A layer is the basic element that makes up a composition, which is based on the concept of a stack. When you unfold a layer, you have access to all of its composing elements as well as the functionalities that allow you to move it, resize it, animate it, change its timing, or control the way it's merged with other elements.
In order to be displayed in the composition, a layer needs a source. Without a source, a layer can contain information but it won't be applied to anything. Is this useful? Actually, it is. Imagine a layer as a box. If you put something in the box, you can move it and its content will move as well. If the box is empty, you can still move it. Later, when you add something inside the box, its content will immediately be positioned at the right spot because the position information is held by the box and not by the object inside of it. The same can be applied to layers. You can add and position an empty layer in the composition, then add or even replace its source later. This distinction is really important as it allows us to split the layer into different concepts:
- Where it is: defined by transform
- What it is: what does it look like? Defined by the source
- What it’s going through: what effects are applied? Defined by modifiers
Other concepts will be described later, such as when it appears, defined by visibility keys and the time offset.
A layer with no source has a different name, depending on which software you use: transform, empty or null. Since the latter is the most used in the world of 2D, Autograph also uses the name null.
A null layer can also be really useful when you want to attach another layer to it, thanks to the concept of parenting. Through parenting, you can animate only one layer in the stack, and drive many.
In this section we will be focusing solely on 2D layers. Other types of layers can contain information about 3D cameras, lighting, or even an entire scene populated with hundreds of objects. It’s also possible to convert a 2D layer into a 3D layer, in order to merge it with these complex scenes, making the boundaries between these two worlds gradually disappear.