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| 引导和跟随的真谛 Keith Lango 关于动画原理的精彩论述 |
| 作者 唬哈 查看 362 发表时间 2008/2/13 16:03 【论坛浏览】 |
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The Zen of Lead & Follow 给作用力，而不是给形式做动画 你们小时候是不是都跟着领头做过游戏？一个小孩做一件事，其他小孩照做？领头的小孩跑 出去到院子里，其他的小孩象跳拉丁舞一样跟着他。好的动画的基础有点象这种对头领的跟 随游戏。问题是我们很多人看不到这种动作，而它却展示在我们的工作中。跟我来，慢慢地 把故事展开吧。 介绍：给作用力做动画 很久以前Walt Disney聘用了一位享有声誉的素描大师，叫做Donald Graham的，来教它的动 画师们关于人物素描，关于捕捉运动等等的课程。Donald Graham先生提到的一件事就是： 动画师应该“给作用力，而不是给形式做动画”。听起来太奇妙了，我们都完全同意。一个小问题是我们经常不能真正领会他讲话的精髓。至少我在很长，很长时间里不是真懂。我的意思是说，嗨，我同意这个观点，它听起来非常正确，非常真实。但是如果你问我细则上如何操作，如何详细说给你听，我只能给你一些结结巴巴的无用解答。令人沮丧的是这种缺乏理解渗透到了我的动画中，就像一只湿漉漉的狗身上的味道一样，你仍然像条狗，但是那味儿。。。呃呜。随着时间流逝，我从其他更有经验的动画师那里学习到了Graham先生一直坚称的观点，我们应该给作用力做动画。就像我们后来发现的那样，这一动画的基本底层规律业已在最近几十年中在许多场合中被详细阐述过。但是从来都没有说到点子上。至少我觉得没有。所以我希望在这里所作的一切或许能有助于更多的人“找到点儿上”。就像我的一贯作风，我认为“嘿，如果我为理解这东西费了不少周折，那么我敢打赌肯定还有其他人愿意这东西解释给他们听。”怀着这个美好的愿望，我提出尝试着旧话重提，并用一些新的方式来表现，希望会有些新意，就算是陈年古董焕发光彩吧。或者，至少，这篇文章提升了我的网站的点击率。不管怎么样，生活还是很美好的呵。 定义 我们应该做动画的作用力是什么？力在字典里是这样定义的： 力：作功或者导致物理变化的能力；能量，力量或者有效功率。很酷。第二条定义也具有启发性。 力(2)：使抗拒阻力运转的力量。 现在，这对于我们动画师来说意味着什么呢？让我们看看第一条定义。 一个力就是导致物理变化的东西。在动画中，这可以是内在的推动力也可以是外在的推动力。一个内在推动力的例子可以是当一个角色开始转体时臀部的移动。一个外在推动力的例子则可以是有人抓住了角色的领子并把他提了起来。所以第一条定义可以看作是主要的或者开始的力。 主要的力：内在或者外在的能量源，导致了物理位置的变化和角色身体的运动。 好的，那么第二条定义呢？使抗拒阻力运转的力量。我们看得出力只有对物体有阻力影响时才能看到。问题又来了，这对于我们动画师意味着什么？好的，基本上它意味着我们可以看见力－－通过一种二手的方式－－力抗拒阻力的效果。那么存在何种阻力呢？阻力的主要类型是惯性和动量。什么是惯性呢？我们再来翻翻词典： 惯性：身体反抗加速度的趋势；静止的身体保持静止的趋势。。。对运动，动作或改变的抵抗或厌恶。 所以惯性是对运动的阻力。基本上，我们可以把它称作物体想要蜷缩呆在那里的特性，直到有力迫使其移动。（你愿意的话，也可称作是物理学的懒惰）。惯性的阻力就是抵抗动起来的阻力。就像早上起床的我一样。 那么，动量呢？ 动量：运动中的物理物体的推动力。。。或者一个沿着直线运动的物体要保持匀速直线运动的状态直到被外力所改变。 在英语中这个词的意思是一个运动中的物体会想要保持在原来方向运动，直到有原因改变这样。在这里阻力是试图使运动的物体停下来或者改变其方向。 所以在观察不同的物体怎样抵抗初始力的影响时，我们能够看到有别于主要力的第二种力在发挥作用。我们把这些有证据的力称作次要力或者反作用力。 反作用（次要）力：当物体反抗原始力的影响时，惯性和动量的效果便是反作用力。 好的，物理先生，这是什么意思呢？ 我知道这东西感觉挺学术的。但是上面的定义的价值在于可以帮助我们掌握力的定义，以及它是怎样体现在动画中的。重申一下，我们有原始力和反作用力。或者如果你愿意，也可以叫主要力和次要力。一个原始力是运动的来源。比如说：臀部转动和猛推。反作用力则体现在身体其余的部分如何抵抗这个运动。比如说：胸部和头部滞后，手臂则拖在后面。所以我们应该理解这两个力在我们创建的动画中在构造一个运动时占了很大的一部分。我们要加深理解就要再看看一些例子。首先，是个简单的鞭子／尾巴的例子。 第一个例子展示了内在的原始力是怎样发挥作用的。第二个例子则使用了一个外在的原始力。（是的，这个例子是用关键帧做的，我很懒，但是没有那么懒！） 在第一个例子中，原始（主要）力来自于盒子的移动。而反作用（次要）力则反映在骨骼链的顺次移动上。每个部分都先是拒绝移动直到它再也不能这样做。这个反抗就是惯性。然后每个部分都会遵循它自己的动量直到它被强迫改变这个动量。这一改变或者来自于主要力方向的改变，或者碰到了它自己的活动范围的极限。当方向的改变发生时，由于动量的存在，改变产生的影响要逐级传递到骨骼链的末梢就需要一些时间。但是它终归还是会发生的。第二个例子则显示了不管原始力来自何处，结果总是会同样发生。所有你需要做的就是决定原始力来自于哪里，并把动作做出来。 看看下面的带注释的屏幕图解，以便对二级骨骼怎样受惯性影响有一个更直观的认识。 图中文字：反作用力3 骨骼的滞后表明了惯性的影响 反作用力2 反作用力1 外在的原始力 注意f-curve流向相反的方向，表明了惯性产生的影响 再看看这些动量在Graph Editor中是怎样通过f-Curve来展现的。 图中文字： 注意骨骼2继续它原来的运动方向 即便骨骼1已经改变了它的运动方向 这是动量产生的效果。 然后这张图片描述了两种力是怎样共同起作用的。 图中文字： 骨骼1带路，是原始的力 骨骼2跟随同时对骨骼1的运动产生反作用 注意动作之间的偏移时间，这就是通常所谓的交迭。（Overlap） OK，我想我懂了。你还有没有更漂亮点儿的例子了，自信满满先生？ 我就知道你会问！看看这个怎么样？一个简单的球和尾巴的例子。第一个片段只显示了运动。每个后续的版本通过ghost指定骨骼的动画路径，高亮显示了力对每个尾巴骨骼的影响。运动都是一样的，只是通过侧重点的不同，来帮助阐释这些力是怎样把自己表现出来的。（没错，这次也是手动key的动画，你可以说它有味儿，就像那只湿漉漉的狗。。。） 注意球体的运动方向是怎样递归的影响到尾部的运动的？尾部对主要力（球体运动）的方向变化产生反作用，只是这种作用由于动量的影响而受到了延迟。但是或迟或早，动量的阻力就会让位于来自球体的主要力的影响。 你真逊！你窃取了那个想法！ 好的，对，当然！所有好的艺术都是剽窃。但是只是证明了我能想到我自己（我的妻子可以证明）。我又炮制了另一个例子：做了几个Pose动画的角色从一个盒子上跳下来。我有意识的让它具有明显的Pose to Pose的印记。我在运动曲线上做了一点工作，但是我没有做任何的偏移或那样的旋转。即便如此，这也是花了心思的工作。唯一的问题是，它没有重量感，节奏也不太对。基本上，它缺少运动背后的力度感，看一看这里。 现在再来看看这个应用了体内作用力的版本。。。 看看这回有多少重量感了？你能发现不同身体部位的拖后吗？（手臂，头部，躯干等等）你能识别哪个部分在什么时候展示了惯性？动量呢？你能指出左手的惯性让位于运动的那一帧吗？那么又是在哪一帧动量被克服，让左手由于重力作用而改变方向呢？你能看出内在的原始力是来自哪里吗？