Flesh Simulation in a Traditional Animation Pipeline Category: System
Total Page：16
File Type：pdf, Size：1020Kb
Category: System Online ID: papers_0411 Flesh Simulation in a Traditional Animation Pipeline Category: System ABSTRACT aware that something is missing and the illusion of life is shattered. As the exposure of movie-going audiences to computer-generated To achieve this illusion of plausible reality, animators frequently creatures has increased, the demand for realism has grown to choose to employ physical simulation, procedurally determining include fine details of motion as well as appearance. Dynamic how creatures’ anatomy should move. Even in the most simulation of computer graphics creatures has become an unrealistic and extreme situations, the laws of physics lead us to indispensable part of their animation, providing high-resolution the most plausible solution. Such simulations can achieve detail which is prohibitively difficult or expensive to animate by extremely high physical fidelity while augmenting, not replacing a traditional means. Previous dynamic systems have been post- traditional animation pipeline. In fact the result of a physical processes to, or replacements for, existing animation pipelines. simulation adds a secondary realism by bringing out the natural Our approach differs from a purely dynamical simulation in that dynamics of a performance, producing emergent elements in the we view physically-motivated procedural animation as adjunct to animation which help to motivate the reality of the story. performance animation. Moreover, our method allows the seamless integration of dynamic flesh simulation into traditional 1. BACKGROUND AND RELATED WORK animation pipelines, without imposing constraints elsewhere in The idea of a physically-based treatment for flesh is not new. The the production process. We present a pipeline which effectively rich history of physically-based dynamics approaches to flesh merges dynamic simulation and traditional animation. We discuss simulation, spawned in part by the seminal works on dynamics for the physical model we use to simulate flesh, the problem of computer graphics by Terzopoulos et al. [19][20][21][22], is control in a visually demanding field, and finally the forward and discussed below. inverse geometric mapping that allows us to readily integrate our methods into existing pipelines. The first successful approaches to the layered construction of deformable animated characters arose in the area of facial I.3.7 [Computer Graphics]:Three- CR Categories: animation. An early approach to dynamic modeling of flesh was Dimensional Graphics and Realism - Animation undertaken by Platt and Badler [15], who wrapped an elastic layer Keywords: Animation systems, physically based animation. of skin around muscles that were attached to bones. Later, Waters constructed a more detailed facial model [25], even using CT data INTRODUCTION AND MOTIVATION to model accurate skin tissue depths [26]. Terzopoulos and In recent years, filmmakers have made increasing use of computer Waters used laser scanners to acquire geometry and color data graphics technology to reincarnate extinct species, give life to from a subject’s head [27] and drove these models using muscle creatures of fantasy, create super-human events too dangerous to actuators with contractions estimated from video sequences of film, and generally serve as a portal into situations viewers could human faces [21]. These models were further improved upon by never witness first-hand, such as menacing dinosaurs or talking Lee, Terzopoulos and Waters, culminating in a multilayered cats. Viewers know that they’re watching creatures and events approach to flesh modeling that anchors elastic volume-preserving based in fantasy, but they are willing to suspend disbelief so long muscles to a base layer of bones, and surrounds these muscles as no subconscious cues force their attention to the unreality of with a dynamic elastic model for skin including both cutaneous the creatures on the screen. However, with increasing exposure to and subcutaneous tissue [11][12]. computer-graphics creatures, viewers’ expectations for realism Other significant efforts include Miller’s [13] simulation of have grown to require that these creatures not only look photo- snakes and worms with a mass spring system in which muscle realistic, but also move in a realistic manner. Even when viewing contractions were modelled by animating the spring tension, and unfamiliar creatures and situations, viewers have a strong intuition Gourret et al.’s use of a finite element method to simulate both a for the way flesh, bones, and joints ought to move. If a creature’s human hand and a deformable object that was grasped by the hand motion is not physically believable and consistent with its [6]. apparent body composition, the viewer becomes subliminally In the medical community, Larrabee et al.’s three-part study of LEAVE BLANK THE LAST 3.81 cm (1.5”) skin deformation, which included cutting tests on live piglets, OF THE LEFT COLUMN ON THE FIRST PAGE demonstrated that finite element simulations can produce FOR THE COPYRIGHT NOTICE reasonably accurate