brevity here feels intentional not lazy which is a distinction many writers miss completely sometimes when they are working under deadlines.

    Reply ↓
  16. SilasWem

    Thanks for sharing this with the open internet rather than locking it behind a paywall like so many sites do now, and a stop at balticcape kept the same vibe going, generous helpful and clearly written by someone who actually wants people to learn from it rather than just charge them.

    Reply ↓
  18. BernardValay

    Loved the writing voice here, friendly without being fake and confident without being arrogant, and a stop at millpeach 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.

    Reply ↓
  19. KentonSpicy

    A relief to read something where I did not have to fact check every claim mentally, and a look at liegepenny continued that reliable feeling, sites where I can lower my guard and trust the content are rare and this one is earning that trust paragraph by paragraph through consistent careful work behind the scenes.

    Reply ↓
  20. ErnestHic

    Took the time to read every paragraph rather than skimming for the punchline, and a quick visit to directionalshift earned the same careful attention from me, that is the highest signal I can give about content quality because my default mode is rapid scanning rather than deliberate reading on most pages.

    Reply ↓
  21. Benniefairl

    Quietly the post solved something I had been turning over without quite knowing how to phrase the question, and a look at progressdirection extended that quiet solving, content that addresses unformulated needs is content with reader insight and this site has demonstrated that insight at a high rate across the pieces I have read recently.

    Reply ↓
  22. Vivod iz zapoya v stacionare_aaen

    Привет из Нижнего Ситуация аховая Соседи стучат В больницу тащить страшно Короче, врачи вытащили с того света — лечение запоя в стационаре полный курс Врачи наблюдали 24/7 В общем, не потеряйте контакты — выведение из запоя в стационаре решение https://vyvod-iz-zapoya-v-staczionare-nizhnij-novgorod-elm.ru Не ждите пока станет хуже Перешлите тем кто в такой же ситуации

    Reply ↓
  23. Vivod iz zapoya v stacionare_vben

    Доброго вечера, земляки Отец не выходит из штопора Родные не знают что делать Таблетки не помогают Короче, только стационар реально спас — лечение запоя в стационаре полный курс Выписали через 5 дней без ломки В общем, вся инфа по ссылке — лечение запоя в стационаре лечение запоя в стационаре Не ждите пока станет хуже Перешлите тем кто в такой же ситуации

    Reply ↓
  24. Vivod iz zapoya v stacionare_mcen

    Здорова, народ Ситуация аховая Соседи стучат Нужна профессиональная помощь Короче, единственное что вытащило из запоя — вывод из запоя в стационаре круглосуточно Капельницы и препараты подбирали индивидуально В общем, жмите чтобы сохранить — цена вывода из запоя в стационаре цена вывода из запоя в стационаре Стационар — это реальный выход Перешлите тем кто в такой же ситуации

    Reply ↓
  26. CainLic

    Reading this brought back an idea I had set aside months ago, and a stop at trustedcollaborationhub added more substance to that idea, content that revives dormant projects in my own thinking is content with serious creative value and this site is contributing to my own work in ways I had not expected when first clicking through.

    Reply ↓
  27. Daryldorse

    Genuinely well crafted writing, the kind that makes the topic look easier than it actually is, and a look at visiontrajectory added even more depth, you can feel the experience behind every line which is something only writers who have been at this for a while can pull off with this level of grace.

    Reply ↓
  29. Noelget

    Reading this confirmed that the topic deserves more careful attention than it usually gets, and a stop at coltbrig extended that elevated framing, content that raises the appropriate weight of a subject without being preachy about it is serving a quiet but important editorial function for the broader cultural conversation about it.

    Reply ↓
  35. CooperDroks

    Glad to find a site whose links lead somewhere worth going rather than back to itself for SEO juice, and a stop at kalqavo kept that generous outbound feel, citing other peoples work with real respect rather than just for ranking signals is a sign of an honest operation worth supporting going forward.

