After Effects has a lot of tools that make your job easier. Taking advantage of the ‘pre-comp option’ is a great time saver, a great management tool.The ‘pre-comp’ tool is quite easy to access and apply and a very easy concept to grasp. Using ‘Pre-Compose’, the last option under your Layer pull down menu lets you group multiple layers together as one and treat them as one. Perhaps you have an animation of a cartoon monkey swinging from the trees. The monkey with his body, arms, and legs is actually made of multiple layers, and these layers have their own animation and relationships. He grabs the vines, swings from one tree to the next; his body sways as he flies through the air. Going Here
Once you have that set of relationships established it would be much easier to manipulate the group as a whole and this is one attribute of pre-composing. You reduce all these layers… to a single layer, a composition. Now you can adjust the position or scale or any transfer attribute of this composition, with a single change, and affect them all. Now, just like we use compositions inside other compositions, as soon as you pre-compose this set of layers, they appear in your timeline sources as a single composition, just what you wanted.However, if you look at your project resources, just below the composition you are working on, you will see this new one, the ‘pre-composed’ composition you just created. What this means is that you can now use this composition with any of your projects, by simply dragging the new composition to your new one.
Having this new composition, what was formerly a set of layers available to all others in your project is great. You saved time; you saved ‘busy work’. In your original composition you were able to manipulate many as one and be able to place this set with a single position change.An even greater application is building groups. Now you have a single object that has the detail you devoted to creating, in our case, a monkey swinging from the trees. Tomorrow you are asked to do a Tarzan movie and you need literally hundreds of monkeys swinging from trees. No problem. Now that your monkey is a composition, you can scale him, rotate him, flip him around so you can easily get many variations of your monkey swinging and while that seems funny, animations with hundreds of flowers, with grass growing all over the plain, and flocks of birds flying by, use this same technique and concept, to get many, from one. And that’s no monkeying around.