Precomposing

Pre-composing is a way to organize elements into groups, rearrange the rendering order, and apply one effect to multiple layers at the same time. The only hard and fast rule is Pre-compose when you need to isolate layers.

Here are a couple of examples that I use all the time:

I want to Camera Track a scene that has a lot of moving objects in the shot that are fouling up the camera tracking. The solution is to duplicate the footage layer, set an in and out point on the duplicate so I am only going to run camera tracking the part that I need to track, then set up a mask to hide the part that is causing problems. I also often increase the contrast and the detail in the shot to give the camera tracker more detail to track. The layer is Pre-composed, moving all attributes to the new composition and trimming the comp to the in and out points of the layers. Camera tracking is performed on the Pre-comp, the origin and ground plane established, reference solids, and a camera are added, then the layer is turned into a Guide layer so it won't render and I have successfully 3D Camera Tracked a shot that would not track in any other way. I probably do some version of this on about half the shots that I run camera tracking on because it gives me better results. It may also be useful when tracking in Mocha AE and even when just using AE's motion tracker. In these cases, the pre-comp is set as a guide layer because you don't want to render it in the final project because of the masks and adjustments you made make the shot unusable, but you need that detail and those masks to make the tracking work.

Another example:

I have a text layer that I want to use as part of a design that is going to involved a half dozen copies of the text layer on different layers for use in different effects. I want the text to always be the same in all of those layers. In this case, I would pre-compose the text layer, then duplicate the pre-comp in the timeline as many times as I needed to. When it comes time to edit the text I just open up the Pre-comp and edit the text and it changes on every other layer saving me time.

A third common scenario requiring pre-composing.

I have several layers that are interacting with each other using blend modes and effects. I need to be able to change the color grading on this group of layers to make them work with the rest of the scene. I cannot use an Adjustment layer because that would affect every layer below it, so I select the group of layers, pre-compose, then I apply my color adjustments to the pre-comp.

Here's a fourth scenario:

I have a bunch of 3D elements in my main comp with lights and camera moves. Some of the layers are forming a virtual set and I may want to rearrange the pieces. Other 3D layers I want to animate separately. I pre-compose the 3D layers that I want to keep together, turn on Collapse Transformations, and the Main Comp Camera and lights act like the layers are still in the Main comp but it is much easier to complete the rest of the animation and adjust the camera moves because that group of layers will no longer be in the way when I am adjusting the other layers in the scene. I can also add effects to the entire group of 3D layers from the main comp.

I'll give you one more:

I have one or more layers that are animated, but I may want to change the timing of the animation and there are dozens, maybe hundreds of keyframes. If I select the layer or layers and pre-compose them. I can then apply Time Remapping to the pre-comp and adjust the timing any way I like without any problems.

I hope that helps you see some of the possibilities. Because you are new I strongly suggest that you spend the first part of your training with the User Guide. The tutorials you find there are pretty well explained and follow recommended workflows. Sadly, most of the newer YouTube tutorials I see are presented by enthusiasts that have discovered or copied what they believe is a 'cool' recipe for some kind of effect. Most of these recipes are poorly explained and some of the workflows are just awful. You have to vet your trainers. Make sure that they know what they are talking about. The number of views is no indication of the quality of the tutorial.

When you run into problems post workflow details, screenshots that show the modified properties of the layers you are having problems with, and system, os, and AE version details. When you are trying to troubleshoot your own problems start by soloing layers and pressing the 'u' key twice to see what has been modified on the layers. Start turning effects on and off or resetting parameters until you find out what is causing the problem. 

Good luck. Time learning proper workflow techniques is never time wasted.

A pre-comp can have time-remap enabled, a single layer cannot (except actual footage). Sometimes you need time-remap, therefore you have to precomp a layer, or a bunch of layers.

Also, I precomp layers when I want them to handle "as one thing" in means of timing and global parametes as positon and scale. If they are a logical unit, I also pack them into pre-comps for hierachy and sense.

Example from last project: a swarm of bees flying around flowers (animated cartoon). It was one bee with a wing flapping animation. Precomped to "bee_animated". This bee_animated was put into a new composition "bees_flying_around" and it's postion was animated, making the wing flapping bee flying around. Then this was duplicated and the position paths have been alterated, to there was a couple of bees flying around, but all referring back to the same bee with the same wing-animation.

This was put into my scene "at the busstop". which was larger than HD, so I could pan around. This busstop-scene was put into "scene 2", where I animated the pan rounds and scene 2 was put into "Master", along with all other scenes of the movie.

master

  - all other scenes

  - scene 2

      - busstop (very large and panoramic comp - animated scale and position)

          - many other layers and precomps

          - bees flying around

               - bee animated (animated position)

               - bee animated (animated position)

               - ...

               - bee animated (animated position)

                    - bee body

                    - bee wing left (animated)

                    - bee wing right (animated)

I once spend a whole day including a layer and adjusting a timing in an animation of many scenes, because I didn't added any pre-comp. I had dozens of layers and most with keyframes and no idea about pre-comps.

It was my first and my last time I didn't precomp.

*Martin

Simple answer: In case of doubt, always pre-compose. Beyond that there's no magic to it. It just takes an understanding of AE's rendering pipeline, which is all explained in the online help and I'm pretty sure there are equally long-winded explanation videos out there. There's a million ways to think of it, but the most fitting analogy most of the time is "to let the paint dry", i.e. flattening the buffer. Painting on an already dry image will yield different results than using wet-on-wet techniques in a manner of speaking. Of course the whole truth is a lot more complicated, but let me be candid: You won#t learn this by watching a million tutorials. You actually have to work on projects to develop an understanding how effects, layer switches and so on interact. If you're focused on the immediate result following a tutorial step by step is just fine, but if you realyl want to wise up on it, you can only do so by practicing. Even for most of us old-timers it took a while to crack that nut....

Mylenium