（臀部升高，被腿部推动）你能否说出在哪里内部的原始力转换为外在的原始力？（当重力追上了在空中的身体，并把他往下拉）你能看出原始力的改变是怎样影响到所有的反作用力的吗？我还花力气清理了运动曲线。你会发现的一件事是当你开始思考在你的工作中力的作用时，你就同样得注意弧线。基于糟糕的弧线的作用力看起来好像是一堆陈奶酪。力要想适当的发挥作用，就要遵循好的运动曲线。你会注意到的另一件事情是第二个片段有更为显著的挤压和拉伸效果。那是因为挤压和拉伸也是力的一个关键指示器。 这里有一个左右对照的动画，以免你有严重的短暂失忆症，记不住他们的区别。 不论如何，第二个结果看起来要顺眼得多了。但是有一个小小的问题，它花时间！第一个动画花了我最多十五到二十分钟来分出大块儿并清理成你所看到的。第二个版本有帅呆了的力的作用，但是多花了我两三个小时。身体的某些部分在每一帧都设了关键帧，为了更好的说明力的作用，也是为了产生更漂亮的弧线。所以这样的动画真是来之不易。你需要致力于精雕细琢出运动的每一个细节。这意味着花时间来确定原始力源自哪里，它怎样影响身体，身体又是怎样产生次级的反作用力，追踪此力来看看新的力在哪里发挥作用。这意味着要辛苦的关注那些细小的事情，比如运动的弧线和身体各个部分的重量等等。哪些部分先动，哪些在后？由于惯性作用，需要延迟多久？跟随自身的动量又需要多久？当受到原始力的影响的时候，它会向哪个方向运动？所有这些都是固定的常规问题，你需要提问，回答并解决他们，以便获得看起来感觉真实可信的动画。流畅的，被适当的力推动的动画。你不能仅仅在Dopesheet中拖拽出几个偏移帧就认为你已经掌握了表现力的诀窍。还有比这多得多得多的工作要做的。如果你想要一个问题列表来问你自己，请查看我的另一篇文章：Life After Pose to Pose。这是继Pose to Pose之后的后续篇－－有力的生命。 等一下，。。。我以前看过这个！ 我希望你最好迟早掌握它。这不是什么新东西，也不是我的想法。这样的规律已经被人们口头传颂了很多年了，也以不同的方式描述过。看，动画书籍已经描写这一运动特性有许多年了。这也被称做：交迭，行动过火，滞后，重量感，连续的骨骼阻断，有时候还会被称作二级运动，等等。但是每一种都在孤立的描述当原始力和反作用力互相对抗时对角色产生的影响。所以这些书籍把规则封装起来，作为诊断优秀动画症状的手段。这些年来我们所作的就是要把规律从上下文中脱离出来.我们把这些做成了检查列表上的条目.我们在我们的动画中强行使用这些规律,却并不知道,好的动画之所以优秀,并不是因为在里面强行塞满了所有的动画规律.而是优秀在它抓住了那一时刻的力的能量.动画规律只是检测你的动画为什么不好的诊断方法.这是个小事,但是把马车架在了马前面,我们就失去了追踪的意义.那么这是否意味着这些书和作者全都错了呢?绝对不是,小子!这只是意味着他们把力分解开了,并作为单一的规律来进行描述,多年来全世界的学习动画的学生也是这样学习和记忆的.事实上是我们欠缺一种感觉,以便把这些单一影响进行通盘考虑,综合分析因果关系,原始力与反作用.我们记得这些力的效果,但是没有弄懂怎样通过这些效果来阐释力的存在.我们在学习字母表前先学习了漂亮的书法!在这里,通过对主要力和次要力的目光更宽泛的观察理解,以及通过一些说明它们是如何应用的简单的实例,我希望能让你更深入的理解怎样使用这些效果来更好的描述力的存在.同样希望,这些动画规律不再彼此孤立的出现在你的动画中,而是作为整个力的动画的一部分而存在,就像Don Graham在五十多年前提到的那样.对于这些力如何彼此交互作用并把能量反映在我们的运动中这一知识点,我们需要有更深刻更广泛的认识和理解. 现在,再去看看你最喜欢的动画吧,去搜寻这个动画师是怎样对肢体应用原始力或驱动力,而肢体又是怎样通过抵抗这个力而展示出次要力的.你会发现这样的好范例无处不在,你会惊呼:太棒了,做的真酷!具有重量感和强烈流畅感对于一个好的动画是非常关键的.所以如果你觉得你的Pose很扎实,你的Timing也相当得体但是你奇怪为何你的动画就是达不到高级水平呢,那么请你看一看你的带领和跟随的力的感觉.在好的动画中我敢打赌你会发现一些精华,当你把规律理解并应用在动画中去,你会更清楚的审视你原有的动画水平,并使自己的能力得到提升. 好了,我今天的傻事干完了.我希望会有所帮助.如果没有,尽可以致电你的议会代表对其提出控告,那可真是天怒人怨呵. Keith 原文： You guys ever play Follow the Leader when you were kids? One kid does something, the other kids follow along? The leader kid goes out into the yard and the rest tag after him like a conga line. At the foundation of good animation is a bit of this Follow the Leader thing. Problem is a lot of us don't see it and it shows in our work. Follow along as I unfold the tale.... Intro: Animating Forces Long ago Walt Disney hired a renowned life drawing instructor named Donald Graham to come and teach his animators about life drawing, about capturing motion, etc. One of the things that Mr. Graham said was that the animators should "animate forces, not forms". Which all sounds just fantastic! We all agree wholeheartedly. One small problem is that we often don't have a stinkin' clue as to what he's talking about. At least I didn't for a long, long time. I mean, yeah, I agreed with it, it sounded so right, so real. But if you had asked me to break it down and define it for you I'd have given you a stammering useless reply. Sadly this lack of understanding permeated my animation like the smell of a wet dog. You still like the dog, but that smell... ewww. As time went by I learned from other more experienced animators what Mr. Graham was on about when he said we should animate forces. And as we'll discover later, I then learned that this essential ground rule of animation HAD been defined for us in detail on many occasions in the intervening decades, but the