Probability distributions of the directions of image sequences generated by 3-D sources in the absence of occlusion. A) Pinhole camera model for assessing the probability distributions of the projected directions of randomly oriented lines moving in all possible directions in a simulated 3-D space. B) Enlargement of the circular image plane in (A). Projected lines with any particular orientation (45º in this example) generated by 3-D lines moving in any direction on the image plane can be sampled to determine the frequency of their occurrence. C) The probability distribution the projected sequences of unoccluded lines oriented at 45º. Angles around the perimeter of this polar coordinate graph represent the projected directions of movement, with 0º indicating the direction perpendicular to the orientation of the line. The distance from the center to any point in the coordinate system represents the probability of image sequences moving in that direction. As might be expected, the frequency of occurrence of projected directions for unoccluded lines is approximately uniform. (After Sung et al., 2008)