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Exploring keyframe interpolation, velocity, and influence

From: After Effects Apprentice 03: Advanced Animation

Video: Exploring keyframe interpolation, velocity, and influence

For those who skipped the last movie and who have access to the project files, we are in project AEA_Advanced Animation, and we're looking around inside comp 00-keyframes 101. To reveal the keyframes, you can press U to reveal all animated properties. And we were just discussing that keyframes contain a lot more than just a particular value at a particular point in time. For one, if you have two different keyframes at different points in time with different values, After Effects will interpolate between those values, vary from one value to the next over that period of time between the two keyframes.

Exploring keyframe interpolation, velocity, and influence

For those who skipped the last movie and who have access to the project files, we are in project AEA_Advanced Animation, and we're looking around inside comp 00-keyframes 101. To reveal the keyframes, you can press U to reveal all animated properties. And we were just discussing that keyframes contain a lot more than just a particular value at a particular point in time. For one, if you have two different keyframes at different points in time with different values, After Effects will interpolate between those values, vary from one value to the next over that period of time between the two keyframes.

In other words, it just doesn't abruptly change when it hits a new keyframe. And if I RAM-preview, you can see this. You can see how scale doesn't jump. It gradually scales the object up and down. And you'll also see that position doesn't jump from keyframe to keyframe; the object actually moves along its motion path. But keyframes can do a lot more than just hold a value and interpolate; they also define how fast those values change from keyframe to keyframe and through the keyframe, and that's what we're going to explore in this movie. Now spatial keyframes like position are really handy because the Comp panel gives us some clues about this interpolation.

Namely, the spacing of these dots along the motion path indicate how fast that object is moving. These dots represent the position of that layer at each frame in time. If I was to move it back here where it's moving quicker and press the Page Up and Page Down keys to step through a frame at a time, you can see how it's jumping from dot to dot along that motion path. So, larger space between dots means a larger change in position from frame to frame and therefore a higher speed. Closer spacing the dots, less change from frame to frame, slower speed.

However, with temporal keyframes, you need to do a little bit of work to reveal this information. If you hold down Option on Mac, or Alt on Windows, and double-click a keyframe, it will open up the Keyframe Velocity dialog. Here is where you see, numerically, what is the speed through that keyframe. In this case, you can see the speed before that keyframe, the Incoming Velocity, is very fast, about 423 pixels a second. And the Outgoing Velocity, the speed after this keyframe between it and the next keyframe, is slower, 147 pixels per second.

Temporal keyframes default to an interpolation type of linear. Linear keyframes have these abrupt transitions in speed from incoming to outgoing. And the reason is it's because their influence is essentially zero. The Influence says, how long should I attempt it to maintain the speed before or after this keyframe? How much do I slow down gradually or speed up gradually coming in or going out? A larger influence says, take your time slowing down to the new speed or speeding up to the new speed.

A smaller influence says ah! Don't spend any time at all, just abruptly change to that new speed. You can edit these values numerically if you like. However, there is a more graphical way of doing this, and oddly enough it's called the Graph Editor. I'll click OK. You'll notice the keyframe type has changed because whenever you edit something in that Keyframe Velocity dialog, it assumes it's no longer linear, even though we left the Influence at zero. I will move my cursor over to this Graph Editor icon. Now we're going to be spending a lot of time later in this lesson going through the Graph Editor in depth, but let me give you a quick overview.

Click on this icon or press Shift+F3 to open the Graph Editor. There are many different ways to customize this view, but for position keyframes, it defaults to showing what is the speed in pixels per second across time. You will also notice that the values are color coded. There is sort of lilac line matches this lilac value of to the left for position. A flat line like this tells me that it is keeping a constant speed, a constant velocity of 423 pixels per second.

When we hit this keyframe, there is a sudden speed change down to the slower speed of 147 pixels a second--very similar to what we just saw in the numeric Interpolation dialog. The gap between these keyframe nubbins, or handles, indicates that there is a sudden jump in speed. If you wanted to smooth out that speed transition through that keyframe, you would need to drag these so that they have roughly the same height. This means that the incoming and outgoing velocity is the same, however, the influence, the amount of deceleration or acceleration entering or leaving a keyframe, is still pretty close to zero.

So if I drag out these handles, you will see I start to smooth out this interpolation. The smoother the line, the smoother the speed changes through our animation. Let me go ahead and RAM-preview this, and watch what happens. You'll notice as we go through this keyframe that we've smoothed, that the speed transition up in the Comp panel is much more gradual. It's not quite the sudden change. But this one we haven't modified does have a very sudden in jump in speed as it hits that keyframe. If I find that the speed change is still a little bit abrupt, that's fine; I'll just lengthen the handle, make it a smoother speed change, and now we'll preview.

Now you see there is a much more gradual transition. There is still a slight bump because I have this scale keyframe in the red down here, which is still Linear. It does not have a smooth transition. So that little bump that you're seeing right through here is actually an abrupt change in Scale value. Again, if I want to, I can go ahead and smooth this transition as well, preview, and now we have a smoother transition through this point in time. Now, as I mentioned, you don't have to use the Graph Editor. I can go ahead and close it and then Option+Double-click or Alt+Double-click on a keyframe and edit these incoming and outgoing velocities.

I can make them the same to make sure I have the same smooth speed, and I can edit the influence. For example, I can enter 50% influence. That's actually a pretty broad speed-up or slow-down through a keyframe. I can do the same thing for my Scale keyframes. They're locked. They're showing me both the X and Y Dimensions, which are the same, and I'll increase the Influence here as well. Tab over. 25% there. You can use either the numeric dialog or the Graph Editor. They're editing the same information that's hidden underneath that keyframe, not just its value, but also the velocity and the influence going through that keyframe--how it interpolates.

The important thing to know is that all that information does exist for every keyframe and manipulating and refining those values is how you create a more refined sophisticated animation, and that's what the rest of this lesson's dedicated to: how you gain more control over your animations and make them more refined and more sophisticated. But I know you're busy, so the next movie is going to show you the quick shortcut ways to get instant gratification and instant refinement in your animation.

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After Effects Apprentice 03: Advanced Animation

32 video lessons · 23404 viewers

Chris Meyer and Trish Meyer
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