dots had never been connected. At least not in my mind. So what I hope to do here is maybe help a few more of us "connect the dots" to complete the picture. As is my usual form, I figured "Hey, if I struggled to understand this for a long time, I bet there's other folks who would love to have this explained to them as well." And with this noble intent in my heart I have set forth to try and re-word this basic idea in some fashion that hopefully will be fresh and shed new light on an age old subject. Or at the very least it'll drive up the hits on my site. Either way, life is good. Definition What is a force that we should animate it? Force is defined in the dictionary as: Force: The capacity to do work or cause physical change; energy, strength, or active power: Pretty cool. The second definition is also enlightening: Force (2): Power made operative against resistance; Now, what does this mean to us as animators? Let's look at that first definition. A force is the thing that causes a physical change. In animation this can be either an internally motivated force or an externally motivated force. An example of an internal force would be the hips shifting as a character begins to turn their body. An example of an external force would be someone grabbing a character by the shirt collar and pulling them. So this first definition of force could be called a Primary or Initiating Force. Initiating (Primary) Force: that internal or external source of energy which enacts a change of physical position and motion in the body of a character. OK, what about that second definition? Power made operative against resistance. Here we see that there are forces that are seen only by their effect on resisting objects. So the question comes again: what does this mean for us as animators? Well, basically it means that we can see forces in a second hand fashion- by the force's effect against resistance. What kinds of resistance are there? The primary types of resistance are inertia and momentum. What is inertia? Again we turn to the dictionary: Inertia: The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest ... Resistance or disinclination to motion, action, or change: So inertia is the resistance to motion. Basically we can call it the overarching desire to be still until forced to move. (a kind of laziness of physics if you will). The resistance of Inertia is the resistance against getting going. Kinda like me in the morning. OK, so what about momentum? Momentum: Impetus of a physical object in motion... or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force. In plainer English that means that an object in motion will tend to want to continue in motion along the same direction until it has a reason not to. The resistance here is to try and stop something in motion and change it's direction or stop it's motion altogether. So the evidence of a second kind of force other than the Initiating Force or the primary force can be seen in how various objects resist the impact and effect of the initiating force. We can call these evidentiary forces Secondary or Reacting Forces. Reacting (Secondary) Forces: the evidence of the effects of inertia or momentum in the body as it reacts to the influence of the Initiating Force. OK, Mr. Physics, What Does This Mean? I know this seems rather