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Chapter 4. Optimizing the Pipeline > Understanding Rendering Order

Understanding Rendering Order

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:

  • 2D layers are calculated from bottom to top of the layer stack

  • Masks, effects, transforms, paint, and type are calculated from top to bottom (as shown by twirling down layer properties)

  • 3D layers are calculated based on their distance from the camera; coplanar 3D layers use stacking order just like 2D layers (but this setup can introduce errors and should be avoided)

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).

Behind the scenes, After Effects is often being much more clever about what to calculate and what not to calculate in order to optimize rendering speed. Thus portions of a layer at the bottom of the stack that are not visible (because they are covered, masked, or otherwise obscured in layers above them) often will not be calculated. The only way you'll ever know is if your render seems faster than expected.

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.

The Transform effect gives you an alternative method to transform a layer prior to adding a given effect; it allows you to select when transforms occur relative to specific effects, all chosen by you.

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.

If you apply an Add mask to a luma matte, the areas outside the mask will be considered to have black luminance data.

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