Outline, Tactile pictures, and Shape-from-Shadow Pictures

Juan Bai
University of Toronto

Tactile pictures often use outlines to depict occluding boundaries, convex corners, or concave corners of common objects, such as a cup, a fork, or a table. A tactile picture, also known as a raised-line drawing, is the result of using a ballpoint pen to draw on a plastic sheet covering a rubber board. Where the pen presses down, raised lines appear on the sheet, and human hand can feel them. D’Angiulli, Kennedy, and Heller’s (1998) studies found blind children and blindfolded sighted children could identify objects in tactile pictures. The children explored around each picture with both hands for a couple of minutes, then gave a suggestion of what they think was in the picture. When they had made a wrong identification, they could often correct themselves when trying to identify the same picture again, without any feedback. This may imply that they might judge how well a suggestion fits the picture to decide whether they stay with this suggestion. Fit is the degree of correspondence between the shape in the picture and the suggested object. Studies with blindfolded adults confirmed that higher fit judgments are related to repetition of the identifications. Some researchers say picture is not suitable for touch. I argue that since human hands can feel occluding boundaries or corners of real objects, they may also be able to appreciate the shapes the outlines represent in tactile pictures. On the other hand, outlines do not succeed in showing shapes from shadows. When we use outlines to trace the borders between the light non-shadow parts and the dark shadow parts, and reduce the luminance of the dark parts to the same luminance as the light parts, vision fails to see the objects. The reason may be that shape-from-shadow pictures use luminance differences to represent objects in shadows, but outlines do not carry luminance information. In most cases, touch can not tell luminance differences, either, on paper or in the real world. Therefore, in touch as well as in vision, outline can depict occluding boundaries and convex or concave corners, but not shapes from shadows.