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Figure 10 Optic flow (A) The flow field for an observer both looking and moving straight ahead toward a flat wall painted with dots. The lines plotted in the flow field represent the velocities of the moving dots on the retina. As the observer moves closer to the wall, the image expands and each dot moves directly away from the center of the screen. The point from which all the flow field vectors emanate is the focus of expansion (FOE). For an observer moving in a straight line in any direction, the flow field contains an FOE that is always located in the direction the observer is headed. Because the observer is moving toward the center of the wall, the FOE is in the center of the flow field. (B) The flow field produced when an observer is translating in a direction slightly to the left of straight ahead. The focus of expansion has now shifted to the left so that it remains aligned with the translation direction. (C) The flow field produced by an observer looking to the left by rotating her eyes (or head) about a vertical axis. Note that moving the eyes to the left causes all points to move predominantly to the right and that there is no FOE for rotational motion. (D) The flow field resulting from the combination of moving straight ahead, as in A, and rotating the eyes to the left, as shown in C. The resultant flow field is the sum of these two flow fields and looks very similar to the flow field in B. Although it contains a location that has no motion, this is not a true FOE and does not correspond to the direction of translation.

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