When the two halves are slid vertically, a set of sign-preserving X-junctions is created along the vertical contour. The junctions are consistent with transparency, and the contour becomes a strong atmospheric boundary between the left and right regions. The two semicircles are
seen within different frameworks. The statistics on the two sides are different. In addition, grouping cues such as good continuation indicate that the left semicircle is connected to
the light region on the right, and the right semicircle is connected to the dark region on the left. Thus there are several cues that conspire to make the two semicircles look quite different.