Illustration of the aperture problem of 3D motion with projections of an oriented line or contour moving in depth.

<p>The left and right eye with nodal points <b>a</b> and <b>c</b>, separated by interocular distance <i>i</i>, are verged on a fixation point <i>F</i> at viewing distance <i>D</i>. If an oriented stimulus (diagonal line) moves from the fixation point to a new position in depth along a known trajectory (black arrow) then perspective projection of the line stimulus onto local areas on the retinae or a fronto-parallel screen creates 2D aperture problems for the left and right eye (green and brown arrows).