  Optic flow is two dimensional, but no special qualities are attached to one
or other of these dimensions. For binocular disparity, on the other hand, the
terms 'horizontal' and 'vertical' disparities are commonly used. This is odd,
since binocular disparity and optic flow describe essentially the same thing.
The difference is that, generally, people tend to fixate relatively close to
the direction of heading as they move, meaning that fixation is close to the
optic flow epipole, whereas, for binocular vision, fixation is close to the
head-centric midline, i.e. approximately 90 degrees from the binocular epipole.
For fixating animals, some separations of flow may lead to simple algorithms
for the judgement of surface structure and the control of action. We consider
the following canonical flow patterns that sum to produce overall flow: (i)
'towards' flow, the component of translational flow produced by approaching (or
retreating from) the fixated object, which produces pure radial flow on the
retina; (ii) 'sideways' flow, the remaining component of translational flow,
which is produced by translation of the optic centre orthogonal to the
cyclopean line of sight and (iii) 'vergence' flow, rotational flow produced by
a counter-rotation of the eye in order to maintain fixation. A general flow
pattern could also include (iv) 'cyclovergence' flow, produced by rotation of
one eye relative to the other about the line of sight. We consider some
practical advantages of dividing up flow in this way when an observer fixates
as they move. As in some previous treatments, we suggest that there are certain
tasks for which it is sensible to consider 'towards' flow as one component and
'sideways' + 'vergence' flow as another.
