After Effects表达式教程：从入门到精通

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"after effects expressions 入门到中级教程" 在影视后期制作中，Adobe After Effects 是一款强大的视觉特效、动态图形和动画编辑软件。表达式（Expressions）是 After Effects 的一个重要功能，它允许用户通过编写简单的脚本来控制图层属性，实现复杂的动画效果。这个教程可能是针对希望提升 After Effects 技能的中级用户，帮助他们从基础开始学习表达式的使用方法。表达式在 After Effects 中的作用： 1. **自动化动画**：通过表达式，你可以创建自定义的动画路径，而无需手动关键帧。这在创建循环动画或响应式设计时特别有用。 2. **关联属性**：表达式可以将一个图层的属性与另一个图层或同一图层的其他属性链接起来，使得变化同步，提高工作效率。 3. **动态响应**：基于时间、位置、颜色等变量，表达式可以创建动态响应的元素，例如根据声音波形创建可视化效果。 4. **简化复杂操作**：对于需要大量手动调整的项目，表达式可以大大减少工作量，通过一行代码解决多步操作。教程可能涵盖的内容： 1. **基础知识**：讲解表达式的基本语法，包括变量、运算符、函数等，以及如何在图层属性上启用表达式编辑器。 2. **常用表达式**：介绍如 `time`、`valueAtTime`、`random` 等常用内置函数，以及如何使用它们创建动态效果。 3. **对象模型**：解释 After Effects 中的对象模型，如何访问和控制图层、属性、效果等元素。 4. **表达式控制器**：利用表达式控制器来创建更高级的交互性，例如通过滑块控制图层属性。 5. **实用技巧**：分享提高效率的表达式编写技巧，如错误检查、优化性能等。 6. **案例分析**：通过具体的项目实例，演示如何运用表达式解决问题，包括标题动画、粒子系统、跟踪和稳定等常见任务。掌握 After Effects 表达式不仅可以提升工作效率，还能增强创意表达。这个中级教程旨在帮助用户理解并熟练应用表达式，进一步提升在 After Effects 中的动画和视觉设计能力。通过深入学习和实践，用户可以创作出更具动态美感和创新性的作品。

Introduction

or “The dog was taken for a walk by me. ” The latter is awkward, but it still

makes sense. We can understand it. But that’s because we’re smart. Computers

are dumb. They only understand commands that follow rigid rules, so when

we write JavaScript, we need to follow these rules.

That actually makes JavaScript much easier to learn than a human language. If

you learn French or German, you need to expend a lot of energy memorizing

irregular verbs and genders. Because JavaScript is so rigid, there are few irregu-

larities. Wiggle is always wiggle. It’s never wiggled, wiggler, or wobble.

You may have heard of JavaScript before. It’s built into web browsers, such as

Safari, Firefox, and Internet Explorer. It’s also built into Photoshop, Illustrator,

After Effects, Dreamweaver, and many other graphics and design applications.

Flash employs a JavaScript-like language called Actionscript. The Expressions

language is based on a subset of JavaScript, with a bunch of AE-speciﬁ c addi-

tions thrown in by Adobe.

If you’re like many designers I’ve met, you’re probably starting to hyperventi-

late about now. “I didn’t sign aboard to be a programmer! ” you’re thinking.

“ Where’s the exit? ”

I started out as a designer with no interest in programming. But I had to do

some web design, and I needed to learn JavaScript to create some animated

effects. My intention was to learn just a tiny bit of programming: just enough

to achieve the effects I was working on. But as my projects grew in complexity,

I gradually found myself learning more and more. After a while, I understood

JavaScript as well as, say, an American might understand French if he’s lived in

Paris for a few years and has had to use it each day to ask, “How much does

that cost? ” and “Where’s the bathroom? ”

When I got into AE, I was thrilled to discover that it spoke the same lan-

guage as web browsers. If it hadn’t, I might never have learned Expressions. I

really didn’t want to learn a second language. Luckily, I didn’t have to. And

as I leveraged my JavaScript skills within AE Comps, my programming skills

got stronger. Eventually, I learned Flash programming, and once again found

myself speaking JavaScript—this time like a native.