academic, but the value of the above definitions will help us grasp the notion of forces and how they are seen in animation. To recap, we have Initiating and Reacting forces. Or if you prefer, Primary and Secondary forces. An initiating force is the origin of the movement. For example: the hips turn & thrust. The reacting force is shown in how the rest of the body resists that motion. Example: the chest and head lag, the arms drag behind. So we should understand that these two forces make up a vast chunk of how we construct the motion we create in animation. For us to get any more understanding we're going to need to look a a few examples. First, a simple whip/tail example. The first sample movie shows how an Internal Initiating Force can play out. The second sample uses an External Initiating Force. (yes, this stuff was hand keyframed. I'm lazy, but not that lazy!)  .....  (click images to play movies) In the first example the initiating (primary) force comes from the moving of the box. The reacting (secondary) forces are seen in the lagging motion of the subsequent joints down the line. Each part is going to first resist movement until it can no longer do so. This resistance is Inertia. Then each part will follow its own Momentum until it's forced to change that momentum. That change will come either by a change of direction in the primary force or by hitting it's own motion range limits. Once this change of direction occurs, it takes a while for that change to effect the subsequent joints down the line due to their momentum. But they are eventually effected. The second example just shows that regardless of where the Initiating force originates, the results play out the same. All that you need to do is determine where the initiating force is coming from and play the action out from there. Look at the following screen grabbed illustration for more tasty insight into how the second joint demonstrates the effect of Inertia....  And then there's this little number relating to what Momentum looks like for you f-curve junkies...  Then this image describing how things play together...  OK, I Think I Get It. Do You Have Any More Nifty Examples, Mr. Big Britches? Why I thought you'd never ask! How about this example, a simple Ball & Tail. The first clip just shows the motion. Each subsequent version highlights how the forces affect each tailing joint by ghosting a given joint's animation path. It's all the same motion, just with different emphasis to help illustrate how these forces play themselves out. (And yes, this too is hand keyed. You can tell because it stinks. Like that wet dog.....)  .......  .....  .......  Notice how the change in the ball's direction has a recursive effect on the tail motion? The tail reacts to that change of direction in the primary force (the ball moving), yet it is delayed due to the resistance of momentum. But sooner or later momentum resistance gives way to the influence of the primary force from the ball. You Lamer! You Stole That Idea! Well, duh! Of course! All good art is stolen. But just to prove that I can think for myself (with my wife's permission, of course) I've cooked up another example of some quickly posed animation of a character jumping down off a box. I purposefully kept this pretty much stock pose to pose animation. I did a little work on a few of the motion arcs, but I didn't do any offsetting of keys or any of that rot. Even so, it's a spunky little bit of business. The only problem is, it feels weightless and the timing just feels wrong. Basically, it lacks any sense of the forces behind the motion. Here, have a look...  