三维建模外文资料翻译--人体动画基础

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1、 - 11 - 外文资料翻译 原文部分 Fundamentals of Human Animation (From Peter Ratner.3D Human Modeling and AnimationM.America:Wiley,2003:243249) If you are reading this part, then you have most likely finished building your human character, created textures for it, set up its skeleton, made morph targets for faci

2、al expressions, and arranged lights around the model. You have then arrived at perhaps the most exciting part of 3-D design, which is animating a character. Up to now the work has been somewhat creative, sometimes tedious, and often difficult. It is very gratifying when all your previous efforts sta

3、rt to pay off as you enliven your character. When animating, there is a creative flow that increases gradually over time. You are now at the phase where you become both the actor and the director of a movie or play. Although animation appears to be a more spontaneous act, it is nevertheless just as

4、challenging, if not more so, than all the previous steps that led up to it. Your animations will look pitiful if you do not understand some basic fundamentals and principles. The following pointers are meant to give you some direction. Feel free to experiment with them. Bend and break the rules when

5、ever you think it will improve the animation. SOME ANIMATION POINTERS 1. Try isolating parts. Sometimes this is referred to as animating in stages. Rather than trying to move every part of a body at the same time, concentrate on specific areas. Only one section of the body is moved for the duration

6、of the animation. Then returning to the beginning of the timeline, another section is animated. By successively returning to the beginning and animating a different part each time, the entire process is less confusing. 2. Put in some lag time. Different parts of the body should not start and stop at

7、 the same time. When an arm swings, the lower arm should follow a few frames after that. The hand swings after the lower arm. It is like a chain reaction that works its way through the entire length of the limb. 3. Nothing ever comes to a total stop. In life, only machines appear to come to a dead s

8、top. Muscles, tendons, force, and gravity all affect the movement of a human. You can prove this to yourself. Try punching the air with a full extension. Notice that your fist has a bounce at the end. If a part comes to a stop such as a motion hold, keyframe it once and then again after three to eig

9、ht or more keyframes. Your motion graph will then have a curve between the two identical keyframes. This will make the part appear to bounce rather than come to a dead stop. 4. Add facial expressions and finger movements. Your digital human should exhibit signs of life by blinking and breathing. A b

10、link will normally occur every 60 seconds. A typical blink might be as follows: Frame 60: Both eyes are open. Frame 61: The right eye closes halfway. Frame 62: The right eye closes all the way and the left eye closes halfway. Frame 63: The right eye opens halfway and the left eye closes all the way.

11、 Frame 64: The right eye opens all the way and left eye opens halfway. Frame 65: The left eye opens all the way. - 12 - Closing the eyes at slightly different times makes the blink less mechanical. Changing facial expressions could be just using eye movements to indicate thoughts running through you

12、r models head. The hands will appear stiff if you do not add finger movements. Too many students are too lazy to take the time to add facial and hand movements. If you make the extra effort for these details you will find that your animations become much more interesting. 5. What is not seen by the

13、camera is unimportant. If an arm goes through a leg but is not seen in the camera view, then do not bother to fix it. If you want a hand to appear close to the body and the camera view makes it seem to be close even though it is not, then why move it any closer? This also applies to sets. There is n

14、o need to build an entire house if all the action takes place in the living room. Consider painting backdrops rather than modeling every part of a scene. 6. Use a minimum amount of keyframes. Too many keyframes can make the character appear to move in spastic motions. Sharp, cartoonlike movements ar

15、e created with closely spaced keyframes. Floaty or soft, languid motions are the result of widely spaced keyframes. An animation will often be a mixture of both. Try to look for ways that will abbreviate the motions. You can retain the essential elements of an animation while reducing the amount of

16、keyframes necessary to create a gesture. 7.Anchor a part of the body. Unless your character is in the air, it should have some part of itself locked to the ground. This could be a foot, a hand, or both. Whichever portion is on the ground should be held in the same spot for a number of frames. This p

17、revents unwanted sliding motions. When the model shifts its weight, the foot that touches down becomes locked in place. This is especially true with walking motions. There are a number of ways to lock parts of a model to the ground. One method is to use inverse kinematics. The goal object, which cou

18、ld be a null, automatically locks a foot or hand to the bottom surface. Another method is to manually keyframe the part that needs to be motionless in the same spot. The character or its limbs will have to be moved and rotated, so that foot or hand stays in the same place. If you are using forward k

