{"id":2659,"date":"2018-05-08T16:48:32","date_gmt":"2018-05-08T16:48:32","guid":{"rendered":"http:\/\/ciraldo.stream\/uwm-vfx1\/?page_id=2659"},"modified":"2018-05-08T21:42:49","modified_gmt":"2018-05-08T21:42:49","slug":"intro-to-expressions","status":"publish","type":"page","link":"https:\/\/ciraldo.stream\/uwm-vfx1\/topics\/intro-to-expressions\/","title":{"rendered":"Intro to Expressions"},"content":{"rendered":"

Intro to Expressions<\/strong><\/h2>\n

 <\/p>\n

So far we have seen that properties<\/strong> can be animated<\/strong> using keyframes.<\/strong> There is an additional way to animate properties called “expressions.”<\/strong><\/p>\n

“Abracadabra!”<\/em><\/strong><\/h3>\n

In After Effects, expressions<\/strong> are almost like little “recipes” or “spells” that can be attached to each individual\u00a0property<\/strong>\u00a0and executed on each frame. They look like text<\/strong> that has been typed into a small text area next to the property in the Timeline:<\/strong><\/p>\n

\"\"
The Rotation<\/strong> property has an expression attached which divides its value in half<\/strong><\/figcaption><\/figure>\n

In the example above, the word “value”<\/strong><\/span> is a special term which refers to the current value of a property.\u00a0<\/strong>The Rotation property was initially set to\u00a0“100”<\/span>,<\/strong>\u00a0and so “value \/ 2”<\/strong><\/span> equals “100 \/ 2”<\/strong><\/span>\u00a0which equals\u00a0“50”<\/span>.<\/strong><\/p>\n

Note that the value “0x+50.0\u00b0”<\/strong><\/span> appears in red.<\/strong> This indicates that an expression has been applied. Also, the blue equals sign<\/strong> is a toggle for turning the expression on or off.<\/strong><\/p>\n

Turning on (aka enabling) expressions for a property<\/strong><\/h3>\n

Expressions are enabled for a property by option\/alt-clicking the stopwatch icon<\/strong> next to a property. Do the same thing to turn the expression off, or simply erase the text inside the text area.<\/p>\n

If you turn off the expressions for a property, all the text in the text area is lost, so hit undo\u00a0<\/strong>if you did that by accident. Sometimes simply toggling the blue equals sign is a nicer way to go, if you simply want to temporarily disable it.<\/p>\n

Properties values that are multiple numbers<\/strong><\/h3>\n

Some property values are simple,\u00a0single numbers,<\/strong> like Rotation<\/strong> or Opacity.<\/strong><\/p>\n

But some property values are combinations of numbers,<\/strong> like Scale<\/strong> (e.g., “100%,111%”<\/span>) or Position<\/strong> (e.g., “640.0,480.0”<\/span>). When typing out values that contain multiple numbers, use the following square-bracket syntax:<\/strong><\/p>\n

\"\"
An expression attached to Position<\/strong> which sets the value to “120, 30”<\/strong><\/figcaption><\/figure>\n

Expressions on more than one line<\/strong><\/h3>\n

Some expressions take up more than one line<\/strong>. You can resize the expression area by click-dragging on the bottom border<\/strong> of the text area. You must separate multiple lines using a semicolon<\/strong> as shown below:<\/p>\n

\"\"
A multi-line expression on the Scale<\/strong> property that cuts the width in half<\/strong> and adds 35% to the height<\/strong><\/figcaption><\/figure>\n

Storing intermediate values in variables<\/strong><\/h3>\n

During an expression, you can store intermediate values in things called variables.<\/strong> Variables are just words or phrases (without spaces)<\/strong> that you make up. (They also can’t start with a number,<\/strong> and there’s certain reserved keywords<\/strong>\u00a0like “value”<\/span>\u00a0that are off limits, too.) The general pattern looks like this:<\/p>\n

variableName = something<\/strong><\/span><\/p>\n

That puts the value “something”<\/strong> into the variable “variableName”<\/strong>.<\/p>\n

Here’s some more concrete examples:<\/p>\n

temp1 = 10;<\/strong><\/span>
\nmyCoolVariableName = [96,205];<\/strong><\/span>
\nspaceshipSize = myCoolVariableName;<\/strong><\/span><\/p>\n

When you need to refer to the individual numbers of a multi-number variable<\/strong> (like myCoolVariableName<\/b> above) the square-bracket syntax looks like this:<\/p>\n

myCoolVariableName[0]<\/span><\/strong><\/p>\n

which is equal to 96<\/span>,<\/strong> or:<\/p>\n

myCoolVariableName[1]<\/strong><\/span><\/p>\n

which is equal to 205<\/span>. <\/strong>As you can see, inside the square-brackets, the positions of the individual values are numbered starting at<\/strong> 0.\u00a0<\/span><\/strong><\/span><\/p>\n

Take another look at the multi-line expression example<\/strong> above to see this syntax in action as well.<\/span><\/span><\/p>\n