Now have a look at this version of the jump down with the application of forces in the body...  See how much weight it has? Can you detect the lagging of various body parts? (arms, hands, head, torso, etc.) Can you identify which parts exhibit inertia and when? How about momentum? Can you pick out the frame at which the inertia gives way to motion for that left hand? How about which frame the momentum is overcome and the left hand changes direction due to the force of gravity? Can you see where the internal initiating force is coming from? (the hips rising up, pushed by the leg). Can you tell where that internal initiating force switches to an external initiating force? (when gravity overtakes the body in the air and pulls him down). Can you see how that changing of initiating force affects all the reacting forces? I also took the liberty to clean up the motion arcs. One thing you'll discover is that when you start thinking about forces in your work you'll need to pay attention to the arcs as well. Forces on bad arcs look like a big ol' pile of old cheese. For forces to flow properly they need to be channeled into good arcs. And another thing you'll notice is that the second clip has more pronounced squash & stretch. That's because squash & stretch is also a key indicator of force. Here's a side by side comparison in case you have severe short term memory loss and can't remember the difference...  Anyhow, the second result is far more desirable to watch. But there's one small catch: it takes work! The first animation took me about 15-20 minutes tops to get blocked in and cleaned up to where you see it. This second version with all those groovy cool forces at work took me about 2-3 hours more. Some of the body parts have a key on every frame to better define the forces as well as the motion arcs. So this stuff doesn't come cheap. You need to be dedicated to ferreting out all the details in the motion. This means taking the time to determine where the initiating force is originating from, how it affects the body, how the body will react in a secondary fashion, then tracking to see where a new force plays it's role. This means painstaking attention to the little things like motion arcs and masses for all the body parts. What goes first? Then what goes next? How long should it be delayed due to inertia? How long should it follow it's own momentum? What direction will it go when it's affected by the change in the initiating force? All these are constant questions you need to ask and answer and solve to have your animation show force in a fashion that feels natural, flowing and properly motivated. You can't just shuffle offset a few keys in the dopesheet and think you've got this forces thing knacked. It takes a ton more work than that. If you want a list of questions to ask yourself, check out my other manifesto on Life After Pose to Pose. This is life after Pose to Pose- life with force! Wait a Minute... I've Seen This Before! I was hoping you'd catch on sooner or later. This