Welcome to a tutorial on basic concepts of animation!
This tutorial will be going over some of the basic terminology and concepts for various types of animation. In today's world, almost all animation is digital in nature, but many of these concepts apply to other types, and even some other fields, like live action movies and video games.
This tutorial will not go step by step on how to create a specific animation. But if you have a tool at hand that would permit you to create one, you may be able to figure out how to make an image move on your own from the information contained here.
NOTE: Example animations may not play at their max smoothness if you are viewing this tutorial on your phone!
What is animation?
Animation is the illusion of movement created by rapid switching of many individual images.
No matter the way those images are generated, this definition always remains the same. You could have a flipbook with many pages with similar pictures, then flip through the pages quickly and have an animation, or you could have a machine like a computer or projector go through images for you. Every live action movie follows this approach as well. A camera takes many still pictures and to look at the movie we play them back at the same speed they were taken.
A single image that is part of an animation is called a FRAME. The speed that the animation plays at is expressed in FRAMES PER SECOND but is more commonly abbreviated as FPS. Sometimes it is also called a FRAMERATE. There are rare cases that speed is expressed in milliseconds for how long an individual image stays on screen (for example, 24fps = ~41ms per frame).
Most of the video is played back at 24fps which is has been the industry standard for a very long time. It's sufficient for our eye to not be able to tell apart the individual frames easily and can be cleanly subdivided if needed. Some newer videos are made for playing at higher framerates (like 60fps or 120fps), but for many cases, 24fps is the ideal option.
How is animation constructed?
While you can make animations by just creating a lot of similar images one after another, knowing the concepts of constructing animation can help understand it better.
Regardless of the method of animation, a crucial building block is the KEYFRAME. A keyframe is a key point in an animation where an important part of the motion occurs. If you were to see an animation with just keyframes, you would still be able to understand what is actually happening, even if the motion is not smooth.
It is usually at this point that it's best to decide how long a specific action should last by adjusting how long each of these keyframes should stay on screen. This affects an important concept of animation, TIMING. Changing these durations can make all the difference in how an action is perceived. At this early stage, unless there is a guide to sync to (for example, music, sound effects or existing animation), the speed may need adjustment later on, when the motion fleshes out, but the general relation between keyframe durations usually shouldn't change.
The more complex the motion, the more keyframes will need to be added to make the animation readable. Sometimes it may be needed to have LAYERED ANIMATION, where each layer has it's own separate keyframes which play in parallel. If layers are connected and used to better convey motion of a character or object, the result is usually called a SECONDARY MOTION.
These layers are very common and easy to use in digital animation, but applying them in traditional animation requires transparent cells and is not nearly as time efficient.
Once the keyframes are well defined, one would proceed to polish the motion and the artwork. The best order can vary, but in my experience, the more dynamic the animation, the more important planning the motion before polishing the art tends to be.
How is motion smoothed out?
While keyframes can convey what the motion will look like, unless there is a very large amount of them, you would need to smooth the motion out by adding more frames between them. This is called INBETWEENING and there are two distinct methods for creating them:
FRAME BY FRAME
Frame by frame animation implies adjusting every frame by hand to create the illusion of motion between keyframes. This can be done by drawing it completely anew or moving it by hand without changing the image.
Interpolated animation is done digitally using animation software. In these cases, keyframes are defined by parameters like position, rotation, transparency and many other things. Afterwards, the software can fill in the motion between these keyframes on it's own, making it as smooth as needed. This is the primary method of creating motion in 3D animation.
A single 2D animation can have both of these methods, even on top of one another, so they are not mutually exclusive. This is the most common way of doing things, and leads to best results. The ratio can be anywhere from mostly interpolated with subtle frame by frame (like blinking), to mostly frame by frame but with maybe everything moving while it flips through the images.
More about frame by frame animation
Frame by frame animation can be quite time consuming, since you need to manually take care of each frame of animation, and in longer animation that is a lot of frames to worry about! But, it allows a lot more control over the motion and there are some effects that can only be created by adjusting individual frames.
