Anatomy of the human face

For any person who is required to animate a character, be it human, animal or otherwise, a thorough understanding of its underlying anatomical structure is essential. In order for an animation to look realistic and feel believable, it is vital that the animation is based on the real life muscular action.

‘I emphasize such basics because the basics must always serve as your foundation before you can deform your shape. Animation is not the re-creation of living things; it is the exaggeration of living things to create more vivid forms.’ Choi, 2003

For facial animation, even basic understandings of how the human skull and facial muscles work (may differ in animals, this section concentrates on human anatomy) will greatly improve the final product. The following is a short discussion of these two aspects.

The skull

The skull plays an important role in the makeup of the face, as the bone structure essentially determines what the exterior shape will look like. The skull also forms the foundation upon what the muscles and skin lie. The skull, or cranium, protects the brain, and it is constructed from eight major plate-like bones. The skull is also made up of fourteen facial bones. These bones provide the major framework for the front of the skull and face. The most important facial bones are the mandible (jaw), the maxilla (upper jaw), the zygomatic (cheek) and the nasal (nose).

The actual modelling of the skull is not always vital when creating a character, as it plays no major role when rigging or animating (though it can help with positioning of bones, etc, during rigging). However, knowledge of the skull is useful when modelling the actual head of a character, as it can help to define its shape.

Front and side views of the human skull.

Black = facial bones, Red = cranial bones, Blue = features of the bones

Images courtesy of http://face-and-emotion.com/dataface/physiognomy/cranium.jsp

The facial muscles

The facial muscles are also known as the muscles of facial expression. Each muscles energy is supplied by one or more nerves, which when stimulated allows the muscle to contract. Facial expression itself is created by sets, or individual, muscles that contract in relation to emotions felt, or other activities such as chewing and talking. The contracting muscles essentially pull the skin of the face into place, creating the relevant expressions. The muscles are groups, or layers, of muscle fibres that are stretched over the skull and facial bones. The facial muscles are grouped into three types of muscles:

· Linear or parallel muscles pull in an angular direction, such as the zygomatic major.

· Sphincter or circular muscles squeeze, such as orbicularis oculi.

· Sheet muscles, which are like a series of linear muscles working together, such as frontalis.

Altogether there are over 25 muscles that are responsible in some way for creating facial expression. The most major muscles that play a role in expression are outlined below.

The frontalis muscle draws the eyebrows and skin of the forehead upwards. It forms wrinkles that run horizontally across the forehead. The frontalis is made up of two muscle parts, the inner frontalis and the outer frontalis.

The levator labii superioris alaeque is located between the cheek and nose. It comes into action when a person is laughing or frowning.

The orbicularis oculi muscle is a sphincter muscle located around the eye. It has three parts – an outer part, inner part and orbital part. This muscle narrows the eye opening and closes the eye orbit. The blink reflex is the action of the inner part.

The nasalis muscle is found in the nose and upper jaw. The compressor part constricts the nostrils, and the dilator part flares the nostrils.

The epicranius muscle is also found in the forehead. It also raises the eyebrows when a person is surprised. The wrinkles produced cross at right angles to the muscle.

The procerus muscle is found running from the nose upwards to the forehead. It comes into action when a person is angry, creating the deep furrows in the forehead.

The Buccinator, or ‘trumpet muscle’, is located at the corners of the mouth. It compresses the cheeks to the teeth, and helps with keeping food in the mouth during chewing.

The zygomatic major and minor muscles are found in the cheeks. They are also known as the smiling muscles, as they are responsible for pulling the mouth into a smile. Large creases (laughter lines) occur at the sides of the mouth when these muscles are put into action.

The depressor labii inferioris muscle is found in the chin, and pulls the lower lip down to bare the teeth. It is used to make negative expressions.

The orbicularis oris muscle is the sphincter muscle surrounding the mouth, determining its shape. It is responsible for creating lip movement during speech, as well as tightening, thinning, pressing and puckering the lips. This is one of the most important facial expression muscles, and all movement of the face affects it, i.e. for every facial expression created, the orbicularis oris will play a part.

The masseter muscle is found in the cheek, and helps to raise the jaw and clench the teeth.

A frontal view of the muscles of facial expression

Image courtesy of http://face-and-emotion.com/dataface/expression/muscles.jsp

After studying the anatomical structure of the face, and observing how the muscles move and skin wrinkles, producing the required expression in an animation should be straightforward and look extremely convincing.

