Submission

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Bridget Dexter

Submission for

Master of Arts (Electronic Arts) by Research

(Animation)

at the

Australian Centre for the Arts and Technology Institute of the Arts

Australian National University

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Bridget Dexter

Submission for

Master of Arts (Electronic Arts) by Research

(Animation)

This submission consists of the following sections:

1. Animation folio, weighted at 80%

1.1 A VHS videotape containing the major animation work, and

two shorter animations.

1.2 Programme notes and information on the folio.

2. Written sub-thesis, weighted at 20%

"Modelling and animation of a realistic computer-generated

3-dimensional human face."

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Bridget Dexter

Animation folio:

Programme notes and information

Submitted with the VHS videotape as partial requirement for the degree of

Master of Arts (Electronic Arts) by Research

(Animation)

in conjunction with the written sub-thesis

at the

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Animation folio:

Programme notes and information

(Submitted with the VHS videotape)

(I) 'Blodeuwedd' 7' 29" November 1999

Animation, modelling, textures, compositing, voice Bridget Dexter

Sound, voice manipulation Tim Kreger

Information

This animation is the major practical work submitted for this thesis. It was created using

the 3-D animation software Side Effects Houdini, and edited using Avid Media Illusion.

Some textures were created using the image manipulation and paint packages Adobe

Photoshop and Interactive Effects Amazon. The completed animation was laid to

U-matic broadcast quality tape using the realtime image processing software ZaplIT.

On a technological level it explores my interest in realistic computer-generated 3-D facial

modelling and animation, to create a virtual character upon whom the animation

focusses. I modelled the human figure and face in the Houdini modeller using NURBS

surfaces, and animated them using Houdini's 'bones' skeletal deformation tools. The dress

was modelled using both the modeller and procedural modelling techniques, and was

animated with mathematical spring deformations. I was also interested to explore the use

of particle system dynamics to depict natural phenomena such as steam and the random

movements of flowers floating through water. A further technological focus was on the

realistic representation of natural objects, such as the flowers, standing stones, and

mirror. My interest in archaeological perceptualisation led me to create the Celtic mirror

as an accurate reproduction of the Desborough mirror, which was made in Britain

between the first century B.C. and early first century A.D. (Cunliffe 1979, 97-98; Duane

1996, 32). Finally, I have used Houdini's expression language to animate many geometric

deformation and procedural texture effects including, respectively, the movement of the

water surface and exploding leo and virgo symbols, and the movement of the mirror

surface and textures of the leo, virgo and libra symbols.

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a Celtic myth. To create an appropriately Celtic setting for scenes where the character appears, I created backgrounds composed from intricate Celtic key patterns (Sloss

1997), which I created in Houdini mathematically using Lindenmayer systems (Prusinkiewicz and Lindenmayer 1996).

The accompanying written sub-thesis focusses on my research in the field of facial modelling and animation. This research, and the way in which I modelled and animated my own virtual character, is described in detail in sections 2, 3 and 4 of the sub-thesis. My artistic focus and a symbolic analysis of the Celtic myth upon which the animation is based are discussed in section 5 of the sub-thesis.

Programme notes

Exhibited 25/11/99: ACAT Performance Night 'Too Weird for You'

This animation is based around the symbolism inherent in the Celtic story of

Blodeuwedd, which forms part of the Welsh collection of myths and histories known as The MabinogioiL Blodeuwedd is an otherworid maiden conjured from flowers and from the water of the ninth (or most powerful) wave. As such she personifies the Virgin aspect of the Triple Goddess, although in her story she also later passes through the Mother and Crone aspects. Her husband, Llew, personifies the Sun God, and in particular its connotations of a dying and resurrecting figure.

Their story is a reflection in microcosm of the macrocosmic points of balance of the Wheel of the Year. The Wheel marks the rhythmic changes of the earth's seasons with its eight important festivals or Sabbats. Blodeuwedd appears mortal but her reflection in the mirror reveals her enchanted origins. In the mirrors of mythology truths about our own existence may be reflected back to us.

Thanks to Tim Kreger for the sound.

Hardware: SGI workstations, PowerMacintosh G3.

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(II) T 2' 03" November 1995

Animation, modelling, textures, compositing, sound Bridget Dexter

Information

This piece was my first 3-D computer animation, and was submitted as part of

the academic requirements for the coursework Graduate Diploma in Electionic

Arts with which I commenced my studies at A.C.A.T., before subsequently

rolling over into this Master degree.

My goals were to become proficient at using the 3-D animation package Side

Effects Prisms, and to explore a number of challenging animation areas. As my

interest lies in the area of realistic animation, these areas comprised the realistic

representation of natural effects such as water, steam, fire, light and shadows. I

used particle systems and point colour algorithms to achieve the running water,

steam and fire effects. For the moving water surface, 1 used a geometric surface

which was deformed using a sine wave expression. I constructed the brazier, cup

and bowl geometry objects using both procedural chains of surface operations

and the Prisms modelling editor.

Programme notes

Exhibited 23/11/95: A C A T Performance Night 'Hysteresis'

This piece was inspired by the Zen ritual of the Japanese tea ceremony. I wanted

to explore the concept of the beauty and haimony which are intrinsic in

simplicity.

Hardware: SGI workstations.

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(III) 'Ozvmandius' 3'20" June 1995

Animation, textures, sound, voice Bridget Dexter

Information

This piece was my first 2-D computer animation, and was submitted as part of

the academic requirements for the coursework Graduate Diploma in Electronic

Arts with which I commenced my studies at A.C.A.T., before subsequently

rolling over into this Master degree.

My goal was to become proficient at using the 2-D paint and animation package

DPaint IV. I explored the representation of painterly surfaces. My focus was on

subtle colour and texture effects used to evoke the atmosphere of a

sandstorm-swept desert, and on trying to achieve the effect of'painting come to

life'. My early interest in facial animation is apparent in the attempt to create the

2-D character of Ozymandius, and to animate it with some basic facial expression

changes.

Programme notes

Exhibited 6/95: ACAT Performance Night 'Metanoia'

The material pleasures and temporal powers of this world are illusory. From dust

do great kings come, and to dust they go. Percy Bysshe Shelley evokes the

recognition of this impermanence in his poem, Ozymandias. This poem was a

favourite of my late dear uncle David Dexter, who used to recite it with gusto.

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Bridget Dexter

"Modelling and animation of a realistic

computer-generated 3-dimensional human face"

A written sub-thesis submitted as partial requirement for the degree of

Master of Arts (Electronic Arts) by Research

(Animation)

In conjunction with the animation folio

at the

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Modelling and animation of a realistic

computer-generated 3-dimensional human face

1 Introduction

This written sub-thesis investigates the problem of modelling and animating a realistic computer-generated 3-D human face. The thesis as a whole includes the folio of animations, and its goal has been to create a major animation work, in which the computer-generated character is artistically immersed in an animation created around the theme and symbolism of the Celtic myth of Blodeuwedd. Blodeuwedd's story forms part of the Fourth Branch of the Mabinogi, from the collection of ancient Welsh mythical and historical tales known as The Mabinogion (Gantz 1978, 110-117, Walton 1972; Stewart 1996, 141-153).

