Adding keyframes to slider controls is a useful way of animating the properties of your lens flare. But there's a way to "program" your lens flare so that properties change automatically as the lens flare moves around. For example, you can make your reflections automatically dim and fade out as the light source reaches the edge of the comp. This kind of animation is called Evolution, and it's possible using the Expressions Library. If you don't know anything about expressions, you don't have to worry because all you have to do to use this feature is cut and paste. When you're in the Expressions Library comp, you'll see a bunch of layers. Each one has a different expression and different slider controls to go with it. To find out what an expression does, just unhide the layer, and you can read a description of it in your Composition window. The most useful expressions are found at the top of the list.
Let's go through the process of using the first evolution effect, called Basic Time or Motion Evolution. What this expression does is just a basic change in value as the layer gets closer or further from the center of the comp. In this example, we'll put this expression on the Brightness slider control for a Reflection Shape Array element. First, we would go to the Expressions Library, select the Basic Time or Motion Evolution layer, and copy the slider controls that appear in the Effects Controls panel. I'll be sure to select the top item first, which is the effect title, then shift-select the bottom slider control to select them all at once. Selecting them in this order will preserve their order when you paste them. Go back to your lens flare comp and select the Reflection Shape Array layer, and paste the slider controls. They will appear at the bottom in the Effects Controls panel.
Next, we'll copy the expression over. Go back to the expression layer. With the layer selected, hit the E key two times fast, which will reveal the bits of code, called expressions. Click on the expression in the Opacity expression box to select the code. Copy it, go back to your Reflection Shape Array layer, Find the Brightness slider control, and alt-click the stopwatch icon to open the expression box. Paste the code into the box. If you want the element to get less bright as the layer moves away from the center, then use a negative number on the Brightness Evolution slider control to make the value decrease. Now, when you move the control layer, you'll see the element automatically dim as it gets further from the center. If you don't want the element to start dimming until it gets further away, enter "600" into the Start Evolution slider control.
You can go much further with evolution and the Expression Library. We changed the brightness in this example but you can put an evolution expression on any slider control or effect that you want. And the different expressions in the library all do different things.
Expressions from the Expressions Library are used throughout the lens flare presets and you'll know them by their titles, which actually use acronyms since there isn't enough room to write it all out. For example, you may note a slider control on an element with the name BTOME on Expansion. That stands for a Basic Time or Motion Evolution on the Expansion slider control.
You can add more than one expression to any lens flare element, but keep in mind that there can't be more than one slider control of the same name on any one element, because the expressions, which point to the slider controls, will only recognize the first one. If you know you want more than one evolution effect on an element and some of their control sliders share the same name, what you can do is edit the names of the slider controls before you copy them over to your lens flare element. When you rename them, it will update the expressions so that they point to the new names. Finally, be sure to always copy the slider controls before you copy the expression, or you'll get an error.
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