『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 recommend this site in a context where my recommendations are taken seriously, and a stop at visionmapping confirmed I should make that recommendation soon, the small but real act of recommending content into spaces where my taste matters is something I take seriously and this site is worth the recommendation.
Saving this link for the next time someone asks me about this topic, and a look at trustfoundry expanded what I will be sharing with them, this is the kind of resource that makes a real difference when you are trying to point a friend to something useful and reliable rather than generic marketing pages.
Worth saying that the quiet confidence of the writing is what landed first, and a look at focuslane continued that quiet quality, confident writing without the loud display of confidence is a rare combination and this site has clearly developed both the knowledge and the editorial restraint to land that combination consistently.
Honest opinion is that this is the kind of post that builds long term trust with readers, and a look at focusguidesgrowth reinforced that perception, the slow accumulation of trust through consistent quality is the only sustainable way to build a real audience and this site is clearly playing that long game.
Народ, подскажите по краске для стен. Выбор огромный, цены отличаются сильно, а по отзывам вообще ничего не понятно: кто-то хвалит, кто-то пишет про плохую укрывистость, большой расход и разводы. Не хочется переплачивать за бренд, но и покупать дешевую краску, чтобы потом всё перекрашивать, тоже смысла нет. Какую краску брали вы и остались ли довольны результатом? А как выбиралась вами краска luxium
chicken road registro ios http://chicken-road90154.icu/
chicken road promo mines http://chicken-road90154.icu/
A modest masterpiece in its own quiet way, and a look at trustaxis confirmed the same quiet quality across the rest of the site, calling something a masterpiece is usually overstating but for content this carefully crafted the word feels appropriate even if the writers themselves would probably resist the label honestly.
chicken road kyc chicken-road90154.icu
Honest assessment is that this is one of the better short reads I have had this week, and a look at ideafocus reinforced that, the bar for short content is low because most of it sacrifices substance for brevity but this site manages both at once which is harder than it sounds for most writers attempting it.
Thanks for treating the topic with the seriousness it deserves without becoming pompous about it, and a stop at clarityfollowsfocus continued that balanced treatment, the gap between earnest and self serious is huge and writers who can stay on the right side of it earn my respect when I find them online today.
Felt the writer respected me as a reader without making a show of doing so, and a look at trustedharborbond continued that quiet respect, this is the kind of small but meaningful detail that separates the sites I bookmark from the ones I close after a single skim and never return to again no matter how interesting the headline.
download aviator ios http://aviator63663.icu/
Now realising the topic deserved better treatment than it has been getting elsewhere, and a look at signalcreatesfocus extended that broader recognition, content that exposes the gap between actual quality and average quality elsewhere is doing the quiet work of raising standards and this site is contributing to that elevation in its own corner.
mostbet aviator қоида mostbet aviator қоида
aviator suporte chat ao vivo http://aviator63663.icu
Came in skeptical of the angle and left mostly persuaded, and a stop at ideasunlockvelocity pushed me a bit further in the same direction, content that can move a critical reader by argument rather than rhetoric is rare and worth pointing out because it indicates real substance underneath the surface presentation here.
aviator crash online https://aviator63663.icu
A memorable post for me on a topic I had thought I was tired of, and a look at signalactivatesdirection suggested the same site can refresh other tired topics, sites that can revive my interest in subjects I had written off as exhausted are doing rare work and this one is clearly doing that for me today.
Now realising the post has been quietly doing important work in my mind for the past hour, and a stop at bondedconnections extended that quiet processing, content that continues to do work after I close the tab is content with afterlife in the mind and this site is producing those long lived effects at a meaningful rate.
chicken road casino online chicken-road90154.icu
However measured this site clears the bar I set for sites I take seriously, and a stop at clutchbulb continued clearing that bar, the metrics I use for site quality are admittedly informal but they are consistent and this site has cleared them on multiple measurements across multiple visits which is meaningful for my evaluation.
Now feeling slightly more committed to my own careful reading practices having read this, and a stop at creativepath 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.
Genuinely good work, the kind that holds up over multiple readings without losing its appeal, and a stop at progresssystem kept that going, definitely a site I will be returning to and probably mentioning to others who work in or care about this particular area of interest today and in coming weeks.
aviator limite pix https://aviator63663.icu/
Looking for similar voices elsewhere has come up empty in my recent searches, and a stop at highlandbond extended the search frustration, the rare site that does what no other does in quite the same way is precious and this one has clearly developed a particular approach that I have not been able to find duplicates of.