The artistic aim of the thesis as a whole is to create an original animation work incorporating the virtual character and based on the symbolism inherent in the Celtic myth. The artistic focus of this sub-thesis is thus on the expressive acting of the character and its artistic integration in the animation.

The technological aim of the thesis as a whole is to develop and refine a practical method of realistic computer-generated 3-D human facial modelling and animation, to construct a reproducible and orderly system for generating lip-synch and emotive expression, and to model and animate a realistic 3-D human face. Another technological goal is to base the virtual model on a real-life human being. The technological focus of this sub-thesis is therefore on the investigation and evaluation of various facial modelling and animation techniques, with the aim of developing and refining a method which is best suited to overcoming the problems associated with constructing a realistic, versatile model and animating it with flexible, believable expressions and speech.

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protocols, a consideration of these high-end methods is beyond the scope of this sub-thesis.

Realistic 3-D facial animation is often viewed as prohibitively complex, and as a result many animators may believe that they are limited t o creating stylised or cartoony characters. This sub-thesis seeks to demystify to some extent the problems involved in this area. Through researching and assessing the area of accessible 3-D human facial modelling and animation techniques, and discussing the conceptual and technological problems encountered in the practical animation work, this work may contribute to making realistic 3-D facial modelling and animation more accessible t o other animators.

This sub-thesis initially discusses the background and applications of computer-generated human facial modelling and animation, to place in context the subsequent investigation and the resulting practical animation work. Human facial computer modelling and animation techniques have many applications in the arts, sciences and entertainment, and their development was the culmination of many interacting scientific, medical,

technological and social influences. This sub-thesis sets the work in context by tracing some of the major applications, influences, developments and turning points.

It then examines some of the main methods of computer facial modelling and animation which have been accessible to the majority of 3 - D animation artists to produce both realistic and cartoon-like virtual 3-D characters. These methods are evaluated in terms of their advantages and disadvantages, their ability to produce a versatile realistic model, and their suitability for developing a comprehensive and reproducible system for generating facial poses.

Following the determination of the most appropriate direction to explore in modelling and animating a realistic 3-D human face, the sub-thesis then focusses on my practical animation work, discussing the technological methods and techniques which I developed and refined to model and animate the virtual human character's face.

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2 Applications and history: a brief tour of 3-D

computer facial modelling and animation

2.1 Applications

3-D facial modelling and animation has important applications in many disparate fields, including the sciences, medicine, art, entertainment, communication, advertising and forensic science. The level of facial animation realism which some practitioners are able to achieve is impressive. One example is seen in the short animated movie clips The Gathering {GXyphX Inc. c. 1998).

One of the major and most visible areas of application of 3-D facial modelling and animation is of course film and television production. Over the last decade the expanding needs of the computer animation industry and the film and special effects (SFX) industries have motivated increasingly sophisticated developments in 3-D facial modelling and animation techniques, using expensive high-end graphics machines (Parke and Waters 1996, 5). In the last few years a number of completely 3-D

computer-generated, character driven animated short features and full length feature films have appeared which have pushed the boundaries of the possible in the area of facial animation. These include Pixar's Tin Toy, Toy Story and A Bug's Life (Pixar Animation Studios 1988, 1995, 1998, 1999), Pacific Data I m a g e s ' ( P a c i f i c Data Images 1998 and n.d ), and Maya's 5/n^o (Alias/Wavefront 1999a). Virtual characters have recently been created which are so realistic that they may soon compete with real humans for some acting and performing jobs. These include the virtual model Webbie Tookay (Elite Illusion2K 1999; Rotzer 1999), and the character Jar Jar Binks in the film Star Wars: The Phantom Menace (Lucasfilm Ltd n.d ).

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Facial modelling and animation have become increasingly important in medicine, particularly to simulate surgical procedures pre-operatively. This has been especially important in planning craniofacial surgery where the facial bones are rearranged, and in understanding and simulating post-operative skin and muscle tissue dynamics, with very detailed models of skin tissue structure having been developed (Parke and Waters 1996, 6). A related function is the use of 3-D facial modelling technologies in forensic science, palaeontology and archaeology t o reconstruct the appearance of a deceased person using just the bare skull as a starting point (Richards 1999).

3 - D facial modelling and animation is becoming increasingly important in the communications and internet industries (Ratner 1998, xi). As the field of

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2.2 History

The seeds of the current developments in computer facial modelling and animation were sown long before the advent of computers, with some early investigations into facial expression analysis, a subject which is fijndamentally important to the successful animation of a 3-D facial model. These studies related emotional states to the interaction of facial muscle actions. Bulwer published such an investigation of facial expression in the late 1640s (Parke and Waters 1996, 12).

Two people in particular subsequently defined the field of facial expression analysis. The first was Duchenne de Boulogne, who published his seminal analytical study of human facial expression. The Mechanism of Human Facial Expression, in 1862 (Parke and Waters 1996, 12-17). This work essentially defines the field, even for researchers today. De Boulogne investigated facial muscle movement by stimulating individual facial muscles with electrodes and documenting the results using photography. As a result of his analysis he classified facial muscles or muscle groups according to the expression(s) they elicited. Charles Darwin extended this work to demonstrate the universality of expressions in man and animals. His theories, published in 1872 in The Expression of the Emotions in Man and Animals, are also still relevant today (Parke and Waters 1996, 13). It was not until the late 1970s that a more precise analysis and parameterisation of human facial expression was undertaken. Ekman and Friesen reported their Facial Action Coding System (FACS), which broke down facial action into 66 small Action Units (Ekman and Friesen 1978, in Parke and Waters 1996, 17). Each Action Unit represented the action of an individual muscle, or a small group of muscles, which resulted in a recognisable facial pose. In combination these facial poses generated defined recognisable expressions (Parke and Waters 1996, 122-127). FACS has subsequently extensively influenced the field of computer facial animation, in the interpretation and construction of realistic facial expressions based on inferring the emotional state from the action of the muscles (Badler, Barsky and Weltzer 1991, 83).

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Subsequently, during the 1970s, the 2-D cartoon animation industry was supported by the development of 2-D computer-assisted animation systems at the New York Institute of Technology, Cornell University, and Hanna-Barbera (Parke and Waters 1996, 4), and research on a muscle-controlled facial expression model was published in 1980 by Piatt at the University of Pennsylvania (Piatt 1980, in Parke and Waters 1996, 4). During the same period work on 2-D facial images was proceeding, with the development of a computerised 2-D photoidentikit system and of computer based techniques for aging facial images (Parke and Waters 1996, 4).

