To truly master After Effects and become an expert compositor, you must understand the order in which actions occur. For example, you need to know whether a mask is applied before an effect and whether an effect is applied to a track matte. You need to understand the render pipeline. For the most part the render pipeline is plainly visible in the timeline and follows consistent rules:
Let's recap that for a moment. In a 2D composition, After Effects starts with the bottom layer, calculates any adjustments to it in the order properties are shown, then calculates adjustments to the layer above it, composites the two of them together, and so on up to the top layer of the stack. If you want to know what order is used to calculate layer properties, you need only reveal them in the timeline (Figure 4.12). Figure 4.12. Just because After Effects lacks a tree/node interface doesn't mean you can't easily see the render order; it's laid out for you in the Timeline window. Layer properties render in top to bottom order (shown here, Motion Trackers, then Masks, Effects, and finally Transforms).
This, then, adds an extra advantage to adjustment layers. They behave just like other 2D layers in the stack, so that they are always rendered after all calculations on layers below them are completed. Effects within layers, on the other hand, always calculate prior to transforms. And what about track mattes? Track mattes (and blending modes) are calculated after all of the other layer properties (masks, effects, and transforms) have been calculated. Of course, before track mattes are applied, their own mask, effect, and transform data is applied to them. Therefore, it should not be necessary to pre-render a track matte in order to see these edits affect the matte.
As I mentioned in the previous chapter, you are taking your chances if you try to apply two consecutive track mattes (in other words, you apply a track matte to a track matte). Sometimes the method works, and the UI does not specifically prohibit you from doing it; however, it's inconsistent. I recommend pre-composing instead. Better safe than sorry.
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