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Expressive and Dynamic Deformation of Animated Computer-Generated Characters

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Expressive and Dynamic Deformation of Animated Computer-Generated Characters EG UK Theory and Practice of Computer Graphics (2006) M. McDerby, L. Lever (Editors) Expressive and Dynamic Deformation of Animated Computer-Generated Characters Paul Noble and Wen Tang School of Computing, University of Teesside, Middlesbrough, U.K. Abstract 3D computer animated characters often lack the communicative power and the expressiveness of traditional animation, especially when used in non-photorealistic graphical representations. Inspired by a talk by Pixar Animation Studios at Animex 2005 [WR05] and based upon the animation and cartooning concepts of lines of action and motion, we present an expressive animation tool that emphasizes and accentuates the intentions of the animator derived from the skeleton-driven animation. Motion and pose-based shape deformations are automatically generated to quickly add an emotive layer to an animated character. This warping of the character geometry also provides a novel solution to the problem of implied motion in non-photorealistic still images. Categories and Subject Descriptors (according to ACM CSS): I.3.7 [Computer Graphics]: Animation 1. Introduction However, it is important to note that, with the exception of ‘Squash and Stretch’, limb-bending serves to merely In the early 1930s, animators at the Disney Studios began accentuate any use of these principles rather than studying their subjects for the first time in an attempt to employing them directly. create the ‘illusion of life’. They pored over film reels, frame by frame, and analysed the anatomy of their characters; even going so far as building a jointed armature of a young deer for reference purposes when making Bambi [TJ81]. Traditional animators have, however, always had a rather flexible view of bone structure. An understanding of anatomy is crucial but of paramount importance are the fundamental principles of traditional animation [TJ81, Wil01], which maintain that bones are there to be squashed Figure 1: Examples of expressive deformation of a and stretched, and joints can be broken [Wil01] if it makes character’s limbs. ©Disney for a more appealing image or dynamic motion. As a result, the limbs of hand-drawn characters are often distorted to accentuate a motion or imply an emotion (see The primary cause of these distortions is the use of what Figure 1). animators and cartoonists refer to as ‘Motion Lines’ and These deformations encapsulate four of the eleven ‘Action Lines’ [LB78, Har94, Har97, Bla94, Whi86, [Las87], or twelve [TJ81], fundamental principles of BPE01]. Action Lines are “the basis for rhythm, simplicity, traditional animation, defined by Lasseter [Las87] as: and directness in animation.” [Bla94]. • Squash and Stretch – Defining the rigidity and mass of As can be seen in Figure 2, these two types of line are an object by distorting its shape during an action. closely related and are typically drawn as smooth curves or arcs. The action line is often thought of as an extension of • Anticipation – The preparation for an action. the spine and, in traditional animation, indicates the overall • Staging – Presenting an idea so that it is unmistakeably pose and direction of a character. Motion lines indicate clear. “the direction of the most accentuated movement of the pose.” [Har97]. When applied to the motion of humanoid • Arc – The visual path of action for natural movement. characters, these motion lines usually define the motion of c The Eurographics Association 2006. 212 P. Noble & W. Tang / Expressive and Dynamic Deformation of Animated Computer-Generated Characters the limbs and, whether consciously or subconsciously, principles and practices is not always a straight forward often lead to the distortion of these limbs in the final matter. illustration (see Figures 1 and 2). One challenge facing many 3D computer animators attempting to produce cartoon-style animations is the fact that most hand-drawn characters simply cannot be modelled in 3D. Traditional animators distort their characters to maximize their aesthetic appeal depending on the direction from which they are being viewed. “Spatially, a hand-drawn character is often ‘cheated’ to emphasize traits of the face or body - to improve legibility of an expression, recognition of character (through silhouette features), and/or design (composition). The character's features are often drawn slightly out of proportion or off- perspective as a result.” [Van01] A sense of three-dimensional solidity is crucial in 3D computer animation, which is often presented by a realistic geometric model. However, a single 3D model cannot encompass all of the possible distortions required. To avoid the need for a new model for each new viewpoint, it