results [8][9][10]. Zajac [29] developed a rather detailed model of muscles and tendons which has been used by a number of authors, e.g. see Chen and Zeltzer [2]. Delp et al. used the Zajac muscle model to develop a computational and graphical framework for simulating human and animal movement [5]. Delp and Loan further developed this into a system called SIMM [3][4] which can be used to model human and animal movement. SIMM was designed for the medical and 1 Category: System Online ID: papers_0411 biomechanical communities, and uses realistic inputs for bones, “loss of volume” and other similar problems are endemic to joints, ligaments and muscle attachments, e.g. bones can be coordinate-based skinning schemes. generated from medical imaging. SIMM can be used to perform During the period 1997 - 1999, enveloping technology was forward dynamics given a set of muscle activation inputs or to significantly refined. Films such as Jurassic Park II (1997), Star perform inverse dynamics given time history of the joint angles. Wars: The Phantom Menace (1999), Stuart Little (1999), and SIMM uses a linear muscle model which is impressively accurate Dinosaur (2000) utilized a layered approach of new technologies. from a dynamics standpoint, but which lacks the volumetric Volume loss and lack of wrinkle definition were attacked by the effects necessary for visual realism. In addition, SIMM does not addition of specified surface shape deformations driven from joint incorporate models for fatty tissue or skin so it is currently not a motions. Dynamics were added through the use of coordinate system that could be used for creature animation. system nulls whose motions were controlled either by hand or via Chadwick et al. [1] designed one of the earliest creature modeling simple dynamical oscillators. These nulls were used as additional systems. They proposed a layered approach to animation where targeting points for the enveloping process. the animator only needs to control an articulated skeleton in order Full physically-based volumetric systems began to appear in the to generate motion. Sederberg and Parry [17] used free-form 1999-2000 time frame as computers became fast enough to solve deformations to model a muscle and fatty tissue layer that was the large number of degrees of freedom inherent in such systems. capable of dynamic deformation using a three-dimensional mass Our approach marks the application of full volumetric dynamics spring mesh. First an animator specifies the motion of the to the problem of skinning and procedurally animating large articulated skeleton, then some of the points are slaved to move models of photo-realistic resolution for filmmaking. In this with the skeleton while the rest are dynamically simulated using approach, volume preservation and treatment of large-scale the mass spring mesh. Finally, the results from the dynamic motions are integrated into a common dynamic framework. The simulation are used to map the control points of the free form surface microanimation is represented by a physically motivated deformation into the new geometric shape. For extreme skin model integrated with the volumetric simulation system. expressions that are hard to capture with physical models, key frame deformations may be sculpted by hand. 2. FLESH DYNAMICS PIPELINE Turner and Thalman [24] built a similar system, and achieved a Our pipeline is focused on the creation of special visual effects for more realistic look by modeling the skin as a separate dynamic feature films. To this end, motivated and constrained by the layer. The skin was attached to the muscle layer using springs existing processes of film production, we start with a model of the that could be adjusted to give the skin a tight or loose look. The exterior of a creature which we intend to create using computer thickness of this spring mesh fatty layer could be adjusted as well graphics. In fact, many authors have overlooked the artistic to model different parts of the creature. aspects of designing creatures. When an artist visualizes a creature in his mind’s eye, the visualization usually concerns the Wilhelms and Van Gelder [28] took a large step towards realism outside appearance of the creature -- including skin, clothing, fur proposing anatomically based modeling. They used a deformed or hair -- accompanied by some intuition for how the creature’s cylinder muscle model with origins and insertions that topology and visible features change as the creature moves. This geometrically deformed to approximately preserve volume. vision is then realized in concept art, typically two-dimensional Scheepers et al. [16] took muscle modeling a step further, illustrations of the creature, which communicate the features, proposing a few types of muscle models in order to anatomically behaviors, and extreme postures of the creature to others. As the represent human musculature. Wilhelms and Van Gelder character design proceeds, maquettes (sculptures) are produced to wrapped a 75,000 node elastic spring mesh around their creature, illustrate the creature, typically at a reduced scale, in a tangible, anchoring it to the musculoskeletal system using springs [28].