Characteristics of the face and important factors to consider

The face itself is made up of many different parts. The majority of research on facial animation to date has consisted of only manipulating the facial mask, but to get a more realistic representation, the entire head needs to be considered as a whole. The facial mask is the area of skin that surrounds the entire external surface of the head. However, this animated alone is not at all convincing. To add realism, other details need to be added such as the eyes, mouth, eyebrows, hair, tongue, teeth and ears.

The eyes play a major role in facial animation, as the eyes can portray a whole level of emotion by themselves. Even using simple polygon spheres to represent them will suffice if necessary. The eyes need to be able to move freely in their sockets, and for extra realism the pupils should dilate and constrict. Any facial animation will look completely lifeless without the eyes.

The lips are not always a necessary detail to be able to recognise a face or help it to express emotion, as this can be achieved purely through the eyes alone. However, the lips are vital if speech is to be used on a character that is based on a real life subject, and they also make instantly recognisable expressions, such as happiness and sadness. The lips must be flexible enough to allow for their wide range of motion.

The teeth play an important role in facial animation where a character requires life-like motion. The teeth affect how the orbicularis oris muscles move, and these muscles are the most important for facial movement. Modelling teeth also increases the realism aspect of the model, as the teeth are sometimes partly visible during speech and other motions involving the mouth, such as chewing. In addition, the teeth help us to form the speech phonemes, and so speech patterns and sounds can change dependent on the teeth we have in our mouth. Missing teeth, or having no teeth at all, restricts the use of certain phonemes and can make speech sound incorrect (like in having a lisp).

Other features of the face that can help with its animation are the tongue, ears, hair, eyebrows and eyelashes. These details are not essential to achieving good facial animation, but they help with our perception of recognising one face out of many, as every person’s features are unique. They also help to give our characters an instant personality and likeness. The model that I will be using to demonstrate my animation system on will be of a complete head, containing all of the above facial details. I want it to look and react as realistically as possible, even if these features are exaggerated. I will need to create teeth for the model as well.

When designing my facial animation system, as well as the facial details of my model, I need to consider other aspects of human face development. These aspects include the difference in facial structure between Men and Women, the difference in facial structure between children and adults, and does the structure of the face change, as we grow older? These questions need to be thought about, as these physiological differences will affect how my facial rig works, if it is to be for a man or woman, a young person or old person.

Physiological differences in facial structure between men and women

	Men	Women
Head	Squared in shape, parts more prominent, higher forehead, lower third of face – appears longer	Less squared, lower forehead
Jaw	Angle more pronounced, more prominent cheekbones, greater distance across cheeks	Angle less pronounced, less prominent cheekbones
Eyes	Ear is farther back	Distance from eye corner to ear is the same as corner of eye to corner of mouth
Mouth	Larger, indentation under lower lip more noticeable	Generally smaller
Nose	Nostrils more dilated, wider	Narrower and more pinched
Brows	Can vary	Can vary

Physiological differences in facial structure between children and adults

The face goes through its most dramatic changes anatomically from birth through to maturity, at about fifteen years of age. The most rapid changes occur in early childhood. The most noticeable change is in the ratio of the face to the head. At birth the face takes up a relatively small area of the head, a ratio of about 1 to 3.5. At maturity, this ratio is greatly reduced to about 1 to 2. The eyes, nose and mouth expand to gradually fill a greater area of the head, and the forehead shrinks as the eyes move upwards. The eyes also become smaller, and the chin becomes larger, protruding forward more noticeably. Children also have softer, shorter noses, which are very narrow across the wings, and smaller mouths. They also have much more rounded cheeks.

How does the face change as we become older?

The face goes through its major changes from birth to puberty, but a few of these changes continue as we gradually become older, although they will be less noticeable. The majority of changes that occur as we age do not affect our underlying bone structure, but our skin and facial muscles. The skin becomes darker, rougher and less flexible as it loses its elasticity, and we begin developing wrinkles and blemishes. The bags under our eyes become more prominent and permanent, and the skin under our chin may begin to sag. Under the skin, the facial muscles begin losing their elasticity as well, causing gradual loss of control of the facial muscles, and fatty deposits may break up, leaving the hollowed cheek look.

All of these changes that occur to the face need to be taken into account when I am designing my facial animation system. This will be fully discussed in the next section of this report.

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