Eyes are the doorway to the soul, and a well rendered eye can be the piece de resistance that deceives the viewer into believing.
Yet frequently eyes that you see in 3D work are modelled as simple spheres with a black circle for the pupil and a coloured circle for the iris.
Well that kind of representation fails the realism test for a number of reasons.
Importantly, eyeballs are made of a few major structures relevent to 3D work. Each one of those structures reflects light and casts shadows.
This play of light and dark is what makes an eyeball convincing.
But first, for the curious here is the animation this tutorial made.
Let's start at the basics.
In this diagram you can see the parts of the eye that 3D modellers should be familiar with.
The cornea is a transparent bubble which covers the iris (the coloured part of the eye).
The iris is a meshwork like a fisherman's net that can widen and narrow the pupil.
The pupil is a hole in the iris. It appears black because inside the eye is shadowed.
The lens of the eye (not labelled) is directly behind the pupil.
The conjunctiva is the white part of the eye. The cornea and the conjunctiva are attached.
- The limbal area is the joining area between the cornea and the conjunctiva. It is important because while the cornea is transparent and the conjunctiva is opaque, the limbus is translucent.
Here we have it all in cross section
In cross section things start to look a little different, don't they?
The first thing we notice is how far each of the anatomical parts are from each other.
Gone is our flattish cartoon eyeball.
Instead we have three dimensional parts of the eye that, when, modelled correctly, will cast a shadow on another part.
Most obviously the conjunctiva is curved, but the iris is also curved.
Look at the above diagram and imagine light source directly above the eye.
The top of the conjunctiva will cast a shadow on the top part of the iris.
The bottom part of the iris, because of its curved shape, will also be in shadow.
In humans frequently these shadows will add to the impression of an attractive darker "ring" around the iris as in Figure 3.
The second thing that we notice is that the parts of the eye have thickness.
This means that, in the transparent parts of the eye (the lens and the cornea), each surface will be a reflecting surface. In other words, the traditional highlight that artists put on the eye is really a series of highlights.
The most dominant part of the reflection comes from the front surface of the cornea. But we also have reflections from
- the back of the cornea
- both surfaces of the lens of the eye (ie front and back)
- from the back of the eye (think of red eye in camera flashes and the reflection of a cat's eyes at night)
This is an effect first noted by a chap called Purkinjie, In FIg 4 we can see candlelight reflection from the front surface of the cornea and from the front surface of the lens. So next time you are in the middle of a candlelit dinner take advantage of the lighting and gaze into your companion's eyes.
And lastly, we also have reflections at
- the tear layer which congregates just above the bottom lid (see Figure 5)
- the tears on the conjunctival surface
- the triangular region in the inner eye (the canthus)
- and for the nitpickers among us, we have internal reflections at the corneal surface, so that a light shone on the right hand side of the eye will be scattered through the cornea and light up the left hand side of the eye in a small triangular area which should be in darkness. (see Figure 6)
You also might find this page handy : Eye Dimensions
A few references for the academically inclined: