# Essential LightWave 3D- P15

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## Essential LightWave 3D- P15

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Essential LightWave 3D- P15: What you have in your hands is, quite simply, a collection of tools and techniques that many professional LightWave artists use every single day doing what we do in our various fields. The tools and techniques explored in this book are essential to creating the caliber of imagery that you see on film and television and in print and video games.

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## Nội dung Text: Essential LightWave 3D- P15

1. Chapter 16 · · · · · · · · · · · · · · · · · · · · · · · Note Figure 16-16 Some have had great success with using limits to restrict how far a bone will bend in a given angular direction. However, without the ability to “cushion” these limits, the resulting motion of an item as it reaches its limit is often sharp and inorganic. So I tend to shy away from using limits whenever possible, opting instead for well-planned models and cleanly executed animation. Number of Axes Solved If you take a look at your own arm or leg, you’ll find that it’s built much the same way When you have two or more joints in an IK as the above illustration. Your bicep and chain operating in 2D space, things tend to thigh can swing back and forth, and up and work quite well. But in three-dimensional down (two axes: heading and pitch), while space, the level of complexity of the calcula- your forearm and calf move only around one tions that IK must figure out rises axis of the elbow and knee, respectively. dramatically. If you want things to work I’ve found that referencing real life in dependably, you’re going to have to curb building riggings helps more than nearly your demands on the IK solver. Figure 16-17 408
2. · · · · · · · · · · · · · · · Layout 3: Character Animation Figure 16-18 anything else in making character setups Rotation Order that do what you’d expect them to do. Character riggings are complex things. While the desire to use IK on shoulder Riggings use a lot of heavy-duty math that or hip joints in order to solve for all three we animators take for granted when we’re axes (heading, pitch, and bank) is tempting, doing our jobs. Usually, we don’t care how I find that often (though not always) this something works, so long as it does. makes for a loose, “swively,” hard-to-con- But, if you’re curious about the more trol arm or leg. Most often, character complex details that make for a good, riggers will leave the bank axis to be con- dependable character rigging, this section trolled manually by the animator using FK. and the next one will fill you in on a lot of This lets you precisely control the position the stuff that goes on behind the scenes. of the elbow or knee joint just by rotating the bicep or thigh. So, the rule of thumb for good, solid IK Note chains in character riggings is: The topmost The “Rotation Order” and “Joint Compensa- item can use IK to solve for a maximum of tion and Muscle Flexing” sections are mainly for the technical directors and the heavy- two rotation axes, and the child item in a duty math-oriented folks who really want to character IK chain should use IK for only one understand why things happen and why cer- axis. tain decisions are made in the crafting of riggings. If this isn’t your cup of tea, don’t worry (there won’t be a quiz on this later); Note just skip to the section called “Flipping.” I don’t recommend using standard IK on a bone chain consisting of more than two LightWave has a fixed order in which it cal- joints (such as for a character’s tail). It cre- ates too many opportunities for the complex culates the rotation of an item: math to give you something you neither 1) Heading expect nor want. IK Booster is the tool for the job if you have something like a tail, 2) Pitch tentacle, whip, or rope you need to animate. 3) Bank 409
3. Chapter 16 · · · · · · · · · · · · · · · · · · · · · · · You can think of this rotation order as if your object (“Arrow” in Figure 16-19) were parented to a hierarchical series of null objects, each null handling one rotation axis. Rotating one null will cause all of its chil- dren to rotate right along with it. Using this logic, I’ve set up a scene in which we’ve got two arrows as children of a Figure 16-19 hierarchical series of nulls. Figure 16-20 Figure 16-21 In Figure 16-21, notice I’ve selected both its rotation is being controlled by a system arrow objects and rotated them: H=20, that solves heading first, then pitch, then P=40, B=60. bank, the order in which the rotations were If an object is rotated H=20, it would applied must be reversed. (LightWave does make sense that to reset its applicable rota- this automatically when you simply reset an tion to H=0, its parent object would need to item’s rotation to 0, 0, 0 — but if you have a be rotated H=–20 (0 = n + [–1 · n]). hierarchical system creating Steadycam-like When you want to “unwind” an object, “floating head” movement, then rotation reset its applicable rotation to 0, 0, 0. When order becomes very important.) 410
4. · · · · · · · · · · · · · · · Layout 3: Character Animation Figure 16-22 I’ve left the arrow objects at H=20, P=40, In each viewport, the left-hand arrow is B=60, and I’ve rotated the parent nulls rotated 0, 0, 0. The right-hand arrow is labeled Heading by –20, Pitch by –40, and rotated 0, –90, 0. The bank axis is a child of Bank by –60. The only difference between the pitch axis, and so when pitch is rotated the two hierarchies is the order in which the +/– 90°, the bank axis swings into exactly rotations are applied. The arrow on the the same plane as the heading axis. In the right applies bank first, then pitch, then top viewports of Figure 16-23, you see two heading, essentially reversing the rotation concentric circles, representing the heading order applied by LightWave. (larger) and the bank (smaller), around the Among other things, rotation order is right-hand arrow. The right-hand arrow is in responsible for the phenomenon known as a condition where rotating in heading and “gimbal lock.” bank produce the same applicable results. This is gimbal lock. Figure 16-23 411
