Abstract
A 3D object such as plural points distributed in 3D space appears flat when rear projected onto a translucent screen. Its 3D shape becomes apparent when it is rotated about an axis parallel to the screen (KDE). The relative motion of the points gives depth information but perspective information is not present. So, the relative depth of points can be perceived but not their depth order. It is impossible to tell which is the front and which is the back of the objects. The 3D object is ambiguous with respect to its reflection in the projection plane. As a result the object periodically appears to reverse in depth as well as direction of rotation. This study investigated if observer's manual control for the stimulus change removes ambiguity of KDE. In the experiment, the stimulus change was coincided with the rotation of a crank handle rotated rightward or leftward by the observer. The result showed that manual control considerably removed the ambiguity: the perceived direction of rotation from KDE coincided the manual control at a high rate. Prolonged viewing, however, made reverse the apparent rotation. We measured duration of initial rotation. The manual control extended the mean duration.