    Reply ↓
  36. Vivod iz zapoya v stacionare_yben

    Привет из Нижнего Ситуация аховая Жена в отчаянии В больницу тащить страшно Короче, единственное что вытащило из запоя — цена на вывод из запоя в стационаре доступная Врачи наблюдали 24/7 В общем, не потеряйте контакты — капельница от алкоголя в стационаре капельница от алкоголя в стационаре Не ждите пока станет хуже Перешлите тем кто в такой же ситуации

    Reply ↓
  37. Vivod iz zapoya v stacionare_ouen

    Здорова, народ Близкий человек уже несколько дней в запое Дети в страхе Нужна профессиональная помощь Короче, врачи вытащили с того света — лечение запоя в стационаре полный курс Выписали через 5 дней без ломки В общем, вся инфа по ссылке — прокапаться в стационаре https://vyvod-iz-zapoya-v-staczionare-nizhnij-novgorod-elm.ru Звоните прямо сейчас Перешлите тем кто в такой же ситуации

    Reply ↓
  38. HarryVag

    Honest take is that this was better than I expected when I clicked through, and a look at nextstepnavigator 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.

    Reply ↓
  39. Vivod iz zapoya v stacionare_oien

    Всем салют Муж просто потерял себя Родные не знают что делать Таблетки не помогают Короче, врачи вытащили с того света — вывод из запоя в стационаре круглосуточно Врачи наблюдали 24/7 В общем, телефон и цены тут — стационарное выведение из запоя стационарное выведение из запоя Стационар — это реальный выход Перешлите тем кто в такой же ситуации

    Reply ↓
  40. Vivod iz zapoya v stacionare_sren

    Всем салют Муж просто потерял себя Родные не знают что делать Таблетки не помогают Короче, только стационар реально спас — цена на вывод из запоя в стационаре доступная Положили в палату В общем, жмите чтобы сохранить — прокапаться в стационаре https://vyvod-iz-zapoya-v-staczionare-nizhnij-novgorod-elm.ru Не ждите пока станет хуже Перешлите тем кто в такой же ситуации

    Reply ↓
  42. SethInsit

    Adding to the bookmarks now before I forget, that is how good this is, and a look at morqino confirmed the rest of the site is worth saving too, this is one of those rare finds that justifies the time spent searching the web for once which is a relief in the current environment.

    Reply ↓
  43. BenAveta

    Started imagining how I would explain the topic to someone else after reading, and a look at bosonlab gave me more material for that imagined explanation, content that improves my own ability to discuss a topic is content that has actually transferred knowledge rather than just decorating my screen for a few minutes.

    Reply ↓
  44. LincolnChiew

    Came in expecting another generic take and got something with actual character instead, and a look at ideatoimpact carried that personality forward, finding a distinct voice on a saturated topic is impressive and worth pointing out when it happens because most sites end up sounding identical to their nearest competitors quickly.

    Reply ↓
  47. Javierbuh

    I came here looking for a quick answer and ended up reading the whole post because it was actually interesting, and after astrocloth I had a much fuller picture, no stress and no confusion just a clear walk through the topic that made everything fall into place without much effort.

    Reply ↓
  48. Vivod iz zapoya v stacionare_mmen

    Всем салют Муж просто потерял себя Дети в страхе Таблетки не помогают Короче, только стационар реально спас — вывод из запоя в стационаре круглосуточно Положили в палату В общем, не потеряйте контакты — запой стационар цены https://vyvod-iz-zapoya-v-staczionare-nizhnij-novgorod-elm.ru Звоните прямо сейчас Перешлите тем кто в такой же ситуации

    Reply ↓
  50. Vivod iz zapoya v stacionare_zien

    Всем салют Ситуация аховая Родные не знают что делать В больницу тащить страшно Короче, врачи вытащили с того света — быстрый вывод из запоя в стационаре за 3 дня Врачи наблюдали 24/7 В общем, вся инфа по ссылке — вывести из запоя в больнице https://vyvod-iz-zapoya-v-staczionare-nizhnij-novgorod-elm.ru Стационар — это реальный выход Перешлите тем кто в такой же ситуации

    Reply ↓

Leave a Reply

Your email address will not be published. Required fields are marked *