is possible to deform a single model depending on the current direction of view [Rad99]. Several deformed models, each linked to a different key viewpoint can be used to warp a base model. At each frame of an animation the base geometry is distorted by interpolating these key Figure 2: Lines of action and motion. ©Christopher Hart deformations to produce geometry unique to a particular (top) ©Marvel (bottom). viewpoint. This technique produces excellent expressive distortions of animated characters but the initial modelling Whereas an artist can intuitively add deformations to a work is highly labour-intensive and is also character- hand-drawn character, the anatomy of most 3D computer- specific. generated characters is more rigid and, rather than a loose Tools can be provided to assist the animator with the task sketch of a skeleton, animators work with virtual joints and of creating the key deformations directly from drawings bones. These skeletal structures incorporate transformation [LGXS03]. Again, this technique relies heavily on the restrictions based on reality: joints have rotation limits, animator creating the distortions and, furthermore, alters bones have fixed lengths, and the associated geometry of the underlying skeletal animation which may alter the the character must follow these rules. In this respect, original intensions and expressiveness of the initial skeleton-driven computer animation struggles to compete animation. with the flexibility and expressiveness of traditional hand- Capturing the essence of traditional animation can also be drawn animation. taken literally [BLCD02]. The motion-capturing of In this paper, we present an animation tool that cartoons allows the work of master animators to be dynamically deforms the limbs of computer-generated retargeted to 3D models. The inherent drawback of this characters based on the pose and motion of their virtual technique is that no new animation is actually created and bones. Our aim is to enable computer animators to quickly there is a finite supply of suitable source material. The add a layer of expressiveness to an animation and possible creation of a coherent piece of animation based upon the applications include use in the computer games industry or retargeted sequences of old films would be almost as a tool for creative animators. impossible. Another problem encountered when trying to produce 2. Related Work cartoon-style animations is how to imply motion in a non- photorealistic still image without using motion blur. With non-photorealistic rendering (NPR) techniques Simulating motion blur in photorealistic animations is a maturing, interest in non-realistic and expressive computer relatively simple matter [PC83] but these techniques can graphics has increased in recent years as 3D computer rarely be used with Non-Photorealistic Rendering (NPR). animators look to the world of traditional animation for Traditional animators and cartoonists use many visual cues inspiration and understanding [SS02, NT04, CPIS02]. The and techniques to convey the motion of objects [Bla94, techniques used in traditional animation are just as relevant Whi86, BPE01]. Speed-lines, after-images, and jagged to computer animation [Las87] but transferring these distortions have been applied successfully in computer c The Eurographics Association 2006. P. Noble & W. Tang / Expressive and Dynamic Deformation of Animated Computer-Generated Characters 213 graphics [KHK03, HL94, LMHB00, SPR94] but the The direction and magnitude of the deformation are deformation of the whole object can also imply motion. As dependent upon the aforementioned motion-line, which is clearly illustrated by the tennis racket in the third frame of itself defined by the existing animation or via several user- Figure 1, the exaggerated distortion of the racket shows its controlled attributes. Consequently, the tool’s success velocity and implies the impact of its motion. This aspect depends upon the quality of the initial animation or the of implied motion in traditional animation, which has been skill of the animator. A good piece of animation should largely overlooked until now, is reflected in our expressive possess inherent lines of action and motion and our task is animation system. to simply extract and help highlight them. The principle behind this work is not to attempt to replicate the creative process, but to act as an animation aid. We cannot hope to 3. Overview procedurally capture the skills used in the creation of a The following sections describe a tool created for 3D piece of animation, and nor do we hope to. This is, as animators that dynamically bends the limbs of computer- always, the task of the artist, but we do aim to help generated animated characters to create the appearance of a emphasize and accentuate the animator‘s intentions. more stylized motion. Our algorithm first
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