Comments within expressions<\/h3>\n

Expression syntax is very precise, exacting, and not always intuitive. So sometimes it’s helpful to store little notes to yourself<\/strong> within the expression. These are called comments.<\/strong>\u00a0After Effects basically ignores anything inside a comment. You can start a one-line comment<\/strong> with double-forward-slashes (ie, ‘\/\/’<\/strong><\/span>) or enclose multi-line comments<\/strong> with a \/*<\/strong><\/span> and a *\/<\/span><\/strong> like this:<\/p>\n

\"\"
Two kinds of comments\u00a0<\/span><\/figcaption><\/figure>\n

How After Effects evaluates the result of an expression<\/strong><\/h3>\n

On each frame, After Effects evaluates each line of an expression\u00a0from top to bottom,<\/strong> and then uses the result of the bottom-most line<\/strong> as the new value<\/strong> of the property. (In one-line expressions, the one line is the bottom-most line.)<\/p>\n

For illustration purposes, here’s a silly expression that has multiple lines for no reason:<\/p>\n

\"\"
A pointless multi-line expression<\/figcaption><\/figure>\n

In the example above, AE evaluates each line from top to bottom, and then uses the bottom-most line as the value for the property. Since the bottom-most line has nothing to do with the lines above it and is simply “12”<\/span>,<\/strong> the property gets set to 12<\/span>.<\/strong><\/p>\n

If you go back and look at the multi-line expression example<\/strong> above, you can see a less silly demonstration. In that example, the bottom-most line is [temp1,temp2]<\/span>,<\/strong> which AE will then use as the final value to assign to the Scale<\/strong> property.<\/p>\n

Keyframes and expressions combined<\/strong><\/h3>\n

Sometimes a property can have both keyframes and expressions<\/strong> enabled. On each frame, After Effects first<\/strong> evaluates the effect of the keyframes, and then<\/strong> applies the effect of the expression. Here’s an example of an property that is keyframed and has an expression enabled:<\/p>\n

\"\"
A property with keyframes and expressions enabled<\/figcaption><\/figure>\n

In the example above, the\u00a0Blurriness\u00a0<\/strong>property is keyframed from 0<\/span> <\/strong>to 10<\/span>.<\/strong> The current time is right in the middle of the keyframes, and so without any expression the value is 5.0<\/span>.<\/strong> With the expression applied, the value becomes 5.0 + 1<\/span>,<\/strong> or 6.0<\/strong><\/span> (seen in red<\/strong>).<\/p>\n

The expression language menu<\/strong><\/h3>\n

Since it can be tough to memorize all the precise ways to write expressions, After Effects provides a handy little menu that will plug in expression code for you. You access it by clicking on the little black triangle<\/strong> to the right of the blue equals sign:<\/strong><\/p>\n

\"\"
The expression language menu<\/figcaption><\/figure>\n

The most commonly used expressions are under the Property<\/strong> submenu.\u00a0For example, to add a loopOut<\/strong> expression to a property, which loops the animation of a property’s keyframes, select Property > loopOut(…):<\/strong><\/p>\n

\"\"
After the loopOut shortcut has been applied<\/figcaption><\/figure>\n

Referencing other properties, and the expression pick whip<\/h3>\n

In your expressions you can refer to the values of<\/strong> other properties.<\/strong><\/p>\n

Each property has a text name<\/strong>\u00a0for use in expressions that is somewhat reminiscent of the way street addresses work. (When you enable expressions for a property, you’ll first see the text name<\/strong> of that property.)<\/p>\n

In the example below, the Opacity<\/strong> of the Bat layer<\/strong> is tied to the Rotation<\/strong> of the Pop layer.<\/strong><\/p>\n

\"\"
Whatever Pop’s Rotation<\/strong> is set to, Bat will use that as its Opacity<\/strong><\/figcaption><\/figure>\n

But<\/strong> remembering the syntax of how other properties are written out is tough, so After Effects provides the expression pick whip<\/strong> to make your job easier. It’s the little spiral icon<\/strong>\u00a0(or snail?) to the left of the expression language menu.<\/strong><\/p>\n

To use it, first make sure the other property you’re trying to refer to is visible in the timeline. Then, with the text cursor inside the expression text area, click and drag<\/strong> on the expression pick whip<\/strong> to reveal the stretchy whip line. Drop the whip onto the other property.<\/strong> AE will insert the correct text name of the other property.<\/p>\n

This is how you can create relationships between properties<\/strong> that go above and beyond what parenting can do. For example, you can have a layer act as a master opacity controller<\/strong> for other layers, and only do keyframing on the master layer:<\/p>\n

\"\"
A master layer with subordinate layers<\/figcaption><\/figure>\n","protected":false},"excerpt":{"rendered":"

Intro to Expressions   So far we have seen that properties can be animated using keyframes. There is an additional way to animate properties called “expressions.” “Abracadabra!” In After Effects, expressions are almost like little “recipes” or “spells” that can be attached to each individual\u00a0property\u00a0and executed on each frame. They look like text that has … <\/p>\n

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