The early 1970s saw the first attempts at computer generated 3-D facial modelling and animation. Parke at the University of Utah developed crude polygonal facial models (Parke 1972, in Parke and Waters 1996, 3). Gouraud, at the same university, developed his smooth polygon shading algorithm, named Gouraud shading, and Parke subsequently used it to produce 3-D facial animation sequences by photogrammetrically analysing real faces to create several Gouraud-shaded polygonal facial expression models, and then interpolating between them (Parke and Waters 1996, 3). Parke later constructed the first parameterised facial model (Parke 1974, in Parke and Waters 1996, 3).

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(Pixar Animation Studios 1988 and 1999; Watt and Watt 1992, 411-412). Around fifty muscle models controlled the movement of approximately 3000 control points in the baby's face, and were in turn controlled by a higher level of macromuscles (Watt and Watt 1992, 411-412).

The late 1980s and early 1990s saw the development of optical range scanners such as the Cyberware optical laser scanner (Parke and Waters 1996, 4; Cyberware 1999; headus (metamorphosis) Pty Ltd c. 1999). These devices, basically an extension of the old photogrammetric techniques, can scan in the complete data of an individual's face to automatically create a realistic polygonal facial model.

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3 Investigation and evaluation of techniques

3.1 Scope

For the practical animation component of this sub-thesis, I decided t o base my animation work around an attempt to model and animate a realistic 3-D computer-generated human face, based on a real-life human template. This is a challenging task, and before I was able to commence work on the project I needed t o acquire a good understanding of the technological concepts involved, and of the different modelling and animation methods which were accessible to me given the limitations of the equipment and time available. The scope of my investigation therefore concerned those techniques which are accessible to the average 3-D graphics artist, as discussed in section 1. This section summarises the area of my investigation, discussing the technological issues involved in modelling and animating a 3-D human face, the methods which I explored, and my evaluations of these methods.

3.2 Modelling a realistic computer-generated 3-D face

3.2.1 Requirements of the model

Modelling a realistic computer-generated 3-D human face combines the skills of drawing and sculpting in an interactive computer environment (Ratner 1998, xi).

The human face is a complex construction of bones, cartilage, muscles, nerves, blood vessels, glands, fatty tissue, connective tissue and skin (Williams et al 1989). It is not yet possible to construct geometric or mathematical models which simulate all these elements. Although such a complete simulation is the eventual goal for medical visualisation, models constructed for realistic character animation certainly do not require all this detail (Parke and Waters 1996, 55).

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The complete head model will consist of a number of parts. These comprise the skin surface of the face and neck (expressive facial mask, including the mouth), eyes, eyebrows, ears, teeth, tongue and hair (Parke and Waters 1996, 63-65; Kelly 1998, 135-139). At a minimum, this model must be constructed in such a way that during animation it is possible t o close and stretch the eyelids properly over the eyeballs, to open, close and stretch the mouth without distortion, t o move the j a w properly, and to move the skin surface without it puckering or gaping . Of course, if super realism is required, there are many subtle movements which must also be considered.

Each of these parts presents its own issues and problems. The mouth and the eyes are the most important areas, as they communicate the most information about a face, and they therefore require more detailed geometry than the rest of the model (Maestri 1996,

135-136, 274-276; Parke and Waters 1996, 63-64, 113; Kelly 1998, 136-139). Care must be exercised from the start to build in the flexibility necessary for their successftil animation. Eyebrows also convey much expressive information. They can be constructed as separate geometry elements pasted onto the facial mask, or they may be simply painted on (Maestri 1996, 141; Kelly 1998, 135). Ears are a complex shape which is extremely difficult t o model (Ratner 1998, 233-236), but they are necessary appendages if a realistic model is required.

Hair is a big challenge, and ideally should be dynamic. One-piece surface hair models are easier to construct, but generally look very artificial (Ratner 1998, 242-244,246). Protocols for dynamic hair models can become extremely mathematically complex (Parke and Waters 1996, 309-335). A successful but accessibly simpler approach is t o model the hair as a collection of individual strands using particle systems, although this can be resource intensive (Maestri 1996,. 145-146). A less resource intensive method is t o use a minimal number of geometric hair strands in conjunction with a procedural texture shader to simulate the individual hair strands (Maestri 1996, 147; Kelly 1998, 314-322). Some 3 - D animation packages, such as Maya and Softimage, have custom-designed hair shaders based on particle systems, which automatically generate hair and make things considerably easier (Maestri 1996,145-146).

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shading and texturing of hair are very difficult because of its complex light and transparency behaviour (Kelly 1998, 259).

Of overriding importance is that the model should be flexible and well constructed to suit the planned method of animation. A well constructed facial model will facilitate easy animation along many of the natural lines of muscle action of the human face (Maestri

1996, 106, 259, 262). These considerations make it fairly obvious that in order to successfully model and animate a realistic human face, it is necessary to have a detailed anatomical understanding of the face, head and neck. The study of anatomy has always been important in painting and sculpture, but it is even more important in animation, where realistic movement is a major goal.

3.2.2 Anatomy of face, head and neck

There are essentially two parts of the head which need to be understood in order to successfully model and animate a human face - the facial skeleton and the facial muscles. The best sources of reference for this study are medical reference manuals such as Gray's

(Williams et al 1989).

The facial skeleton, which is the skeleton of the actual face, forms part of the skull, the other part being the cranium. The facial skeleton is of particular interest in 3-D modelling as it is the hard framework to which the muscles and skin are attached. Its only freely jointed structure is the mandible or jaw (Maestri 1996, 104; Parke and Waters 1996, 22-23; Kelly 1998, 133-134).

The face is a complex collection of muscles, known as the muscles of facial expression, which pull and stretch the facial skin in various ways. They are superficial muscles which physically interweave with each other and work synergistically, not independently. The majority of them are anchored to specific fixed points of attachment on the skull at one end and to the facial layer of subcutaneous fat and skin at the other, although some, such as the orbicularis oris around the mouth, are attached to subcutaneous fat and skin at both ends and are not anchored to the skull at all (Maestri 1996, 104-106; Parke and Waters 1996,31-33).

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n

eyebrows by the cornigalor supercilii, the action of frowning by the frontalis, the laughing mouth shape by the zygomatic major, the smihng mouth shape by the risorius, and the variety of lip shapes required for speech, or for pursing or narrowing the lips, by the orbicularis oris. The movement of the jaw involves the coordinated action of a number of the muscles which are attached to it. Where super realism is the aim, it may also be necessary to study the action of the muscles of the tongue, scalp, ears, cheeks and neck, as well as the overall construction of the muscle, subcutaneous fat, and skin layers (Parke and Waters 1996, 34-51).

For successful realistic facial animation, it is also necessary to understand the ways in which these muscles move in order to construct a model which will move easily along the same lines that the muscles are pulling, allowing accurate simulation of their movement. In terms of facial movement, there are three types of muscles (Parke and Waters 1996, 33-34, 228-235; Maestri 1996, 106; Ratner 1998, 290). The linear or parallel muscles pull in an angular direction. Examples include the zygomatic major and corntgator supercilii. The elliptical or circular sphincter-type muscles squeeze in a radial fashion, such as pursing of the lips produced by the orbicularis oris. The sheet muscles are a series of linear muscles spread over a broad area. An example is the frontalis. Once the structural principles of the face and head have been understood, it is time to investigate the methods and techniques which can be used to model a human face, and to evaluate them in terms of their suitability for producing a realistic face capable of realistic expression and movement.