is not new information. This is not my idea. This stuff has been under our noses for decades, described in a different way. See, animation books have been describing this activity in motion for years. It's been called overlap, overshoot, lag, weight, successive breaking of joints, sometimes erroneously called secondary action, etc. But each of those are isolated descriptors for the effects of initiating and reacting forces as they work against the bodies of our characters. So the books have encapsulated Principles as a means of diagnosing the symptoms of good animation. What we've done over the years is divorce the Principles from their context. We've made them items on a checklist. We force these principles into our animation without understanding that good animation isn't made good by force feeding all the principles onto the work. Rather good animation is good because it captures the energy and force of a moment. The Principles are just a means for diagnosing why your animation isn't good. It's a slight thing, but by putting the cart before the horse we've lost track of the meaning. Does this mean these books and writers have been wrong all this time? No way, dude! It just means they broke down the forces and then described the effects as singular principles, which animation students the world over have been learning and memorizing for decades. The thing is that we've lacked a sense of context in which to USE these singular effects as an interweaving tapestry of cause and effect, initiation and reaction. We memorized the effects of forces without knowing the context of how to use these effects to illustrate force. We learned good handwriting before we learned the alphabet! By describing this larger overview of primary and secondary forces and showing a few simple examples of how they can be applied I hope to give us a little bit better understanding as to HOW to use these long described effects. Hopefully now these principles are no longer isolated items to add to your motion willy nilly, but are instead just parts of the overall animation of forces that Don Graham talked about more than 50 years ago. We need to think of the larger context of how these forces interact with each other to effect their power on our motion. Now go ahead and watch your favorite animation some time, looking just for how the animator used this idea of the initiating or driving force in the body and how the secondary forces were revealed as the body resisted the forces. You'll see this stuff EVERYWHERE and you'll soon realize "Holy Smokes, this stuff is cool!". That and it's really a key to animation that has weight and a strong sense of flow as well. So if you think your poses are solid and your timing is fairly decent but you're wondering why your stuff just isn't hitting that top shelf level, take a look at your forces of lead and follow. I'm willing to bet that you'll find some gems there that, when understood and applied in context, will help you step off that plateau you've been sitting on. OK, I'm done making a right fool of myself for today. I hope this has helped. If it hasn't, then please feel free to call your congressional representative to lodge a complaint. This indeed could very well be a national outrage. -keith |
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