Adobe 12040118 Tutorial - Page 556

You can apply Expression Controls effects to any layer; however, it is useful to apply them to a null layer, which you can simply use as a controlling layer. You then add expressions to properties on other layers to take input from that control. For example, you can add the Slider Control effect to a null layer (Null 1), and then apply this expression to the Position properties of multiple layers: position+[0,10*(index-1)*thisComp.layer("Null 1").effect("Slider Control")("Slider")] In this example, as you drag the slider, each layer with this expression moves. The layers with greater index numbers (layers toward the bottom in the Timeline panel) are shifted more than the layers with lower index numbers, at 10-pixel intervals. You can set keyframes for the slider on the null layer, and all of the other layers animate accordingly. It is often useful to rename each instance of an Expression Control effect to indicate its use. For example, renaming an instance of the Color Control effect to sky color makes it easier to tell what the effect is controlling. You can rename each instance of an Expression Control effect just as you rename any other effect: select it in the Timeline panel or Effect Controls panel and press Enter on the main keyboard (Windows) or Return (Mac OS). To modify the range of a control, right-click (Windows) or Control-click (Mac OS) the underlined property value for the control and choose Edit Value from the context menu. Additional resources about Expression Controls effects Jerzy Drozda, Jr. (Maltaannon) provides a simple explanation and demonstration of the Angle Control effect and Expression Controls effects in general on the Motionworks website. Aharon Rabinowitz's three-part "After Effects Expression Controls" series of video tutorials on the Creative COW website introduces the Expression Controls effects and shows some basic examples of their uses. Harry Frank provides a clock project on his graymachine website, in which the movements of the hands are connected to an Angle Control effect on a null layer that controls the time. Todd Kopriva provides instructions for using the sampleImage method and the Point Control effect to monitor colors for a specified point during color correction on his After Effects Region of Interest blog. Convert an expression to keyframes To the top In some situations, it's useful to convert an expression to keyframes. For example, if you want to freeze the values in an expression, you can convert the expression to keyframes and then adjust the keyframes accordingly; or, if an expression takes a long time to evaluate, you can convert it to keyframes so that it renders more quickly. When you convert an expression to keyframes, After Effects evaluates the expression, creating a keyframe at every frame, and then disables the expression. In the Timeline panel, select the property on which the expression is written and choose Animation > Keyframe Assistant > Convert Expression To Keyframes. The expression language To the top The After Effects expression language is based on JavaScript 1.2, with an extended set of built-in objects. After Effects uses only the core standard JavaScript 1.2 language, not the web browser-specific extensions. After Effects contains its own set of extension objects-such as Layer, Comp, Footage, and Camera-that you can use to get most of the values in an After Effects project. Though the expression language is based on a scripting language, a subtle but important difference exists between a script and an expression: Whereas a script tells an application to do something, an expression says that a property is something. For more information about JavaScript, see a JavaScript reference resource. When creating expressions, keep in mind the following: The value of an expression is the value of the last statement evaluated. JavaScript is a case-sensitive language. Semicolons are required to separate statements or lines. Spaces between words are ignored, except within a string. In JavaScript, a value stored in an object is called a property. However, After Effects uses the term property to refer to layer components as defined in the Timeline panel. For this reason, After Effects refers to JavaScript properties as either methods or attributes. In general practice, the difference between a method and an attribute is that a method usually does something to create its output (return) value, whereas an attribute simply refers to an existing value to determine its output (return) value. You can tell a method from an attribute most easily by looking for the parentheses following the method name, which surround any input arguments to the method. An object is an item that can contain other objects, attributes, and methods. Compositions, layers, and footage items are examples of objects. Specifically, compositions, layers, and footage items are global objects, which means that they can be referred to in any context without reference to some higher-level object.