3.2.3 Computer-generated surface types

When modelling a face, the first question to decide is what surface type is to be used to construct the expressive facial mask. A computer-generated surface for a face can be constructed from two main surface types, polygons or splines (Parke and Waters 1996, 58; Maestri 1996, 107-108, 113;Ratner 1988, xii; Kelly 1998, 150-151).

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polygonal network, the vertices are organised into a rectangular array and then uniformly and regularly connected with triangles or quadrilaterals. In an arbitrary polygonal

network, the surface is formed of networks o f polygons whose vertices are arbitrarily and irregularly connected up as required (Parke and Waters 1996, 61-63). A polygonal mesh is a series o f connected polygons with m * n topology, which share common

vertices and edges (Hill 1990, 527-528; Penner and Vinovich 1996, 2:356).

Parametric or spline surfaces are mathematically based on polynomials which describe the curves o f the surface plane (Parke and Waters 1996, 58; Penner and Vinovich 1996,

2:368-375; Watt and Watt 1992, 65-107; Kelly 1998, 150). They can be built from a series o f curved 2-D lines, called isoparametric curves or splines, which are subsequently joined or skinned to form a surface. The surface thus consists o f a matrix of

isoparametric curves (Ratner 1998, 12, 198; Penner and Vinovich 1996, 2:374-375; Hill 1990, 115). Parametric surfaces include bezier and N U R B S (nonuniform rational

B-spline surface) surfaces (Parke and Waters 1996, 58; Penner and Vinovich 1996, 2:374-375; Watt and Watt 1992, 73-107). These surfaces are defined and controlled using arrays o f control vertices whose action defines small areas of the overall surface.

Smooth transitions are generated between the control vertices by a series o f

mathematical blending functions, allowing the small areas to join together with specific

surface continuity characteristics (Parke and Waters 1996, 59-61; Penner and Vinovich 1996, 2:368-369; Ratner 1998, 12-13).

The majority of computer-generated facial models to date have been based on polygonal

surfaces (Parke and Waters 1996, 61; Kelly 1998, 150), although spline surfaces are today becoming the preferred way to create digital flesh as they can give very realistic resuhs (Maestri 1996, 113; Ratner 1998, 198-199; Kelly 1998, 150).

Polygonal surfaces have a number o f advantages (Maestri 1996, 107-108; Kelly 1998,

151). They are in some ways easier to model than spline surfaces, allowing for instance

more flexibility when cutting holes for eye sockets or nasal passages. They are also easily

displayed and efficiently handled by all generally available graphics computers (Parke and Waters 1996, 61), thus lending themselves to near real-time updating. There are some

major disadvantages to using polygonal surfaces for the expressive facial mask, however

(Maestri 1996, 108; Ratner 1998, 198-199; Watt and Watt 1992, 4-5; Kelly 1998, 157).

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unless a large number of polygons are used in the construction of the face surface, there will be visible discontinuities and an overall unnatural look t o the face. However, if a smooth surface with a large number of polygons is constructed, the model becomes very resource intensive to manipulate, in terms of database access and rendering time. Furthermore, dense polygonal surfaces can be difficuh to control when animated, being prone to crimping and puckering.

Parametric surfaces overcome these technical problems associated with polygonal surfaces, and can produce very realistic results. They produce models with smooth, natural, curved surfaces defined using relatively few control points, and this low vertex density allows easier control when defining the overall curves of the facial surface, as well as at animation time (Maestri 1996, 113; Ratner 1998, 12-13, 198-200; Watt and Watt 1992, 65-66; Kelly 1998, 151). Organic curved surfaces can be easily modelled using splines, by first constructing several key curved isoparametric or spline 2-D curves, and then joining or skinning these curves together t o form the surface (Ratner 1998, xii; Hill 1990, 115). This method allows optimal positioning of the isoparametric curves t o ensure that the model will move easily along the natural lines of action of the muscles when animated. However, parametric surfaces have their own set of disadvantages (Parke and Waters 1996, 60; Maestri 1996, 108, 113, 128; Kelly 1998, 162). The optimal sparse density of the control vertices does not support very detailed surface definition, and it can therefore be difficult t o build into the model the extra surface detail and control which are required around the eyes and mouth. T o add in local detail, complete rows and/or columns of control vertices, extending over the whole face model, must be added. This greatly increases the control vertex count, making the model more resource intensive to work with. Additionally, it is difficult to model even moderately deep creases in the face, as these involve a break in the surface continuity. Furthermore, there can be problems with stubborn irregularities and creasing in the parametric surface, due t o individual isoparametric curves standing proud of the surface (Ratner 1998, 198-200); this problem is especially seen in areas of more densely spaced control vertices, and can cause fijrther surface problems during animation.

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(Maestri 1996, 126; Parke and Waters 1996, 60-61; Birn 1999b). This can present its

own set of problems, such as gapping at patch edges during animated movement.

3.2.4 Approaches to 3-D facial modelling

There are many different ways to model facial surfaces, ranging from the very simple to

sophisticated methods at the cutting-edge of computer graphics research. This sub-thesis

examines some of the more common methods which are accessible to the ordinary 3-D

graphics artist.

Constructing a realistic virtual human face is a significant modelling challenge. We are so

used to examining human faces that the slightest awkwardness in the model will be

immediately obvious. The methods used can be divided into two groups, those which

make use of automated 3-D surface measuring techniques, and those which rely on

manual interactive techniques such as surface sculpting or assembling faces from

components (Parke and Waters 1996, 66).

3.2.4.1 3-D Surface measuring techniques

3-D surface measuring techniques use technology to measure and digitise the surface

contours of a real-life human face or of a facial sculpture. There are three techniques

used today. The first two are only briefly described as the requisite equipment was not

available to me for this animation. The third technique is considered in more detail.

Photogrammetric techniques capture surface shapes photographically (Parke and Waters

1996, 67, 72-78; Yuencheng, Terzopoulos and Waters 1995, 55). The face of the live

model or sculpture is directly marked with horizontal and vertical contour lines, and

multiple simultaneous photographs are then taken from various points of view. The 3-D

surface points are then computed from this comparative data. This technique is quite fast

and so is conducive to the use of a live model.

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positioned. This technique best suits working from a sculpture of the face rather than a live model, as it requires the subject to remain still for quite long periods.