19、inematics, then this could mean keyframing practically every frame until it is time to unlock that foot or hand. 8.A character should exhibit weight. One of the most challenging tasks in 3-D animation is to have a digital actor appear to have weight and mass. You can use several techniques to achiev

20、e this. Squash and stretch, or weight and recoil, one of the 12 principles of animation discussed in Chapter 12, is an excellent way to give your character weight. By adding a little bounce to your human, he or she will appear to respond to the force of gravity. For example, if your character jumps

21、up and lands, lift the body up a little after it makes contact. For a heavy character, you can do this several times and have it decrease over time. This will make it seem as if the force of the contact causes the body to vibrate a little. Secondary actions, another one of the 12 principles of anima

22、tion discussed in Chapter 12, are an important way to show the effects of gravity and mass. Using the previous example of a jumping character, when he or she lands, the belly could bounce up and down, the arms could have some spring to them, the head could tilt forward, and so on. Moving or vibratin

23、g the object that comes in contact with the traveling entity is another method for showing the force of mass and gravity. A floor could vibrate or a chair that a person sits in respond to the weight by the seat going down and recovering back up a little. Sometimes an animator will shake the camera t

24、o indicate the effects of a force. It is important to take into consideration the size and weight of a character. Heavy objects such as an elephant will spend more time on the ground, while a light character like a rabbit will spend - 13 - more time in the air. The hopping rabbit hardly shows the ef

25、fects of gravity and mass. 9. Take the time to act out the action. So often, it is too easy to just sit at the computer and try to solve all the problems of animating a human. Put some life into the performance by getting up and acting out the motions. This will make the characters actions more uniq

26、ue and also solve many timing and positioning problems. The best animators are also excellent actors. A mirror is an indispensable tool for the animator. Videotaping yourself can also be a great help. 10. Decide whether to use IK, FK, or a blend of both. Forward kinematics and inverse kinematics hav

27、e their advantages and disadvantages. FK allows full control over the motions of different body parts. A bone can be rotated and moved to the exact degree and location one desires. The disadvantage to using FK is that when your person has to interact within an environment, simple movements become di

28、fficult. Anchoring a foot to the ground so it does not move is challenging because whenever you move the body, the feet slide. A hand resting on a desk has the same problem. IK moves the skeleton with goal objects such as a null. Using IK, the task of anchoring feet and hands becomes very simple. Th

29、e disadvantage to IK is that a great amount of control is packed together into the goal objects. Certain poses become very difficult to achieve. If the upper body does not require any interaction with its environment, then consider a blend of both IK and FK. IK can be set up for the lower half of th

30、e body to anchor the feet to the ground, while FK on the upper body allows greater freedom and precision of movements. Every situation involves a different approach. Use your judgment to decide which setup fits the animation most reliably. 11. Add dialogue. It has been said that more than 90% of stu

31、dent animations that are submitted to companies lack dialogue. The few that incorporate speech in their animations make their work highly noticeable. If the animation and dialogue are well done, then those few have a greater advantage than their competition. Companies understand that it takes extra

32、effort and skill to create animation with dialogue. When you plan your story, think about creating interaction between characters not only on a physical level but through dialogue as well. There are several techniques, discussed in this chapter, that can be used to make dialogue manageable. 12. Use

33、the graph editor to clean up your animations. The graph editor is a useful tool that all 3-D animators should become familiar with. It is basically a representation of all the objects, lights, and cameras in your scene. It keeps track of all their activities and properties. A good use of the graph e

34、ditor is to clean up morph targets after animating facial expressions. If the default incoming curve in your graph editor is set to arcs rather than straight lines, you will most likely find that sometimes splines in the graph editor will curve below a value of zero. This can yield some unpredictabl

35、e results. The facial morph targets begin to take on negative values that lead to undesirable facial expressions. Whenever you see a curve bend below a value of zero, select the first keyframe point to the right of the arc and set its curve to linear. A more detailed discussion of the graph editor w

36、ill be found in a later part of this chapter. ANIMATING IN STAGES All the various components that can be moved on a human model often become confusing if you try to change them at the same time. The performance quickly deteriorates into a mechanical routine if you try to alter all these parts at the same keyframes. Remember, you are trying to create human