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I understand how you animate something (say bones location and rotation in an armature). Doing so creates an action; that action can be renamed in the Dope Sheet. In the NLA Editor I can create a NlaTrack from that action, but what is that for?

To quote the manual:

The NLA editor can manipulate and repurpose actions, without the tedium of keyframe handling. Its often used to make broad, significant changes to a scene’s animation, with relative ease. It can also repurpose, and “layer” actions, which make it easier to organize, and version-control your animation.

Ok all that sounds great, but what is it really used for and how does one reuse an action in the NLA Editor?

As may be quite obvious from my question, I have not done much animation, nor used the NLA Editor.
So what is the NLA Editor used for?

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    $\begingroup$ David, is it really you? Could a specific tutorial like "Blender Tutorial - Creating and Editing Actions for Re-use in Animations and Games " help? youtube.com/watch?v=Gb152Qncn2s $\endgroup$ – m.ardito Oct 13 '16 at 8:48
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    $\begingroup$ @m.ardito yes it is really me :). While that vid is good, I knew everything in it, and he does not touch the NLA Editor. So my question still stands what is the nla editor used for? $\endgroup$ – David Oct 13 '16 at 13:56
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    $\begingroup$ Congratulations on asking the 22,222nd question on B.SE :) $\endgroup$ – gandalf3 Oct 15 '16 at 19:59
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I use NLA editor in blending and smoothing animation after inserting a new keyframe. So for example, if I have a bone that already has an animation, then I paused the animation at frame 100 and rotated this bone 90 around x-axis and inserted a keyframe there for rotation, when I replay the animation, you will find that the bone at frame 100 will snap and rotate 90 around X-axis then snap back at frame 101. To make this inserted rotation looks smooth, I add the action in the NLA editor and then make a smooth interpolation using NLA's editor Blend In and Blend out as shown below. So it's actually very handy with armature's animation modification. enter image description here

Also another main usage of it is to blend two actions together (like walking and waving a hand,...) so that two animations are blended together in a very nice and smooth way. I hope this helps :)

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Short answer: High-quality character animation is hard work even when it is done by experts. A lot of work goes into every second. Nonlinear animation is an attempt to speed things up, but note that it could sacrifice some quality.

Longer answer: When you start animating, you can be successful using only keyframes, but after a while you find yourself animating similar motions again and again, and you want to reuse your work, just like you reuse node groups. You also want to be able to mix and blend overlapping motions in a nondestructive way. You might want to create an animation layer for the broad movements and one for the refinements. Or a layer of face motions and one for the body, and combine them according to the story. Nonlinear animation is used for achieving these goals.

Note that the quality of automatically reused and blended motions might not be as good as the quality of a hand-crafted keyframe animation, but they could be fine for background characters. It could be a tradeoff between quality and time.

The following video explains the "how" part with an example: https://www.youtube.com/watch?v=W4TkA5P5Qes

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The NLA editor is used for editing the position and interaction between actions in an animation. It's main use is for managing, editing, and organizing groups of actions instead of having to deal with huge numbers of keyframes.

Using the Dopesheet, an animator can create an action for an object that consists of any number of keyframes. An action for a complex character with a rig can have thousands of keyframes for even a simple action. Thus, managing the entire action via the dopesheet isn't practical. When I say manage, I mean perform an action on the action that doesn't require editing of individual keyframes. You can think of it like edit mode versus object mode. A 3d object is a group of vertices, an action is a group of keyframes. Object mode is useful because you can easily reposition, scale, or duplicate objects, as well as apply certain effects to them. The same goes for the NLA editor. You can adjust the start and end times of an action very easily, as well as apply effects, like blending two action together.

The ability to deal with each action as a whole in the NLA editor makes it very powerful when combining several actions into a longer animation. Animations often also contain repeated actions. In the NLA editor, you can duplicate an action with ShiftD and reuse it as often as you want. It's the same as instancing an object. It allows the animator to reuse an action many times while still being able to edit it easily.

In an animation, the process would usually involve creating certain actions in the dopesheet, then combining them in the NLA editor. Since you can select and move entire actions very easily, it makes adjusting the general timing of your animation much easier than moving all of the individual keyframes.

Another key feature of the NLA editor is the ability to organize an animation. Since there can be multiple tracks (essentially layers) you can separate certain kinds of actions, or actions for certain parts of objects. You can also blend between layers, so the animator can blend between two actions smoothly.

If you've ever used the VSE in Blender you will notice a large number of similarities between the purposes of each editor. The point is to be able to manage the entire animation without having to deal with each keyframe, just like the point of the VSE is to manage the entire video without having to deal with each frame individually.

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