Laser scanning techniques, using laser scanners such as those produced by Cyberware and others (Cyberware 1999; headus (metamorphosis) Pty Ltd c. 1999), work on the same principle, but use a laser which scans the surface topology t o produce a cylindrical regular polygon mesh of the subject (Parke and Waters 1996, 67, 78-84; Maestri 1996, 126; Yuencheng, Terzopoulos and Waters 1995, 55; Kelly 1998, 214-222). The scan can also simultaneously capture surface colour information (Yuencheng, Terzopoulos and Waters 1995, 56-57, 60-61). This technique is very fast and so can use a real life human face as a model. Laser-scanned models are available on the internet (Cyberware 1999; headus (metamorphosis) Pty Ltd c. 1999).

Although using one of these models initially seems much easier than modelling a complex surface like a face from scratch, there are many problems associated with using the cylindrical meshes produced by laser scans (Parke and Waters 1996, 67, 79-84; Maestri 1996, 126; Kelly 1998, 222-236). For example, the cylindrical mesh must be translated t o Cartesian coordinates, and this results in missing surface information and thus holes in the model, particularly at the top of the head where the cylindrical mesh does not merge. Furthermore, the vertex count of the mesh is so dense that the model can not be practically animated, and it is difficult to thin the amount of data without unacceptably distorting the contours of the facial surface. Fine details such as eyelids and ears are often not captured in sufficient detail by the laser scan and may need to be remodelled, and the overall surface resulting from the scan is noisy, marred by bumps and

irregularities which must be smoothed out. Finally, the polygonal surface type results in a model which does not lend itself to moving easily along the natural lines of action of the facial muscles. A considerable amount of work is required to turn the laser data into a useable 3-D model, either by applying surface smoothing algorithms and data thinning strategies, or by rebuilding the model by fitting parametric curves or an adaptive polygon mesh t o the laser scanned surface (Parke and Waters 1996, 80-84; Yuencheng,

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3.2.4.2 Interactive surface sculpting

Interactive surface sculpting involves manually building and refining the facial surface within the computer, essentially the way a sculptor would do it, generally using some commercially available modelling software package (Maestri 1996, 107; Ratner 1998, xi-xii) such as Houdini (Side Effects Software Inc. 1999), Maya (Alias/Wavefront

1999b), Softimage (Avid Technology Inc. n.d ), LightWave (NewTek c. 1999) or 3D Studio M A X (Autodesk Inc. c. 1999), although a modelling programming language may be used by more technically-minded animators or researchers (Parke and Waters 1996, 87). There is an endless number of ways t o interactively model a face, limited only by the imagination. This sub-thesis looks at some general categories of construction.

One of the most commonly used methods is to assemble a face from separate, simplified component parts. The underlying face shape is a simple primitive such as a sphere, and simple shapes representing eyes, nose and mouth are pasted onto this (Maestri 1996, 106-107; Parke and Waters 1996, 85-86). This method is easy and flexible, but it is most suitable for simple, stylised, cartoon-like characters rather than a realistic facial model, and so will not be considered further.

For realistic results, the facial model needs to be constructed with a continuous skin which encompasses the main expressive features and which moves and flexes as a whole (Maestri 1996, 107).

If a polygonal surface type is to be used, one effective method t o interactively construct a facial model is to build up the face topology by clumping metaballs together. Metaballs are a polygonal data type with a defined field of effect based around a point; when two metaballs overlap in space, their field effects are added together and they fiise (Penner and Vinovich 1996, 2:357-359; Ratner 1998, 74-75). Using separate, appropriately sized metaballs positioned for the skull, cheeks, nose, chin and so on, a facial metaball model can be built up, and the whole is then converted into a polygonal surface (Maestri 1996,

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time-consuming process, and the method may therefore be more suited to creating

stylised facial models (Ratner 1998, 74-78, 75).

As mentioned in section 3.2.3, parametric or spline surfaces are increasingly preferred for

realistic facial models because of their smooth surfaces, lower vertex count, and easier

control of surface curve construction and animation. An important point to bear in mind

when modelling spline surfaces is that the minimum effective number of control vertices

should be used; the simpler the structure of the model, the easier it will be to animate

(Maestri 1996, 113; Ratner 1998, 200).

One method of interactively modelling a spline based head model is to start with a simple

parametric sphere and progressively deform and refine it, by moving control vertices and

adding isoparametric curves in areas where more detail is required, until the desired

facial surface is achieved (Maestri 1996, 113-119; Ratner 1998, 203, 230-232). The

mouth and eye cavities can be formed by creating 'pouches' which balloon inward.

A parametric sphere has its isoparametric curves running 'north-south' and 'east-west' in

the same way that lines of longitude and latitude are marked on a globe of the Earth, and

the construction and animation of the head model will be affected by how the

'north-south poles' of the sphere are oriented with respect to the topology of the model

There are three alternative placements, each having their own implications for

construction and animation (Maestri 1996, 114; Ratner 1998, 203). If the 'north pole' is

placed at the top of the head, it will be intuitively easier to accurately model the face

surface, particularly in the eye and nose areas, because the face is cross-sectioned by the

vertical lines of the isoparametric curves, and it will also be easier to base the model on a

real-world template. However, since the vertical splines flow counter to the natural

muscle lines, animation will be more difficult, particularly for the mouth where an

elliptical sphincter-type muscle (the orbicularis oris) plays an important movement role

This disadvantage can be overcome by placing the 'north pole' at the mouth of the model,

and the 'south pole' at the base of the neck or back or the head. The 'lines of latitude' of

the sphere then encircle the mouth, mimicking the natural lines of action of the

orbicularis oris, but the direction of flow of the isoparametric curves makes modelling

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longitude' now follow their contours more closely, but once again the isoparametric

curves do not run in the same direction as the mouth muscles, and there will be problems

with the animation of the mouth.

A variation o f the sphere method is to progressively deform and refine a parametric

cylinder rather than a sphere. The openings at each end correspond the 'north-south

poles' of the previous method. One opening is placed at the mouth, to form the mouth

cavity, and the other at the base of the neck. An advantage of this method is that the

mouth cavity is automatically generated and does not need to be expressly modelled as a

pouch construction.

Rather than progressively deforming a sphere, a more accurate and effective method of

constructing a head model is to imagine a series of vertical 2-D contour lines slicing

through the surface of the face. The 2-D contour lines can be distributed across the facial

surface in two different ways, cutting either radial wedges or parallel slices through the

head. Taken together these multiple contour lines, modelled as isoparametric curves,

define the facial surface. The 2-D isoparametric curves - the contour lines - are then

skinned or lofted to form a spline facial surface (Birn c. 1999a; Maestri 1996, 120-125;

Ratner 1998, 205-229). An advantage of using this method is that only half the face

needs to be modelled. This half is then mirrored about its central axis to generate the

other half Another advantage is that the modelling process can begin with the modelling

of an isoparametric curve which describes the profile of the face, making this method a

less abstract way o f working, and facilitating the process of working from a real-life

template (Ratner 1998, 205, 223; Bim c. 1999a).

The resulting parametric head model has a 'north pole', and its isoparametric curves

mimic 'lines o f longitude' and 'lines o f latitude', in the same way as previously discussed

for the parametric sphere, method. The placement of the 'north pole' relative to the head

topology will have the same construction and animation implications as those described

for that method.

