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.A coordinate framework for optic flow and disparity Glennerster and Read Author Summary "Optic flow" refers to changes in the visual images we receive as we move through a scene.For example, as we drive along a street, the buildings flow past us and distant objects expand in our field of view. This information can tell us both about how we are moving and about the 3D structure of the scene. Conversely, "binocular disparity" refers to the differences between the views seen by our two eyes due to their slightly different positions in their head.Binocular disparity enables us to perceive the distance to objects, even while stationary. Both flow and disparity are rich sources of information, both for humans and other animals, and for robot systems such as autonomous drones and cars. Generally, they have been studied separately, and a separate set of vocabulary has been developed for each. Yet mathematically the two are fundamentally related: optic flow compares views seen by a single, moving eye at two different points in time, while binocular disparity compares views seen simultaneously by two eyes at different positions. Here, we develop a common language for describing both.It is particularly appropriate for eyes that fixate as they move, a universal feature of biological visual systems.A coordinate framewor...