Eye Movements 3 - Saccades and Fixations

Great Article about the different ways of animating eyes from Synchrolux.


"Convergence
is a motion of both eyes relative to each other that ensures that an object is still foveated by both eyes when its distance from the observer is changed. The closer the object is, the more the eyes point towards each other. This movement can be voluntarily controlled, but is normally the result of a moving stimulus.
Saccades
are the principal method for moving the eyes to a different part of the visual scene, and are sudden, rapid movements of the eyes. Saccades can be initiated voluntarily, but are ballistic: that is, once they are initiated, their path of motion and destination cannot be changed. Visual input is suppressed during a saccade.
Pursuit motion
is a much smoother, slower movement than a saccade; it acts to keep a moving object foveated. It cannot be induced voluntarily, but requires a moving object in the visual field. One frequent failing of thoughtless animation is having the eyes demonstrate pursuit motion when there is no object being followed by the character’s eyes.
Nystagmus
is a saw-toothed pattern of eye movements that occurs as a response to the turning of the head (acceleration detected by the inner ear) or the viewing of a moving, repetitive pattern (the train window phenomenon). It consists of smooth `pursuit’ motion in one direction to follow a position in the scene, followed by a fast motion in the opposite direction to select a new position. This is an eye movement that has probably never been animated, and if it was, it would probably get rejected by the supervisor or director because it would look so odd.
Drift and microsaccades
occur during fixations, and consist of slow drifts followed by very small saccades (microsaccades) that apparently have a drift-correcting function. These movements are involuntary, and their function is in question.
Physiological nystagmus
is a high-frequency oscillation of the eye (tremor) that serves to continuously shift the image on the retina, thus calling fresh retinal receptors into operation. Physiological nystagmus actually occurs during a fixation period, is involuntary, and generally moves the eye less than 1°. As with microsaccades, you’d need to be in an extreme close-up for this to register, but it’s another reason the human eye looks ‘alive’ in live-action extreme close-ups.
Rolling
of the eyes is a rotational motion around an axis passing through the fovea and pupil. It is involuntary, and is influenced by among other things the angle of the neck. Although this is also something we can safely ignore as animators, some rigs will automatically provide this when using the ‘eye-target’ controller.
Of the above, it’s really the first three we’re concerned with. Because of the precision that CG animation allows, we can (and should) pay much more attention to these types of movements than is typically done in hand-drawn animation.
Convergence is fairly straightforward. In a properly rigged character, convergence should happen naturally as the eye target is moved close to the character’s face. If a rig lacks this function, or the eye-target control isn’t being used, it should be a simple matter to manually add in some convergence when a character is looking at something very close. You don’t need to go too far with this to portray convergence.
Pursuit is also straightforward. I like to use the eye target to match the motion of the object being tracked, and I like to make sure the head motion (if the head is rotating in the direction of the object being tracked) is somewhat out of phase with the eye movement, otherwise the sense of the eyes actually moving in pursuit motion is lost.
There really aren’t any timing considerations to convergence and pursuit, but there are for saccades. I’ve heard various rules of thumb regarding how many frames* a saccade should take, and how long the eyes should fixate between saccades. Rather than regurgitate those, I’ll lay out some data from physiology studies. I encourage readers to do their own research — look closely at reference like I’ve posted above, and see what the eyes really do."