The positions of the poles will be determined by how the original vertical 2-D contour

lines were distributed across the facial surface. Vertical contour lines which mimic 'lines

o f longitude' cutting radial wedges around the head will result in a head model which has

the 'north pole' placed at the top of the head (Birn c. 1999a; Ratner 1998, 205-213).

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through the head from ear to ear will result in a head model which has the poles at the

ears (Ratner 1998, 223-225; Maestri 1996, 120).

It is also possible to employ this method using a series o f horizontal 2-D contour lines

cutting parallel horizontal slices through the head from the top to the bottom. The 'north

pole' is then placed at the top of the head (Ratner 1998, 225-229). However, apart from

being intuitively less straightforward, this method forfeits the advantages of being able to

commence the modelling process with the profile curve, model easily from a real-life

template, and model only one half of the face while mirroring the other half

In some modelling packages, such as Houdini, it is possible to construct a parametric

head model with a continuous skin by assembling separate components using hierarchical

B-spline patches, which were described in section 3.2.3. Using this method, difficult

elements such as noses, mouths and eyes are individually modelled in detail as separate

'patches', and then pasted onto a featureless head model, or assembled in other ways

(Maestri 1996, 126-135; Ratner 1998, 233-246; Birn 1999b). This method overcomes

the 'north pole' problem, as mobile features may be constructed according to their own

natural contours and lines of muscle action, without reference to the rest of the model.

As previously mentioned, the area o f 3-D facial modelling is the subject of research, and

sophisticated experimental methods are constantly being presented at forums such as

S I G G R A P H ( A C M Special Interest Group on Computer Graphics 1999). Research

continues into ways of overcoming the problems of using laser scanned models, including

work on the conversion of polygonal surfaces to NURBS surfaces, the use of active

adaptive polygonal meshes, and data thinning and surface smoothing algorithms. An

interesting method o f interactive modelling which is still fairiy experimental and not

widely used at this stage is intensity painting, whereby a grayscale 2-D image is

interpreted as a 3-D surface relief map using the intensity at each pixel of the 2-D image

as the z-depth o f the surface at that point (Parke and Waters 1996, 89). Another

direction dispenses with detailed modelling of geometry altogether, instead using a

simplified head shape onto which are projected 2-D raster images which create the

illusion of animated geometry (Kelly 1998, 533-534). However, these methods are

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3.3 Animating a realistic computer-generated 3-D

face

3.3.1 Requirements of the animation

Animating a 3-D computer-generated face is a challenging animation task. Because we are vei^ good at detecting unrealistic human facial movements and expressions, poor observation or shortcuts in the animation are immediately obvious (Maestri 1996, 259).

As discussed in section 3.2.1, the way a model will be animated is to a large extent determined by the way in which it has been constructed. The job of animation will be made either harder or easier by the modelling choices which previously have been made (Maestri 1996, 259-260,262). The capabilities of the software available to the animator will also play a crucial role in determining how the facial model will be animated (Maestri

1996, 260; Ratner 1998, 290).

To summarise, creating realistic facial expressions depends on an understanding of the anatomy of the bones and muscles of the face, and the way in which they operate, as described in section 3.2.2. The three expressive facial regions are the eyes, mouth and eyebrows (Maestri 1996, 135-136, 141, 274-276; Parke and Waters 1996, 113; Ratner

1998, 290), and the model should have increased detail and flexibility around these areas t o allow easy movement along the natural lines of action of the muscles involved. At a minimum the eyeballs must be able to rotate, the eyelids to close and open, the mouth to open, close, purse and stretch and the eyebrows to raise and lower. For more realistic animation, other movements are required, such as movement of the jaw up and down.

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The successful animation o f a 3-D head model can be considered akin to the job of directing the performance of an actor. The animator needs to develop an understanding of how meaning and emotion are communicated through facial expression and muscle

movement, as well as o f how a text or dialogue is interpreted using speech movements and facial expression to convey meaning and emotion. In order to reproduce believable

expression on a 3-D model, the animator needs to become an observer o f the human facial expressions involved in emotions and speech, and to develop a thorough knowledge o f acting and emotion (Maestri 1996, 259).

3.3.2 Facial expression: emotions and speech

The artistic focus of this thesis is, among other goals, on the expressive acting of the character. To generate the 'acting' of a virtual character, an understanding of how facial

expression conveys emotion and speech postures is important.

Research on facial expression has concluded that there are only six universal categories

of facial expression, which are recognised across all cultures. These categories are sadness, anger, joy, fear, disgust and surprise (Parke and Waters 1996, 113-116,

235-239; Maestri 1996, 274-275). Of course, each of these six categories encompasses a wide range o f variation in intensity and idiosyncrasy.

These universal expressions can be generated by using only a limited set of muscles behaving in a coordinated and synchronised way (Parke and Waters 1996, 235-239; Watt

and Watt 1992, 412). This mainly involves combinations o f the three expressive regions of the face: the eyes, mouth and eyebrows (Maestri 1996, 135-136, 141, 274-276; Parke and Waters 1996, 63-64, 113; Ratner 1998, 290). These are therefore the high priority

features when it comes to animation.

Reproducing these six universal expression categories and their variations in a believable

manner on a 3-D model requires the animator to develop an understanding o f one o f the systems which have been developed to describe and categorise the basic facial muscle

actions and their effect on facial expression, such as FACS discussed in section 2.2 (Parke and Waters 1996, 17, 235-236). FACS underpins most facial muscle action systems used in animation (Parke and Waters 1996, 17, 129, 131).

However, being able to technically reproduce these universal expressions on a model

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content which will make the difference between mere technical skill and a realistically involving character (Thomas and Johnston 1981, 473-478). In order to achieve this, the animator must develop an understanding of the craft of the actor, and of how the right combination and sequence of certain facial gestures and expressions, coupled with the right timing, can evoke specific personalities and emotions and bring the character to life. The animator should ensure that the emotional state and the thought process of the character are clearly defined. It is important to continually ask the questions 'What am I trying to say at this point?', 'What do I really want t o show?' and 'How do I want the audience t o react?' (Thomas and Johnston 1981, 507).

Facial animation also requires the reproduction of the mouth poses and movements associated with speech. The task of modelling mouth poses and animating speech movements t o synchronise with a speech track is known as lip synchronisation or lip-synch (Maestri 1996, 279-280). Lip-synch is similar to the art of lip reading. Both are based on observing and categorising the visually distinguishable phoneme classes associated with speech (Parke and Waters 1996, 261-265; Maestri 1996, 281). There are eighteen visually distinct speech postures which are used repeatedly during speech, involving the lips, teeth, tongue and jaw (Parke and Waters 1996, 262-264). For animation where a lesser degree of realism is required these eighteen are often simplified down t o seven or eight basic mouth positions (Maestri 1996, 282-283; Ratner 1998, 291-292).

The traditional approach to lip-synch is to deconstruct the pre-recorded speech track into its visually distinct phonemes, determine at which frames of the animation these

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this field (Brand 1999; Parke and Waters 1996, 260, 268-280). However, these high-end techniques are beyond the scope of this thesis.

3.3.3 Approaches to 3-D facial animation

There are a number of fijndamentally different approaches t o facial animation. Most widely used facial animation approaches depend on the process of interpolation, which in its simplest form is analogous to the key-framing approach of conventional animation (Parke and Waters 1996, 105). By one means or another, the desired facial poses are specified at particular keyframes, or points in time, and the computer then generates, or interpolates, the appropriate poses for the frames in between (Parke and Waters 1996, 107-108; Watt and Watt 1992, 412-413; Maestri 1996, 277-278).

One obvious method of achieving this is to directly manipulate small sets of vertices whose positions determine the geometry of the expressive features of the facial model For example, specific point vertices of the model may be defined as point groups and directly animated using local region interpolations such as rotation, scaling and position offsets to produce the desired range of movement in each expressive feature (Parke and Waters 1996, 106, 127-128, 187-198; Maestri 1996, 264; Ratner 1998, 293-294). In combination these local manipulations define recognisable expressions. This method has been considerably refined in some animation packages, such as Softimage and Houdini, where selected vertices can be grouped and named, and then animated by referring to the group name (Maestri 1996, 264). Its advantages are that it is direct and can deliver a wide range of expressions (Parke and Waters 1996, 188). Subtle animation effects are possible because it is easy to tweak the model in small ways (Maestri 1996, 264). However, this method is not ideal as it has little anatomical basis (Parke and Waters

1996, 128), and despite its directness it is inefficient Each time a particular expression is required it must be recreated, making the method both labour and resource intensive (Maestri 1996, 264).

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A common method of generating a library of facial poses and interpolating between them is known as shape animation, or morphing. The original, neutral-expression model is directly reworked in the modeller to create a number of different variant models, each with a different expression pose. These different expression models are then stored as a library of poses (Parke and Waters 1996, 105, 108-110; Maestri 1996, 260-263; Kelly

1998, 545-555). The character in the 1985 animation short Tonyde Peltrie was animated by interpolating between a library of twenty key facial expressions (Watt and Watt 1992, 412-413).

This method works for both spline and polygon models (Maestri 1996, 260), and has the advantage of being conceptually simple. However, the initial creation of the library of different expression models from scratch is a labour intensive process, and may be impossible if the model has not been designed to be easily reworked. Using some animation packages, the library is memory resource intensive, as the storage of each key facial expression may require the complete specification of the model geometry (Parke and Waters 1996, 105, 187), even those parts of it which are the same as adjacent key expressions - of^en the major part of the data. This introduces a lot of redundancy into the approach (Watt and Watt 1992, 413). Furthermore, the range and subtlety of the resulting animation will be limited by the number of expression models which were originally created for the library; a prohibitively large number of poses is required for realistic facial animation (Parke and Waters 1996, 110; Maestri 1996, 260). Finally, strict morphing between the poses can look mechanical (Watt and Watt 1992, 413; Maestri 1996, 260). Limitations of subtlety and mechanical blending are not a problem with some animation packages, such as Houdini, Softimage and Maya, which are able to combine weighted blends of a number of expression models to form new poses (Maestri 1996, 261-263). The method also becomes more flexible if it is overlaid with a direct manipulation technique, such as the point manipulation method described previously (Parke and Waters 1996, 188; Maestri 1996, 264).

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128-130). Although the actual muscle actions are realistically reproduced, no attempt is made to exactly simulate detailed facial muscle anatomy and tissue dynamics (Parke and Waters 1996, 106, 128).

One way of implementing a pseudomuscle based method is animating the expressive facial mask using the hierarchical skeletal deformation tools, also called 'bones', which are available in some animation packages, such as Houdini and Softimage (Maestri 1996, 264-270; Penner and Vinovich 1996, 1:200-211; Ratner 1998, 295-296; Kelly 1998, 534-544). Controlled movements of these 'bone' tools simulate the movement of facial

muscles, flexing and moving the skin of the model in essentially the same way that real-life bones and muscles move the skin of a real person. Using knowledge of anatomy to determine their positions, the 'bones' are placed at the main muscle control points of the face, and are anchored to the skin. Based on knowledge of the facial muscle actions underlying facial expression, as categorised by FACS, the 'bones' are moved tangentially to the surface of the face, with rotational movement, mimicking the lines of action of the major muscles. The model's facial surface stretches and flexes to follow their movements (Maestri 1996, 264-270; Ratner 1998, 295-296).

Using this technique, it is not the model geometry itself which is animated, but the positions of the ends of the 'bones' which are anchored to the skin. Thus just these positions need be stored to specify the particular facial poses required in the library of key facial expressions (Maestri 1996, 264, 273).

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stretches the cheek 'bones' move to produce a subtle stretching and flexing of the expressive facial mask.

The method does have some disadvantages (Maestri 1996, 264, 266-270; Kelly 1998, 534), the main one being that it is a complex, fiddly and time-consuming procedure to construct the facial 'bone' structure, adjust the 'bone' pivots and axes of rotation, and specify the 'bone' hierarchies. The initial construction process requires great attention to detail because if the 'bones' are not set up correctly, their movement will cause unnatural crimping or bulging of the facial surface. Furthermore, interpolation between keyframes may produce unexpected and unrealistic results, and therefore the 'bone' set-up must be extensively tested over a wide range of expressions before animation time (Maestri 1996, 264, 266).

Research is continuing into ever more realistic anatomically based methods of facial animation, simulating the characteristics and dynamics of facial muscles and skin and their effects on facial expression. One such method is a muscle based approach. In 1988 Pixar's animation short Tin Toy used a parametric muscle model involving around fif^y muscle models controlling the movement of approximately 3000 facial control points, and being in turn controlled by a higher level of macromuscles (Watt and Watt 1992, 411-412). Research in this area has continued since then. Realistic muscle based animation uses mathematical mass-and-spring models to simulate detailed facial muscle anatomy and muscle and skin tissue dynamics. Based on complex - although of necessity simplified - models of facial bone, muscle, connective tissue and skin structures, these facial animation systems can be highly complicated (Parke and Waters 1996, 106, 131, 228-235; Yuencheng, Terzopoulos and Waters 1995, 57-59). Muscles with elastic properties, interconnected by virtual springs and patterned after FACS, have directional properties to mathematically reproduce the characteristic movements of linear muscles which pull and sphincter muscles which squeeze (Parke and Waters 1996, 228-235; Yuencheng, Terzopoulos and Waters 1995, 58-59), and three layer deformable lattice structures realistically simulate skin (Parke and Waters 1996, 169-178, 242-257; Yuencheng, Terzopoulos and Waters 1995, 57-58).

