Abstract

This article explains the function of layers and keyframes, two fundamental concepts in all modern animation software. It is intended for students considering university-level study in animation. Layers are an organizational tool, functioning like stacked transparent sheets that allow animators to separate elements such as backgrounds, characters, and foreground objects. This method, derived from traditional cel animation, permits independent editing of scene components. Keyframes are markers on a timeline that define the start and end points of a change to an object’s properties. These properties include position, rotation, scale, and opacity. The article details the process of “tweening,” or interpolation, where the software automatically generates the frames between two keyframes to create the illusion of smooth motion. This process is a digital automation of the traditional “in-betweener” artist’s role. Finally, the article contrasts the application of these tools in 2D and 3D animation environments. It describes how 2D software, such as Adobe Animate, primarily uses layers for visual separation, while 3D software, like Autodesk Maya, employs animation layers to manage complex character rigs by separating attributes non-destructively. Understanding these two concepts is the basis for all digital animation production.

Keywords: animation, keyframes, layers, tweening, digital animation

Digital animation, from simple web banners to complex feature films, is built on two core concepts: layers and keyframes. These tools are the primary mechanisms used to organize a scene and create motion over time. For any student exploring animation programs, a firm grasp of their function is essential.

Understanding layers for organization

In digital animation software, layers are the primary tool for organizing assets in a scene. They function like a stack of transparent sheets. An animator can place a background image on the bottom layer, a character on a middle layer, and foreground elements like rain or dust on a top layer.

This concept originates from traditional 2D cel animation. In that process, animators painted characters onto clear acetate sheets called “cels.” These cels were then physically placed over a static, painted background and photographed one frame at a time. Digital layers replicate this workflow.

The practical benefit is isolation. By separating elements, an animator can edit one asset without affecting any other. A character can be moved across the screen on its own layer while the background layer remains completely static. Layers also determine the stacking order, controlling which objects appear in front of or behind others. Programs like Adobe Animate also use specialized layers for masks or camera movements.

The function of keyframes for movement

Keyframes are the tools used to create motion. A keyframe is a marker on a timeline that defines the start and end points of a smooth transition. It records the specific value of an object’s property at a precise moment in time. These properties include:

  • Position: The object’s X, Y (and in 3D, Z) coordinates.
  • Scale: The object’s size.
  • Rotation: The object’s orientation.
  • Opacity: The object’s transparency.

To make an object move, an animator sets a keyframe at the start of the action (for example, Frame 1, Position A). The animator then moves to a later point in time (for example, Frame 24) and sets a second keyframe (Position B). The keyframes themselves do not contain the motion; they only store the “key” poses.

How keyframes and tweening create motion

The motion itself is generated through a process called “tweening.” The term is a digital shorthand for “in-betweening.” In animation history, senior animators drew the main “key” poses of an action. Junior artists, known as “in-betweeners,” were responsible for drawing all the individual frames that connected those key poses.

Modern animation software automates this labor-intensive task. When two keyframes are set, the computer calculates all the intermediate frames needed to create a smooth transition. This automatic generation is also called “interpolation.” The animator can control the timing of the motion by moving the keyframes closer together (for a faster action) or farther apart (for a slower action).

Application in 2D and 3D software

The principles of layers and keyframes are universal, but their application differs between 2D and 3D software.

In 2D programs like Toon Boom Harmony, animators set keyframes on a layer to interpolate the position, angle, and size of a 2D drawing. The layer system is primarily used for visual organization, separating characters from backgrounds.

In 3D software, the complexity is much higher. A 3D character “rig” (a digital skeleton) can have hundreds of controllable parts. Here, animators use “animation layers.” This system, found in software like Autodesk Maya, allows an animator to keyframe different attributes on different layers non-destructively. For example, a character’s walk cycle (leg movement) can be on one animation layer, while the arm movements are on a second layer, and a head turn is on a third. The animator can then blend these layers, adjusting their influence without deleting the underlying keyframes.