A key tool in creating this type of animation is the ONION SKIN. The onion skin refers to seeing the silhouette of the previous frame through the current one. Traditionally this was done by using a light table, which would light the sheets of paper so they would get transparent and you could see what came before. In digital animation, this is a lot simpler and you have more control as to what will be visible and how far into the future or the past. The onion skins usually also get a color tint depending if they are next or previous.
Common method of frame by frame inbetweening is to make a frame in the time half between two keyframes and place it in the middle using the onion skin as a reference. Afterwards, you would repeat the process between one of the keyframes and the newly made inbetween frame, and continue it as many times as needed to smooth out the animation.
As you may already realize, if an animation runs at 24fps and you need to draw almost every frame on your own, a single minute of animation would require drawing up to 1400 images. Though motion wouldn't always be happening and you can reuse frames so the number would be lower, you should also realize that for most animations, there is no need to animate at the maximum play speed. In fact, in today's animation it's very rarely animated by hand at full 24fps. But doing so would be called ANIMATING ON ONES. Most of the animation is done by ANIMATING ON TWOS, which means, having each frame actually stay on the screen for twice as long, effectively making the animation appear as if it's running at 12fps. Depending on situations, animation also gets animated on threes and fours, meaning at effective 8fps and 6fps respectively.
You can also sometimes make it so that each layer plays at different speeds in the same animation, giving the illusion that everything is animated at a higher speed.
In this example, the hair is animated on fours when idle, the body movement is animated on twos and the tail, the crank and the bubbles are animate on ones (even though they are interpolated and not frame by frame)
You can speed things up and slow things down by adjusting how long a frame should appear on the screen, and choosing where the next inbetween should be added. That way you can make the animation EASE IN or EASE OUT, or both. If you make an inbetween and choose to put more frames before or after it, it can lead to the motion favoring one side or the other.
So, what if a motion needs to be faster than the framerate allows? When even if you animate on ones, you can't have the motion appear clear enough? Then you can use a SMEAR or a MOTION BLUR to convey the motion, by leaving a trail of the motion so our eye will think it's moving faster than it can properly track. Smears are used in frame by frame more, while blurs can be easier to add when the computer interpolates.
More about interpolated animation
Interpolated animation can be very time efficient and you can achieve very smooth motion with a lot less effort. But, for it to look natural and appealing, you still need to fiddle with the parameters, and there are some situations where getting the desired result using just interpolation is almost impossible.
The key component of interpolated animation would be the RIG. The rig is usually made out of various connected pieces that will be able to move individually, like a skeleton. You can also have only one piece that moves, but you would get the most out of interpolation if you have more. It's debatable if having just one also counts as a rig, but they all share the same purpose, which is to have something for the computer to easily calculate the motion of.
Computer software allows for a lot of options for just manipulating layers over time. Examples of some of things you can adjust:
transformation - rotation, position, scale, skew
filters - transparency, hue, tinting, blurs
deformation - based on an internal skeleton for the individual layer, adding ripple effects, morphing
timing - of all the other changes (easing, smoothness)
There are some other effects that can be made by computer software and would be almost impossible to do by hand. The most common being PARTICLE EFFECTS. Computers can generate a lot of small graphics that will move and replicate according to given parameters. This can extend to it even using other animations as blueprints, so you can have very impressive results with relatively little work put in.
Another drawback of using interpolated animation is that it is very reliant on the software used. Which means that you need to learn how these features are used for each software separately, even if there are some common elements. And not all software is capable of all of these options, so there are complications in getting the result you need if you are not prepared.
Conclusion and references
No matter the approach and the tools used, there are many principles that apply to all forms of animation. I went over some of the common and some specific ones, but best way to understand how things work is to give it a go yourself!
Here are some resources that may come in handy for those starting out:
12 principles of animation
Techniques of animation
Basics on using OpenToonz animation software