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translating this data to drive tiie animation of a 3-D facial model (Parke and Waters 1996, 106, 111-113, 287-307). Virtual actors - computer generated characters - can have their facial expressions controlled in real time by the facially expressive acting of

real-life actors. The real-life data is collected by using interactive input devices such as data probes attached directly to the main facial muscles of the real-life actor (for

real-time), or by using laser or video based motion tracking systems which track markers placed on the actor's face or analyse feature lines and boundaries (Ratner 1998, 299, 302; Parke and Waters 1996, 106, 111-113, 287-307). Voice puppets whose facial gestures

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4 The practical work: modelling and animating a

realistic computer-generated 3-D face

4.1 Software and hardware

The software which I had available for this project was the 3-D modelling, animation and compositing package Houdini, which has been developed by Side Effects Software (Side Effects Software Inc. 1999). This is a film and SFX industry-standard, high-end animation package which is especially renowned for its procedural approach, its strength in the area of particle system animation, and its scripting and expression languages. It is also suitable for realistic character animation, although it is probably not as intuitive in this area as some other packages. The strengths and weaknesses of this package influenced a number of my decisions when deciding how to approach the modelling and animation of a 3-D computer-generated human face.

Modelling and animating a realistic human face places big demands on the computer equipment used. The realistic model of a human face is a complex set of geometries, and its manipulation and animation require fast high-end computers with a lot of available RAM. I would have been unable t o undertake this project if I had not had access to the high-end Silicon Graphics workstations at the Vizlab, A.N.U. Supercomputer Facility (ANU Supercomputer Facility n.d): the Onyx2 Infinite Reality Engine, Onyx Reality Engine, 0 2 RIOK and Indigo2 High Impact. Even these high-end workstations sometimes imposed speed or memory limitations, and these limitations also influenced some of my modelling and animation decisions.

4.2 Modelling the face and head

4.2.1 A Real world template

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face of the person from a number of angles, particularly fiill face, three-quarter view, and profile.

The person whom I chose as my model is Elizabeth Siddal (1829-1862), who was a model for many of the Pre-Raphaelite painters of last century, as well as being a painter herself (Gaunt 1975; Hawksley 1999, 178, 180; Surtees 1991, 7-11). The many available paintings of her allowed me to study her facial structure from a number of angles (Hawksley 1999, 46-47, 57, 97, 119; W o o d 1994, 25, 31, 33, 97; Surtees 1991). With her tragic life with its sacrificial overtones, and her background as a Pre-Raphaelite model, Lizzie personified the right emotional atmosphere required for the symbolic Celtic myth which was the inspiration for my animation, as discussed in section 5.

4.2.2 Surface type

On the basis of the investigation described in section 3 , 1 decided to construct the 3-D head model using a parametric N U R B S surface. In Houdini this is a standard surface type, integrated into all the software functions, and efficient to work with (Penner and Vinovich 1996, 2:374-368-374). It has the advantage over polygonal surfaces that the smooth and natural curved surfaces necessary for the expressive facial mask can be modelled using relatively few control vertices, thus minimising potential computer speed and memory limitations, as discussed in section 3.2.3. This low vertex density generally allows easier modelling control, and the N U R B S isoparametric curves can be optimally positioned to facilitate movement of the model along the natural lines of action of the muscles. Given Houdini's strength in the area of parametric surfaces, I did not consider a polygonal surface t o be a real option for this project. Furthermore, the point count which would have been required to obtain a realistically smooth polygon surface would have made the model prohibitively resource intensive.

4.2.3 Modelling approach

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pictures o f realistic models which had been constructed using a similar technique (Birn

1999a).

I decided on this approach for a number of reasons. Basing the modelling process on a

series of vertical 2-D isoparametric curves made it easy for me to match the contours of

the 3-D virtual model to the key facial contours of the 2-D pictures of my real world

model. Another advantage was that this method allowed me to model just half o f the

face, mirroring this to complete the other half Furthermore, this method was for me the

most intuitive way to approach the modelling process. When one attempts to model a

realistic head by iteratively deforming and refining a sphere or cylinder, the entire model

is present in the modelling window from the start, and the amount of geometric detail

(points and isoparametric curves) quickly becomes visually overwhelming and confusing.

By using my chosen method, I was able to focus on modelling one part o f the overall

structure at a time, with the advantage that the modelling window remained relatively

uncluttered until much further on in the modelling process. Finally, it seemed to me more

intuitive to construct the expressive facial mask as one whole piece of geometry, and not

to use a series of hierarchical B-spline patches (see sections 3 .2.3 and 3 .2.4.2), given my

need to base my model on a series of 2-D key contours of the entire face. Although

patches are available and work efficiently in Houdini, puckering at the patch edges has

been reported as a potential problem at animation time.

Placement of the 'north pole' of the isoparametric curves at the top of the head has

certain disadvantages in terms of mouth flexibility in the later animation process, as noted

in section 3.2.4.2. However, I chose this construction topography despite the potential

for later problems because the radial sections of vertical 2-D contours made it intuitively

much easier to reproduce the key 2-D facial contours of my real world model. Given my

initial lack of experience in building 3-D facial models, this immediate advantage

outweighed potential later problems.

Although I had access to laser scanned facial models on the internet (headus

(metamorphosis) Pty Ltd c. 1999; Cyberware 1999), I did not consider abandoning the

interactive modelling process and using one o f these. As discussed in section 3 .2.4 .1,

there are many problems associated with using the polygonal meshes produced by laser

scans, and downloading and inspecting these models proved to me that the amount o f

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interactively modelling a head from scratch. It is also possible to purchase or download ready-made realistic head models on the internet (Birn 1999c; Platinum Pictures Multimedia Inc. c. 1999; Discoe n.d.). I did not consider this t o be an option, both for cost reasons and because I would learn nothing about the modelling process using this approach.

4.2.4 Modelling procedure, problems and solutions

I began the modelling process for the expressive facial mask by scanning several views of my real world model into the computer. Using a profile view of Elizabeth Siddal as a background image in the Houdini modelling window, I constructed a 2-D profile isoparametric curve which copied the profile line. The basic process then was t o copy this 2-D curve and rotate it around the central vertical axis of the head, at each rotation sculpting the curve to represent the facial contour appropriate for that angle, and matching the line to my real world model at the three quarter and front views.

Using this approach, I encountered and overcame a number of problems. As predicted in section 3.2.3, using a N U R B S surface type it was a difficult and time-consuming task to achieve detailed surface definition around the eyes, nostrils and mouth. Since Houdini was unable to regularly skin the 2-D contour curves if they did not have the same number of points, it was not possible just to increase the number of control vertices in only those areas requiring more detailed definition. Therefore, all the 2-D contour curves were eventually constructed with the same number of control vertices, resulting in horizontal strips of denser isoparametric curves passing across the eyes, nostrils and mouth. These became congested in featureless flat areas such as the cheeks, and resulted in stubborn surface irregularities which were only overcome with much patient point manipulation. After skinning, many extraneous control vertices were eventually consolidated together, thus reducing the overall number of points in the model.