Introduction Errors in Image Intensity Errors in Line Estimation Errors in Movement Erroneous Shape Estimation Errors in Movement Ouchi Illusion Figure 1 The Ouchi pattern consists of two rectangular checkerboard patterns oriented in orthogonal directions -- a background orientation surrounding an inner ring. If fixating on the pattern, slight movements of the eyes cause a segmentation of the inset pattern, and motion of the inset relative to the surroundings. The illusion is even stronger if one slightly moves the pattern as shown below. You can experiment with the pattern by dragging it with the mouse in different directions. The illusion is experienced for almost all movements, but it is strongest for diagonal motion. Remember, the vision system in the first step estimates the component of the motion vectors perpendicular to edges, the so-called normal flow. The tiles used to make up the pattern are longer than they are wide. Thus, in any small region there are many more normal flow measurements in one direction than the other. In a second stage the vision system estimates optical flow using as input the normal flow measurements in a region. Since the tiles in the inset and its surroundings have different orientations, the estimated regional optical flow vectors are different (the have different bias) The difference between the estimates in the inset and the estimates in the surrounding are interpreted as independent motion of the inset. The following figure shows the estimated regional optical flow vectors for a movement of the pattern along the first meridian (up and right). The estimated flow is smaller in length than the actual flow and it is closer in direction to the majority of normal flow vectors in a region (that is the direction perpendicular to the longer edges of the tiles). Figure 2 The next figure shows the bias vectors (that is, the difference between actual and estimated flow) Figure 3 The motion of the pattern is derived from the motion vectors in the surrounding area. The independent motion is derived by projecting the difference vectors (inset minus surround) on the direction perpendicular to the longer edges of the inset tiles.