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Animating with an NLA
There are two ways to use NLA systems. The first is to simply combine existing motions to create new content. For those who use lots of motion capture, this is probably the primary way of working. A good example might be in a martial arts or fight scene. You can take a library of fighting moves and combine them to create a realistic sequence.
The big downfall with using canned motion is that it never quite fits the shot exactly. That's where the real power of a good NLA will come in handy. An NLA will allow you to add a layer of motion to fix the discrepancies.
Another way to create animation is to use the NLA as a tool that enhances the traditional keyframing process for creating motions from the ground up.
NLA Tools
With this in mind, here are the major tools that an NLA offers over a traditional keyframe system:
Speed changes—Probably the most helpful tool afforded by an NLA is the capability to retime animation on the fly. This could be as simple as slowing down a character's walk as she notices something or as complex as completely changing the character of a motion. Being able to retime motion on the fly allows animators to tweak animation much more quickly.
One thing with changing the speed of a motion is that you will still need to consider physics. Gravity will still pull a character downward at the same rate, no matter how much the speed of other motions slow down. If you slow down an entire clip, it may look like you hit the slow motion button.
Layering—Another very common operation is layering animation. If you had a neutral walk, for example, you could layer the character speaking and gesturing on top of this.
Layering is a great way to create animation. The animator can start with the broad motions and then add layers to get additional detail. This gives the animator the flexibility of having each pass on a different layer, allowing the capability to tweak each layer individually.
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Blending—A blend operation allows the animator to transition from one motion to another—say, from a walk to a run. It is essentially creates an automatic in-between for every curve in the cycle. Most blend algorithms are not very intelligent—they simply find the shortest distance between two points. This means that you need to be sure that you blend things in the right place (see Figure 3). If your run cycle ends with the left foot forward, but your run cycle begins with the right foot forward, then the blending from one to the other would make the character walk backward for a half step. As mentioned previously, standardizing the motions will help considerably.
Figure 3 Make sure that your cycles start off on the same foot. Transitioning from the walk to the run will cause trouble because the cycles start on different feet.
Most software allows animators to define a curve that dictates the rate of transition. Just like the animation curves used in keyframe animation, these allow for animators to slow-in and slow-out the curves.
Many animators animate in a "pose to pose" manner, meaning that they animate using major poses to define the action. Once the poses are created, the animators then adjust the timing between the poses to fine-tune the animation. In an NLA, the poses can simply be treated as single-frame "tracks." The animator can then copy these poses to the timeline and create blends between the poses.
Equalizing—This is a tool introduced in Softimage XSI but that will see the light in other packages. An equalizer breaks down the motion cycles into frequency bands. The animator is then presented with a control panel where he can adjust the amplitude of each of these frequency bands. Much like how graphic equalizers from the audio world amplify and mute audio frequencies to sculpt the quality of sound, an equalizer allows artists to amplify and mute frequencies of motion to sculpt the quality of animation.
In a walk cycle, for example, the arms will swing at a fairly regular frequency. By amplifying or dampening this frequency, the arm motion can be exaggerated or muted to the animator's content. In practice, equalizing provides artists with a series of "exaggeration" sliders for motion. Moving a slider toward a high value amplifies a range of motion, making that part of a character appear more exaggerated. Moving a slider toward zero dampens a range of motion, making other parts of a character appear more sluggish.
Offsets—Offsets allow animators to readjust and align tracks within a scene. This is particularly of concern for those working with motion capture, where individual motions will usually be captured at different places on the stage. By using offsets, these motions can be repositioned so that they start and stop in the same place, allowing them to flow smoothly into the next clip.
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