5. Chapter 16 · · · · · · · · · · · · · · · · · · · · · · · Now, if you feel you’re having a hard At the bottom of the Bone | Properties win- time wrapping your mind around this prob- dow, LightWave has a set of Muscle Flexing lem, don’t feel bad; you’re not alone. Even and Joint Compensation settings for each the best IK solving engines have a difficult bone. These help LightWave preserve the time with this. volume of the character’s joints as bones When the rotation axis that is recorded bend and the object’s mesh (or “skin”) second (pitch) nears/exceeds +/– 90°, IK flexes. can start to have problems. As the pitch angle nears/exceeds +/– 90°, the item can instantly “spin” around to face the opposite direction. Often, this will happen suddenly, over the course of a single frame. This is Figure 16-24 called flipping. Here, you see the difference between the Note character’s hand when Muscle Flexing and Joint Compensation are active for the bones There are as many ways of setting up a character rig as there are people to do the of the fingers, and when they are not. setups. Everyone will find his own sets of The important thing to know about Mus- rules that work for him. cle Flexing and Joint Compensation is that Over the course of your career, you will hear many opinions as far as the “best” way these functions only work with the bones’ of doing things. In my experience, there is pitch axis! no “best way.” There are only ways that work more easily for the ways in which your mind solves problems. Try what sounds interesting, keep it if it works for you, and always keep your mind open to finding new and better ways of working! Joint Compensation and Muscle Flexing Note This section on joint compensation and mus- cle flexing gets into some pretty heavy-duty LightWave and mathematical concepts. If you’re just starting out, or if you have little desire to get into the “hard-core” aspects of character rigging, please feel free to skip this section, moving on to the next section, called “Flipping.” Figure 16-25 412
6. · · · · · · · · · · · · · · · Layout 3: Character Animation Okay, so what if you want to use Muscle to +/– 90°, you essentially “swap” that Flexing and Joint Compensation with a item’s heading and pitch axes. bone that is only supposed to rotate in heading, as with the forearm? Note LightWave allows you to set/record the pivot rotation for an item as a way of helping to combat gimbal lock. A character’s thigh points more or less straight downward, meaning that by default its rotation is nearly P=–90 right from the start. By using Setup | Modify | Orientation | Record Pivot Rotation (the hot key for this is ; don’t forget to use uppercase), you can tell LightWave to store that item’s cur- rent rotation as the rotation setting for that item’s pivot. The result of the Record Pivot Rotation tool is that the value of the item’s previous rota- tion is stored as the pivot’s rotation. Because the item’s pivot now assumes the previous rotation information, the item itself now lies along that particular set of angles when it is set to H=0, P=0, B=0. So, the item’s rota- Figure 16-26 tion is then set to 0, 0, 0, and a keyframe is created to hold the change in rotation data. Under Modify | Rotate | Rotate Pivot, you In short, using Record Pivot Rotation means that a bone can still look like it’s can manually change the angle that pointing straight up or straight down (P=+/– LightWave Layout thinks of as that particu- 90°), but it will animate from the perception lar item’s H=0, P=0, B=0. If you change that that direction is H=0, P=0, B=0. the Bank setting of an item’s pivot rotation Figure 16-27 413
7. Chapter 16 · · · · · · · · · · · · · · · · · · · · · · · Note Flipping In changing the rotation of an item’s pivot, IK relies on a heavy-duty set of calcula- you must do so on that item’s rest pose, the tions. Even the best IK solvers still have frame where your character exists without any changes to his default rigging position issues when things get really complicated. whatsoever. The biggest problem with even the best IK is flipping, where a joint will spin 180° over the course of one frame. Note Foresight and planning in both your rig- You can also use this technique of altering ging and animation, coupled with a strong the Bank setting of an item’s pivot rotation IK solver such as LightWave’s, will usually to help control IK instabilities, which happen keep flipping to a minimum. most often when an item using IK to solve I find that I can avoid most flipping by: for both heading and pitch approaches or exceeds +/– 90° in its pitch axis. If you notice that your character requires a • Keeping the goal object at a distance of at least one-third the length of the bicep or greater range of movement in the pitch axis thigh away from the bicep or thigh’s point of of an item that uses IK for both heading and pitch, and you’re getting headaches from rotation (the shoulder or hip, respectively). the IK misbehaving, you can set the item’s pivot rotation for its bank axis to +/– 90°, so • Keeping the bicep or thigh from near- ing/exceeding +/– 90° in the pitch axis. then the greater range of movement occurs on the heading axis, which now falls in the same plane that used to be pitch! • Keeping the goal object well away from the area behind the bicep or thigh as that bone would lie in its rest pose. • Not trying to have my character assume a pose that would be painful to do in real life. Figure 16-28 414
8. · · · · · · · · · · · · · · · Layout 3: Character Animation Figure 16-29 Figure 16-30 Note and predictably through many productions. I find that if I’m having problems with flip- It’s always best to learn the rules well ping, I’m usually trying to put the character before you start seeing what happens by into a pose that my own joints would com- breaking them. plain about. If you find you absolutely, positively have to have a particular, “painful” pose, you can • IK is only dependable when solving rotations for a maximum of two items within save a lot of time by making a “special-pur- an IK chain. pose” rigging that can more easily move through that pose. • On any item controlled by IK, let IK solve for a maximum of two axes. • In a chain of two items, the child item should only use IK to solve for one of its “Standard” IK Rules axes. These rules are more like guidelines. They can be bent and broken as you see fit to suit • Always give your two-item IK chain a little “suggestion” in knowing which direc- the needs of your particular IK setup. tion it should bend by pre-bending those However, that being said, these rules items. have kept my characters moving smoothly 415