You may have no interest in becoming a computer programmer, but if you learn

some JavaScript, even if you learn it kicking and screaming, you’ll have new tools

that can help you out in many of the applications that you use every day.

Introduction

xvi

AE lets you use JavaScript in two different ways: Expressions and applica-

tion scripting. Expressions let you manipulate effects and transforms in the

Timeline. In other words, they are tools to help you create animation and

compositing effects. Application scripting allows you to control After Effects

itself. With scripting, you can add new commands to AE’s menus. You could,

for instance, add a command that automatically renders out a Comp at 3:30

on Wednesday and e-mails you when it’s done. Such scripting is also avail-

able in Photoshop and Illustrator to help you extend the feature sets of those

applications.

Because this is an Expressions book, that’s all I’m going to say about applica-

tion scripting. But it’s nice to know that, once you learn Expressions, you won’t

have to learn a whole new language for scripting. (If you want to play with

some sample scripts, you can ﬁ nd them in the File  Scripts menu.)

One more note about JavaScript. It’s not the same as Java. Java is a completely

different language (used mostly on web servers and never in AE). Not surpris-

ingly, people always confuse Java and JavaScript with each other. JavaScript

is not a “lite” version of Java. JavaScript is as distinct from Java as Spanish is

from Portuguese. Why do they have such similar names? Because the company

that developed JavaScript (Netscape) had a partnership with the company that

developed Java (Sun Microsystems). Rumor has it that the Sun developers drink

a lot of coffee. Go ﬁ gure.

DO I NEED TO BE A PROGRAMMER?

No. Although you write Expressions in JavaScript, which is a programming

language, you don’t need to be a programmer to craft useful Expressions. You

don’t even need to know much about JavaScript. In fact, the beginning chapters

of this book will show you tons of useful techniques you can use without any

programming knowledge at all. And although it’s true that you can go on to do

even more useful things if you do know how to program, you’re not limited to

boring, 101-type effects if you don’t. You can create complex, original effects

without coding.

In fact, most of the time you won’t even have to type. AE includes an amazing,

magical tool called the Pick Whip, which allows you to create JavaScript with

your mouse. You drag, point, and click; AE writes the code for you. You don’t

even have to understand the code. It just works.

Introduction

xvii

For readers who want a deeper understanding, I’ve included advanced steps in

later chapters. You can decide, in your own time, whether or not you want to

tackle these chapters. It’s my sincere hope that this book will be useful to you,

even if you only read 50% of it. (Of course, I’d be thrilled if you read the whole

thing.) For those of you who do want to learn it all, I promise to hold your

hand through the process and gradually guide you from the simple to the com-

plex, without making any sudden leaps.

Even if you never delve into complex JavaScript yourself, you can still use other

people’s scripts. JavaScript is just text, and text can be copied and pasted. Back

when I ﬁ rst started to use Expressions, I needed to create a pendulum. But I had

no idea how to write such an Expression. Luckily, I was able to ﬁ nd one online.

I read it and was totally bafﬂ ed by it. It looked like a scary mathematical equa-

tion. For a second, I felt panicked. Then I calmed down, selected the text of

the Expression in my browser, copied it to the clipboard, switched over to AE,

and chose Edit  Paste. Voila! My pendulum swung back and forth beautifully.

I had no idea how it worked, but it worked, and that’s what mattered most.

This book is for those of you who don’t care how it works, as long as it works.

It’s also for those of you who do care how it works and want to be able to cus-

tomize it and write other Expressions that are just as complex. This book is for

the auto mechanic. But it’s also for the guy who just wants to drive his car to

work and doesn’t really care what goes on under the hood.

DO I NEED TO BE GOOD AT MATH?

No. Many Expressions don’t use math at all (even though, in the end, they

spit out numbers). If you have to type at all, you’ll type friendly words like

“ wiggle. ” Of the Expressions that do use math, most use simple arithmetic. So

you may have to strain your brain by adding and subtracting, on occasion.

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