Picked up a couple of new ideas here that I can actually try out, and after my visit to clarityconstructor I have even more notes saved, this is the kind of resource that pays you back for the time you spend on it which is rare to come across in this corner of the web.
Thanks for taking the time to write this, it is clear that some thought went into how each point would land, and after I went through securebonding I had a better grip on the topic, real value without the usual marketing noise people have to put up with online when searching for answers.
1win сколько выводят на мегапей https://www.1win15750.icu
Glad the writer did not feel the need to argue with imaginary critics in the post itself, and a stop at motionoptimizer kept the same focused approach going, defensive writing wastes the reader time and confidence on positions that did not need defending and this post has clearly avoided that common failure.
Saving this link for the next time someone asks me about this topic, and a look at bondedprime expanded what I will be sharing with them, this is the kind of resource that makes a real difference when you are trying to point a friend to something useful and reliable rather than generic marketing pages.
References:
Lollybet Kontakt images.google.pt
Reading this on a long flight and finding it the best thing I read across hours of trying, and a stop at bondedpath 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.
Following a few of the internal links revealed more posts of similar quality, and a stop at navisbond added more to that growing pile, sites where internal links lead to more good content rather than to more of the same recycled material are sites with depth and this one has clearly built that depth carefully.
Excellent post, balanced and well organised without showing off, and a stop at growthunfoldsforward continued in that same vein, this site has clearly figured out the formula for content that works for readers rather than for search engine ranking signals which is harder than it sounds today and worth real recognition from anyone.
Now appreciating that the post did not require me to agree with the writer to find it valuable, and a look at steadfastnetwork maintained the same useful regardless of agreement quality, content that informs even when it does not convince is content with broader utility and this site reads as useful even when I disagree.
Generally my attention drifts on long posts but this one held it through the end, and a stop at trustharvest earned the same sustained focus, content that defeats my drift tendency is content with substantive pulling power and this site has demonstrated that pulling power across multiple pieces in a session that has now run quite long actually.
Quietly enjoying that I have found a new site to follow for the topic, and a look at strategicbonding reinforced the small pleasure of the find, the discovery of new high quality sources is one of the more durable pleasures of careful internet reading and this site has been generating that discovery pleasure at multiple points already today.
1win поддержка http://1win74019.icu
jessica rabbit hentai
Probably this is one of the better quiet successes on the open web at the moment, and a look at claritypowersprogress reinforced that quiet success quality, sites that are doing well without making a noise about doing well are the sites I most respect and this one has clearly chosen the quiet success path consistently throughout.
Once I had read three posts the editorial pattern was clear, and a look at directionfuelsmotion confirmed the pattern from a fourth angle, sites where the underlying approach reveals itself through accumulated reading rather than being announced are sites with real depth and this one has that quality clearly visible across multiple pieces consistently.
Approaching this with the usual skepticism I bring to new sites and being slowly persuaded, and a stop at bondedalliancenetwork continued that gradual persuasion, the careful path from skeptical reader to genuine fan is the only one I trust and this site has walked me along that path through patient consistent quality across pieces.
Thanks for putting in the work to make this approachable, plenty of sites cover the same ground but most do it badly, and a quick visit to forwardmotionclarity confirmed this one stands apart, simple language and useful examples without anyone trying to sell me anything along the way which I really appreciated.
Reading this slowly to give it the attention it deserved, and a stop at progressneedsdirection earned the same slow read, choosing to read slowly is a small act of respect for content quality and very few sites earn that respect from me but this one did so without any explicit ask which is the cleanest way.
mostbet бонусы для Кыргызстана http://mostbet90088.company
References:
Lollybet Casino Einzahlung old-site.bsau.ru
A relief to read something where I did not have to fact check every claim mentally, and a look at momentumvector 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.
A piece that did exactly what it promised in the headline without overshooting or underdelivering, and a look at claritycreatesmomentum continued that calibration, alignment between promise and delivery is a basic editorial virtue that many sites fail at and this site has clearly mastered the matching of expectation and substance throughout pieces.
1вин aviator играть http://1win83326.icu