Adobe After Effects 7.0 Studio Techniques

Expressions

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Conditionals and Triggers

Also called If/Then statements, conditionals open up extra possibilities by allowing you to use any circumstance

that expressions can recognize—pretty much any numerical or Boolean state anywhere in the Timeline—as a

condition for an event or a series of events to occur.

Triggers often use conditional statements; they wait for a specific event to occur and use this to trigger another

event. The possible uses for triggers are seemingly endless.

Trigger a Dissolve

Here’s a simple trigger that needs no conditonal statement: A composition marker triggers a one-second filmic

dissolve (this will work equally well with layer markers). This expression could just as easily be set up to trigger

a different operation, such as a simple opacity fade to black or white, but once you understand the basics, you

can adapt it however you like.

Tip

A project containing a dissolve and a fade to black, each using the expression described here, can be

found on your disc as 10_dissolve.aep.

A layer marker can be set by highlighting a layer and pressing * at the frame where the marker should appear.

You can use this shortcut to set a marker interactively while looping a RAM Preview.

Add the Levels (Individual Controls) effect to the layer. Reveal controls for Input Black and Output White;

you’ll apply the same expression to each, with the variation that they will animate in opposition. The expression

for Input Black is

mark = marker.key(1).time;

linear(time, mark, mark+1, 1, 0)

Close-Up: The Dissolve

A fade to black is typically accomplished either by reducing Opacity of a source layer (or layers) from

100% to 0% over time, or by fading up a single black solid layer from 0% to 100% over time. This very

linear transition lacks the visual interest of a true film dissolve, which can have various characteristics

depending on the type of film stock and so on, but which takes on more interest by fading in a nonlinear

fashion. This section teaches you how to use an expression to do whatever you want, and for free

throws in a simple technique to mimic the response of a film dissolve. As is shown beginning in the

next chapter, lowering exposure in 32-bpc HDR naturally behaves like a film dissolve.

The inversion for Output White is

mark = marker.key(1).time;

linear(time, mark, mark+1, 0, 1)

Here’s what’s happening. This is the first example in this chapter of a variable. This one is called mark, named

for the fact that it identifies a marker (and in expression of the author’s vanity). How do you know it’s a marker

and which one it is? The variable’s argument tells you; the marker portion signifies a layer marker, key(1)

specifies the first one, and time specifies the time at which that marker occurs.

The linear(t, tMin, tMax, value1, value2) command is so compact and efficient that it’s easy to forget

it even exists. It has five (count ’em) arguments, all required, that identify, in order, the

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• Time that the effect evaluates

• Time to start a linear transition

• Time to end a linear transition

• Value at the start

• Value at the end

Tip

You can apply an expression directly from the Effect Controls panel by Alt/Option-clicking on the

stopwatch, the same way you would in the Timeline panel. This can be convenient because the property

with the expression is revealed in the Timeline as well, where you can edit the expression.

Each expression first evaluates the current time (time). When that time reaches mark (marker 1), After Effects

performs a linear transition from 0 (0%) to 1 (100%), and the transition lasts one second (until mark + 1,

or one second past marker 1). The result is visible in the Graph Editor and the composition itself (Figures

10.19a and b).

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Figures 10.19a and b. Expressions are set for Input Black and Output White (a). Note

how the look differs from a simple fade to black (b). Although it’s difficult to spot in

a printed figure, the highlights become richer while the blacks deepen, as opposed to

the entire image turning gray.