Abstract
Normally line drawings of objects are presented on a flat pictorial surface and viewed binocularly. Cues for the flatness of the surface are in conflict with the 3-D information provided by lines and junctures. As a result, the experience of objects in space created by line drawings is weaker as compared to stereoscopically presented displays. To test this notion, a line display was created using a wire object that presented three sides of a skeletal cube pointing away from the viewer. We have observed (2008 VSS Demo Night) that this object can create several illusions. When presented monocularly, it is almost always perceived as convex. When the viewers move, motion parallax should indicate the concave nature of the object, but surprisingly line cues dominate. Instead of perceiving the object as static, it is seen as rotating so that the vertex follows the moving viewer. In addition, the white background surface is perceived as coming forward in depth with solid internal surfaces moving with the wire frame.
Twenty-three participants took part in the study to test the consistency of this illusion. They were asked whether the object was convex or concave, static or moving, a solid object or an empty frame. Participants viewed the object monocularly against a white background and binocularly against a striped background. In the binocular condition, viewers reported the object as concave (93.5% of trials), static (84.8%), and empty (95.7%). However, in the monocular condition, viewers reported the object as convex (89.1%), moving (89.1%), and solid (98.9%). In addition, participants in the monocular condition reported that subjective surfaces of the object moved with the wire frame (81.5%). When information for the pictorial surfaces is removed, line drawing cues can generate a powerful illusion of 3-D structure and can overcome the effects of motion parallax.