http://archive.divergentshadows.com/dv/ae/notes/keyframe_basics.shtml
Keyframes
Keyframes are the basis for nearly all modern forms of animation, and I consider them to be the single most important concept in After Effects...simply put, if you don't understand how keyframes work, you'll never get much done in After Effects.
The concept of keyframes goes back to traditional hand-drawn animation. In a Disney cartoon, for instance, every single frame used to be drawn by hand. This can be very time consuming when you consider that a typical film runs at 24 frames per second - that's 1440 drawings per minute of animation, and approximately 130,000 drawings for a typical feature length movie! It's prohibitively time-consuming and expensive to have your best artists drawing all of those pictures, so studios needed a way to distribute the work. Using key frames allowed them to do this. The lead artist would draw only the most important frames in a sequence. For instance, if a character is jumping over a log, the lead artist would draw the first frame as the character prepares to jump, a second frame showing the character in mid-air over the log, and a third frame showing the character landing on the other side of the log. Once these three frames(the keyframes) are completed they can be sent off to a junior animater(today they might be sent overseas) who's job is to draw all the frames in between the keyframes. This allowed the animators to work more efficiently and made feature length animation feasible.
Today, using a program such as After Effects, we can set the keyframes and let the computer draw all the in-between frames(this is called tweening). Although we aren't actually drawing anything in AE, the basic concept is the same. You set a keyframe for each property you want to animate on a layer, move to a different point on the timeline, then change the layer's properties at the new time. Once you change a property a second keyframe is set, and now the computer can calculate the change between those two keyframes.
After Effects will set keyframes for you automatically, but first you need to tell it you want it to do so. When you toggle down a layer property you will see a tiny stopwatch icon to the left of the property's name (1). If you click this icon, you will see a small diamond appear on the timeline at the playhead's current position (2). Keyframes are now turned on for that property and the diamond represents the first keyframe. Next you need to change the current time by moving the playhead to a different location on the timeline. Once you have done this, change the property which you turned on keyframes for. You should see a new diamond appear on the timeline at the playhead's new position - this is your second keyframe (3). Now move the playhead back to the first keyframe and press the spacebar to preview your comp - you should see the property you keyframed change smoothly between the first keyframe and the second.
Important things to remember about keyframes
Keyframes are specific to the property and layer you turn them on for. So if you turn on keyframes for scale, and then change the position of a layer, the change in position will not be recorded. You need to turn them on for each property you wish to animate, on each layer you wish to animate. If you are familiar with Flash you will see that this is one of the significant differences between the two programs; After Effects' way of working with keyframes is far more flexible than Flash's because the keyframes apply to individual properties and not the layer as a whole.
If you turn off keyframes for a layer property (by clicking the stopwatch icon again), all the keyframes you've set for that property will be removed. This can be a good way to reset a layer if you want to start over, but it can also mean a lot of work to redo if you didn't mean to turn them off. Remember you can hit Apple-Z to undo your last action in case you do something like turning off your keyframes by accident.
Remember that keyframes represent change over time. If you set a keyframe, then change the property without going to a different time in your comp, you will just change the value at the same keyframe. In order to crate animation you need at least 2 keyframes separated by a period of time, with different values at each keyframe. The minimum time period between keyframes in After Effects is one frame. However, the more frames you have between keyframes the smoother your motion will appear, but more frames also means more time and thus your property change will occur more slowly.
You can set as many keyframes as you like on a given property. You can also turn on keyframes for as many properties as you would like on a given layer.
I've just covered the basics of keyframing here - however, you'll find as you work more with AE that these basics are something you will use in every AE project you undertake. Next week we'll focus on manipulating keyframes - moving, copying pasting, deleting, and using the different kinds of keyframes.
Growing vine AE tutorial in 3 parts
https://www.youtube.com/watch?v=0ZG_8kwoQeU
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