Tracking and Separation (Visual Depth Perception 11)

Fixation on causes the eyes to rotate because of the observer's continuous translation. We measure the tracking angle counterclockwise (+) from the axis (head aim direction) and call the time rate of change needed to maintain fixation the "pursuit". This is , the time derivative in the sense of calculus.

In terms of the eye parameters = node percent, = interocular distance, and = eye radius, the derivative is

.

This peaks at (when the denominator is largest):

.

The observer's translation also causes the angle separating the fixate and distraction to change, causing motion of the image of D on the retina. The time rate of change in angle from the distractor to the fixate is a "dynamic parallax" describing the moving retinal image of . This derivative is

This simplifies greatly at the time , when the eye crosses the axis:

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When the distraction is in line with the eye, , and has the simple form

and the ratio of retinal motion over pursuit is

.

The basic case of the motion/pursuit law for relative depth from motion parallax is

.