The,Application,Of,Secondary,Motion,in,Animation in The way

　　Abstract:An important contribution of action in animation comes from secondary motion. We apply the technique of secondary motion into animation to refer to passive motions, which are generated in response to the movements of character and objects in the scene or any environmental forces. This paper aims to introduce the application of secondary action in animation. An animation of walk from my own projects is introduced as an example, and human body Falling down is used to compare and contrast the different actions involved in secondary motions.
　　Key words:Secondary Motion Computer Animation Actions Overlapping Human Body Motion Flexible objects Passive Motion
　　
　　1.What is Secondary Motion
　　Actions have sequences, most movements in an action are not simultaneous, there is a lead and there are movements that follow behind, these movements may overlap in time or simply we say time delay. Some specific definitions described from books and Web sites as a reference:
　　(1)“The secondary motion in a shot is the motion that echoes or complements the primary motion. It often starts as a reaction to a primary motion and through time becomes the new primary motion”.
　　(2)“Secondary motion sometimes called overlap, which indicates motion that is not in line with the main animation of the character".
　　Going through those definitions I come up with my own words to describe it, it is much easier to understand, particularly for readers who are not professionals.My explanation is as follows:
　　Part of an object has stopped moving while the other parts are continuing to move. If we observe a jumping animal with big ears and large tail, it is apparent that the ears and tail do not move at the exact same time as the body moves, and they do not stop at the same time as the body stops. The movements of different parts of the body always have time delay, one section of an object influences the other parts. As a result, it is called secondary motion.
　　2.An illustration
　　One visual example that I have drawn to study the technical illustrates the working sequences of secondary animation. These are the key frames in this 2D animation.(See right image)
　　It is much more obvious to work out the passive motion from the above example. The shape of the object keeps changing during the movement. In Frame 16, when the arm moves backward, second part of the stick chain is led backward and followed by the third part, whereas in Frame 24 when arm moves forward, the second stick is forced to move forward while the motion of third part is still backward. This means when the lead part stops movement or changes in direction, the lagging part will continue to move, and will not change direction immediately. This principle can be applied to anything flexible.
　　As far as secondary motion concerns, there should be an earlier motion in the first place. The earlier motion is called the primary motion that dominates the others. That is why we say actions have sequences. "The primary motion in a shot is the motion that captures the audience"s attention. The primary motion in a character is the motion that carries the action forward."
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文 　　The bellow example in Figure 1 shows clearly the first motion and the second motion occur in this two parts object. The force comes from the big blue ball to move forward, which is the primary motion. In contrast, the small red ball which is the second part is dragged backward.
　　3.Human body motion
　　(1)Walking
　　Human body contains many different parts and it is complex and flexible object. When the body starts to move, the correct reaction from each different section of the body is very important. It is also a key factor for a character when we animate, because this brings character alive.
　　Following example (Figure 2) shows two types of walk represented as character 1 and 2. Both of them are in the same pose and same frame. Very obvious differences can be seen on head, neck, upper body and swinging arms between character 1 and 2. With character 1, the joints between head and the upper body and between arm and hand are straight, whereas character 2 shows the flexibility of human body. This is because fewer actions have been applied to character 1 than character 2.More human motions are involved in the second figure and the line of action is much stronger. There always have time delay and influences on different section of human body "In the movement of complex object different parts of the object move at different times and different rates.”
　　(2)Falling down
　　Different movement of human body will result in different reactions. When people are falling down, the movements of many parts of the body are less dominated by the brain. Therefore, when the body is out of balance, it will fall off on the ground naturally without any other influencing forces.
　　Action sequences are emphasized again in this shot. The structure of a human body is composed of different sections. When fall happens the bottom hits the ground first and then followed by the upper body, the two arms and the head will be the last parts hit the floor. After this action is finished, the motion does not complete, the first motion will pass on to the other parts of the body to become secondary motion. This means the motion of the main body is stopped, but the neck, head, arms and legs are still in motion. They hit on the floor, and then bounce back, finally the whole body will stop to move at all.
　　4.Conclusion
　　To sum up, the readability of actions sometimes depends on the secondary motion. The examples described above demonstrate that the importance of applying secondary motion in to animation and the relation between primary motion and secondary motion, in particular, when animating a complex objects. It does make animation more convincing and believable; it also increases the realism of computer-generated work and keeps the interest of the viewer.
　　
　　References:
　　[1]Isaac V. Kerlow. THE ART OF 3D COMPUTER ANIMATION AND EFFECTS. Third Edition,2004.275.
　　[2]David Rogers, Animation: Master 2002: A Complete Guide. 2002.108.
　　[3]Isaac V. Kerlow. THE ART OF 3D COMPUTER ANIMATION AND EFFECTS. Third Edition,2004.275.
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