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Skin and Eyes
The skin, formally referred to as the cutaneous system, is the body’s largest organ, measuring around two square metres in an average sized adult, and weighing around 5kg. It is the thinnest around the eyelids (around 0.2mm – 0.5mm, and sometimes thinner) and you’ll find the thickest region on the heels (around 4.0mm).
The skin provides a protective barrier between the body and what is very frequently a hostile environment. The skin is composed of 3 main layers: the epidermis above and the dermis below, and beneath these is the subcutaneous layer, which is rich in fatty tissue and carries the major blood vessels and nerves to the skin above.
Doing justice in CG to this complex organ, which appears so varied from person to person, and from one part of the body to the next, is a labour of love. Take a look at your face in the mirror: see how colour varies from feature to feature, how some parts are oilier and have deeper pores than others, how lines form around the eyes and corners of the mouth. If you are working on a creative project, then your approach to producing digital skin can be more artistic than technical: you have a look in mind, which can be backed up by reference, and you can strive to get that look by repeated tinkering with texture and displacement maps, whilst being very conscious of and informed by the biological make-up of the skin.
However, if you need to match your reference one-to-one then, you’ll need to work with less creative freedom. You’ll need to work more technically and be more grounded to your photographic reference.
The epidermis in most areas of the body is less than 0.2mm thick. This figure does rise, however, in those areas such as the soles of the feet, which are exposed to a lot of pressure and friction. The epidermis is composed of between 3 and 5 layers, depending on the area of the body. The cells at the surface (stratum corneum) of the skin are dead and are continuously being shed then replaced by cells which have migrated from lower layers in the epidermis. Each month, our outer skin is replaced, and this is a process which continues even while we sleep.
Take a step back to think about that. The very layer of which we find beautiful is in fact dead! Strange but true.
Skin flakes are called squames, which means scales, and we shed around 25,000 flakes or so a minute.
Before we reach the dermis, there is the dermal-epidermal junction, a membrane which effectively glues the dermis and the epidermis together.
Moving deeper, the dermis is sometimes known as the “true skin” and is much thicker than the epidermis, varying from 0.2mm (eyelids and penis) to 4.0mm (soles and palms) thick. The dermis contains nerves and nerve endings, muscle fibres, hair follicles, sweat and sebaceous glands, and blood vessels. It performs a number of functions such as heat regulation, protection from mechanical injury, and the processing of sensory information such as pain or heat.
Now, if you take one thing away from reading this article, it would be this: if a sliver of skin, say a millimetre thick around the arm, is peeled back, that is where you would find all of your skin colour. That sliver of epidermis is everything about you that defines what you may refer to as “colour”. That’s it.
All the fights, battles and wars we’ve had over the centuries, all because of such a small sliver of what, some people feel, defines you. And what defines skin colour? Sunlight.
Skin colour is a reaction to sunlight. Yes, it is more complicated than that on a scientific level; we are talking over a hundred and twenty genes level of complication when it comes to skin pigmentation in mammals, but don’t let that one millimetre of flesh build a barrier between you and your fellow man. Enjoy skin for what it is there for: to protect you, to keep things in and out, to keep you warm, to give you a sense of touch, of pleasure, of togetherness.
Making up our skin is a molecule called melanin, formally known as eumelanin. This molecule is found throughout the living world and is one of the oldest in biology. Melanin colours skin, hair, the feather of a bird, the scales of fish, and even the colour of fruit.
So, take that into consideration: you and your fellow ape, elephant, eagle, dolphin, whale, and yes, even that banana are all connected!
Remember then, you have more in common with your fellow human being, creature and plant than you may think.
But back to skin shaders, and of additional importance when considering shaders, is the oily film which is visible on the surface of the skin to a greater or lesser degree from person to person and depending on the area of the body. This film is composed of secretions from the sweat and sebaceous glands mixed with the cells being shed from the epidermis. Because the surface film reflects light, and because blue light barely penetrates, you’ll generally see a hint of blue reflected.
The adult eyeball measures around 2.5 cm in diameter. Only one sixth of the eyeball is exposed; the remainder is recessed and protected in its orbit, and based on the digital texturing and shading work we will do, it is this sixth that we will focus on. We will look, therefore, at the sclera, iris, pupil and cornea.
The sclera is a tough white fibrous tissue that almost encircles the eyeball, giving it shape and protecting its inner parts. The visible, anterior part of the sclera is the “white of the eye”.
The iris, or the coloured portion of the eye, is made up of circular and radial muscles which are arranged in a doughnut shape, the hole in the middle being the pupil, through which light enters the eye. A principal function of the iris is to regulate how much light enters the posterior cavity of the eyeball through the pupil. When light stimulates the eye, the circular muscles of the iris contract, making the pupil smaller. When the eye needs to see in poor light, however, the radial muscles of the iris contract, thus increasing the size of the pupil.
Covering the iris and the pupil is the cornea, which is a transparent coat protected externally by an epithelial layer.
- The Body – Illustrated: A Guide for Occupants by Bill Bryson – If you’ve ever wondered what really goes on inside your head, heart and beyond, Bill Bryson will help you understand the miracle of our physical and neurological make up.
- Skin – The Histology Guide – Useful information about the skin.
- National Library of Medicine: Skin – More useful information about the skin.
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