Automatic Expressive Deformations for Stylizing Motion Paul Noble* and Wen Tang† School of Computing, University of Teesside Figure 1: A run cycle. Above: before. Below: after expressive deformations have been applied. Abstract 3D computer animation often struggles to compete with the flexibility and expressiveness commonly found in traditional animation, particularly when rendered non-photorealistically. We present an animation tool that takes skeleton-driven 3D computer animations and generates expressive deformations to the character geometry. The technique is based upon the cartooning and animation concepts of ‘lines of action’ and ‘lines of motion’ and automatically infuses computer animations with some of the expressiveness displayed by traditional animation. Motion and pose-based expressive deformations are generated from the motion data and the character geometry is warped along each limb’s individual line of motion. The effect of this subtle, yet significant, warping is twofold: geometric inter-frame consistency is increased which helps create visually smoother animated sequences, and the warped geometry provides a novel solution to the problem of implied motion in non-photorealistic still images. CR Categories: I.3.7 [Computer Graphics]: Animation; I.3.5 [Computer Graphics]: Curve, surface, solid, and object representations Figure 2: Examples of expressive limb deformations in Keywords: expressive deformations, cartoon animation, non- cartoons. © Hart (top) [Hart 1997]. (Used with permission.) photorealistic rendering, stylizing motion and joints can be broken [Williams 2001] if it makes for a more 1 Introduction appealing image or dynamic motion. As a result, the limbs of hand-crafted animated characters (both pencil and computer- Traditional animators have always had a rather flexible view of generated) are often distorted to accentuate a motion or imply an bone structure. An understanding of anatomy is crucial but of emotion (see figure 2). Our aim is to mimic these distortions in paramount importance are the fundamental principles of order to imbue 3D character animations with a degree of the traditional animation [Thomas and Johnston 1981; Lasseter 1987], expressiveness and fluidity found in traditional animation. which maintain that bones are there to be squashed and stretched, Commonly seen in 2D animation, and now becoming more *e-mail: [email protected] common in 3D, the primary cause of these distortions is the use of †e-mail: [email protected] what animators and cartoonists refer to as ‘Motion Lines’ and ‘Action Lines’ [Lee and Buscema 1978; Hart 1994; Hart 1997; Blair 1994; White 1986; Brooks and Pilcher 2001]. Action Lines are “the basis for rhythm, simplicity, and directness in animation.” [Blair 1994]. As can be seen in figure 3, these two types of line are closely related and are typically drawn as smooth curves or arcs. They are used by artists as a visual aid to add dynamism and maintain consistency of motion between frames. It is far easier to animate a few curves and have a character follow them than it is capture to cartoons themselves [Bregler et al. 2002]. The motion- capturing of cartoons allows the work of master animators to be retargeted to 3D models. The inherent drawback of this technique is that no new animation is actually created and there is a finite supply of suitable source material. The creation of a coherent piece of animation based upon the retargeted sequences of old films would be almost impossible. The most recent work on CG cartoon animation involves applying an inverted Laplacian of a Gaussian filter to a motion signal [Wang et al. 2006] to automatically generate three of the principles of traditional animation; anticipation, exaggeration, and follow-through. This elegant technique produces excellent results for a range of animations but on more complex character animations can lead to excessive changes in the motion data. Exaggerating the motion of animations that contain interacting characters may be so fundamentally altered that they no longer work. For example, an animated punch could, after filtering, no longer land on its intended target. Figure 3: Lines of action and motion. Perhaps the biggest challenge facing many 3D computer © Hart [Hart 1997]. (Used with permission.) animators attempting to produce cartoon-style animations is the fact that most hand-drawn characters simply cannot be modelled to animate every joint. The action line is often thought of as an in 3D. Traditional animators distort their characters to maximize extension of the spine and, in traditional animation, indicates the their aesthetic appeal depending on the direction from which they overall pose and direction of a character. Closely related to action are being viewed but a single 3D model cannot encompass all of lines, motion lines indicate “the direction of the most accentuated the possible distortions required. To avoid the need for a new movement of the pose.” [Hart 1997]. When applied to the motion model for each new viewpoint, it is possible to deform a single of humanoid characters, these motion lines usually define the model depending on the current direction of view [Rademacher motion of the limbs and, whether consciously or subconsciously, 1999]. Several deformed models, each linked to a different key often lead to the distortion of these limbs in the final illustration viewpoint can be used to warp a base model. At each frame of an (see figures 2 and 3). animation the base geometry is distorted by interpolating these key deformations to produce geometry unique to a particular Whereas an artist can intuitively add deformations to a hand- viewpoint. This technique produces excellent expressive drawn character, the anatomy of most 3D computer-generated distortions of animated characters but the initial modelling work is characters is more rigid and, rather than a loose sketch of a highly labour-intensive and is also character-specific. Tools can skeleton, animators work with virtual joints and bones. These be provided to assist the animator with the task of creating the key skeletal structures incorporate transformation restrictions based on deformations directly from drawings [Li et al. 2003]. Again, this reality: joints have rotation limits, bones have fixed lengths, and technique relies heavily on the animator creating the distortions the associated geometry of the character must follow these rules. and, furthermore, alters the underlying skeletal animation. In this respect, skeleton-driven computer animation struggles to compete with the flexibility and expressiveness of traditional Another problem encountered when trying to produce cartoon- hand-drawn animation. style animations is how to imply motion in a non-photorealistic still image without using motion blur. Simulating motion blur in In this paper, we present an animation tool that dynamically photorealistic animations is a relatively simple matter [Potmesil deforms the limbs of computer-generated characters based on the and Chakravarty 1983] but these techniques can rarely be used pose and motion of their virtual bones. Our aim is to enable with Non-Photorealistic Renderings. Traditional animators and computer animators to quickly add a layer of expressiveness to an cartoonists use many visual cues and techniques to convey the animation and possible applications include use in the computer motion of objects [Blair 1994; White 1986; Brooks and Pilcher games industry or as a tool for creative animators. 2001]. Speed-lines, after-images, and jagged distortions have been applied successfully in computer graphics [Kawagishi et al. 2003; 2 Related Work Hsu and Lee 1994; Lake et al. 2000; Strothotte et al. 1994] but the deformation of the whole object can also imply motion. As clearly With non-photorealistic rendering (NPR) techniques maturing, illustrated by the tennis racket in the third frame of the lower interest in non-realistic and expressive computer graphics has sequence in figure 2, the exaggerated distortion of the racket increased in recent years as 3D computer animators look to the shows its velocity and implies its motion. This aspect of implied world of traditional animation for inspiration and understanding motion in traditional animation, which has been largely [Strothotte and Schlechtweg 2002; Chenney et al. 2002]]. The overlooked until now, is reflected in our expressive animation techniques used in traditional animation are just as relevant to system. computer animation [Lasseter 1987] but many of these principles and practices are so closely tied to their medium that transferring 3 Overview from pen-and-ink to computer graphics is not always a straight- forward matter. The following sections describe a tool that dynamically bends the limbs of computer-generated animated characters to create the A direct method for infusing computer animation with the appearance of a more stylized motion. Our algorithm first expressiveness commonly found in cartoons is to apply motion- determines prospective ‘motion-lines’ based on the key-framed e e e e Figure 4: A Walk Cycle. Top: traditionally animated © White [White 1986]. (Used with permission.) Middle: reproduced Figure 6: Convex (left) and concave (right) distortions and using 3D animation software. Bottom: the same animation their relationship to vector e with subtle limb-bending applied creative process, but to act as an animation aid. We cannot hope to procedurally capture the skills used in the creation of a piece of animation, and nor do we hope to. This is,