Proposal of Quadruped Rigging Method by Using Auxiliary Joint

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1 , pp Proposal of Quadruped Rigging Method by Using Auxiliary Joint Cho-Hye Jeong 1, Kyu-Don Choi 2, Hyun-Seok Lee 3 1 Department of Visual Contents, Dongseo University, jh0214@gmail.com Divison of Digital Contents, Dongseo University, kyudon@gmail.com 3 Divison of Digital Contents, Dongseo University, Corresponding Author: hslee@gdsu.dongseo.ac.kr Abstract. This paper intends to present a new type of rigging method through experiment to ensure that a quadruped character in 3D animation can implement more realistic movement. Particularly, there have been many problems in animating the hind leg of a quadruped animal character with four joints. To solve this problem, through an experimental study for IK structure, upper joints of the leg are moved by IK, an auxiliary joint is made and linked with IK and Aim Constrain is used to move the lower joint according to the IK movement of the auxiliary joint to ensure that the basic structure follows the rigging structure of a two-legged character but the movement of a quadruped character can also be implemented. It is expected that this method can implement more efficient, realistic and natural movement of a character. Keywords: Quadruped character, IK, Aim Constrain, Auxiliary joint 1 Introduction Recently, personified animals which act as supporting roles increasingly appear in many animation films and the importance of animal characters in the entire story sequence increases. Particularly, quadruped characters increasingly appear such as Maximus, a horse character in <Tangled, 2011> and Sven, a reindeer character in <Frozen, 2014>, and therefore, the natural movement of these characters are required. The process that joints are positioned in the manufactured model by referring to the skeletal structure of an actual animal and they are made controllable to provide movement in animation work is called Rigging. Animal characters are classified roughly into two-legged ones and quadruped ones and quadruped animal characters require higher technological level in rigging and animating. For the requirement, this paper intends to present a more effective method for the rigging system on the basis of analysis on rigging and experimental study. ISSN: ASTL Copyright 2015 SERSC

2 2 Contents 2.1 Rigging Method for a Quadruped Character Rigging of a character is completed roughly by three phases of Joint, Skinning and Set-up. This three-phase work process has been examined with the example of a horse polygon model in order to analyze the joint work of a quadruped character. For the joint work, joints are made by referring to the skeletal structure of an actual horse as shown in Figure 1. Even for the similar quadruped characters such as a horse and a dog, different joint work is required according to the skeletal structure of each animal. Fig.1.Joint structure of a horse character (left), Skeletal structure of an actual horse (right) Once joints are created for the positions of every bone, the skinning phase has to be completed where joints and a polygon model are combined. The skinning phase is completed by assigning the areas of an horse shape of polygon model to be moved at the same time as each joint moves and adjusts weight values. Once skinning work is completed, controllers are created for the smooth control of each joint which makes movement. The process that each controller is linked with a joint to make movement through controllers is called setup. Then, a switch for switching between IK (Inverse Kinematics) and FK (Forward Kinematics) is made for use according to the need and necessary setup is performed with Polevector, Expression, Set Driven and etc. 2.2 Bringing up a Problem Fig.2. Basic-posture sitting of a quadruped character (left), Sitting posture (right) We intend to bring up two problems which occur in rigging work which uses IK handle for a quadruped character. 64 Copyright 2015 SERSC

3 First problem: For the dog, a quadruped character in Figure 2, IK was linked with all four joints which forms a leg. By using this method, B and C in Figure 3 move simultaneously according to IK, and even if Polevector is used, the upper part can be controlled but the lower part cannot be controlled, which cause a problem in making a certain posture. Fig. 3. Joint structure of a hind leg in Maya (left), Simple leg structure of a quadruped character (right) Fig. 4. Movement of three control curves in lower part Second problem: For the character in Figure 4, IK was linked with three upper joints to move the upper part of the leg. Although controllers have been made to control the lower part separately, it can be found in Figure 4 that three different controllers move similarly. Besides, if each controller is moved, B in Figure 3 which is moved by IK moves together for all controllers. In this structure, animators cannot implement predictable movement easily. Copyright 2015 SERSC 65

4 Table 1. Two technical limits due to IK connection Method Work Process Result First problem Linked with all four joints which form a leg Polevector Constrain is linked with the second joint position among four joints. It is difficult to control the lower part Second problem Linked with three upper joints among those which form a leg There are control curves which can control the lower part. Although there are three lower partcontrolling curves, their movements are similar and the upper part moves together when the curves move, which makes it difficult to predict movement. 2.3 Experimental Study In past days, these problems were not solved, resulting in animators being prevented from making desired movement or making it by many times of trial and error. In this experimental study, a leg structure which can implement four-leg movement was made to solve the problems by being based on the leg rigging of a two-legged character which is easy to control and expressing the joint structure of a quadruped character. Fig. 5. Joint structure of a tiger character in Maya (left), Simple leg structure of a quadruped character by using auxiliary joint (right) 66 Copyright 2015 SERSC

5 In Figure 5, IK was linked with joints A~C so that the basic movement can be implemented by the upper IK and joints A' ~B' ~D' were made to connect IK with joints A' ~D'. An auxiliary joint called A'' was made at the same position as A' and made parent. Also, up_vector joint called D'' was made at the same position as D, and by using it, C was Aim Constrained to A''. While Aiming at A'', C moves according to IK of A' ~D'. Due to auxiliary joint A'', it is possible to make IK-like movement even if IK is not linked with joints A-D and a variety of movements can be implemented as the angle of C-D can be adjusted according to the position of A''. Besides, offset joint D'''was made at the same position as D to ensure that the below-ankle part can be controlled separately. Fig. 6. Movement of leg joints (left), Angle adjustment of C-D (middle), Movement of ankle (right) To solve first problem that all four joints move according to the movement of IK, making separate control impossible and second problem that the upper part moves together when the lower control curve moves, this paper proposes a method that the basic movement is implemented by the IK of joints A-C but auxiliary joint A is made to ensure that joint C can move according to the IK movement of A'-D' as shown in Figure 5. It can be found that this method can solve two above-mentioned problems. 3 Conclusions This paper has discussed about the rigging method of a quadruped character with many restrictions. It is judged that this experimentally verified rigging method of quadruped characters which is based on two-legged character rigging and uses auxiliary joint can be used later as a joint rigging method which can express a variety of movements of quadruped characters. It is expected that this method can implement the natural movement of a hind leg of quadruped characters and this study can be used as a reference for related rigging work. References 1. Kyu-don Choi and Cheol-yeong Choi, Rigging System Manufacturing Technology and Method for Facial Capture of Digital Creature, The Animation Society of Korea, Vol.0, No.0, pp (2012) Copyright 2015 SERSC 67