Lines and Shapes-from-shadows

1. Lines and Shapes-from-shadows Juan Bai Purpose: Where do lines fail in vision? What is its implication for the function of lines in touch?

2. Lines and Surfaces We see objects by means of their surfaces, and source of illumination. We touch objects by means of their surfaces, i.e., no brightness information is involved. For example, touch can often accurately discern this line drawing as depicting a cup.

3. surface borders shadow border Lines, Surfaces, and Shadows Lines copying surface edges give us surface edge impressions (e.g., roof, hill, etc. in the image below). However, lines copying shadow borders do not give us the impression of the darkness of shadow (e.g., the shadow on the ground; from Kennedy, 1993).

4. surface borders shadow border Contour, Line, and Outline Contour = reflectance/emission change on a surface. A line has two contours, one on each side. Lines copying surface edges can be called ‘outlines.’ The image below shows that outlines cannot show shadows (Kennedy, 1993).

5. Hering’s ‘Outlined’ Shadow penumbra umbra b a Hering (1874) might contend that for the cup’s cast shadow in (a), shadow perception fails in (b) because the penumbra is lost to the dark line (after Goldstein, 2002).

6. Outlined Sharp-contoured Shadow a b But shadows without penumbras (a, adapted from Mooney, 1957) convey information about a girl’s face, and dark lines in (b) impair percept of the same face. Note that shadowed areas in (b) are darker than illuminated areas (Cavanagh & Leclerc, 1989), yet shape-from-shadow perception is diminished.

7. Shape-from-Shadow: Figure-Ground? Here is another shape-from-shadowed face (adapted from Mooney, 1957), shown with a color image.

8. figure-ground along a contour figure-figure along a contour ground-figure-ground along a line figure-figure along a line figure-ground-figure along a line Lines, Contours, and Kinds of Borders Here are different kinds of figure-ground segregations along borders (Kennedy, Juricevic, & Bai, 2003):

9. Theory of Dark Lines and Shadows b d a c Line version (a), negative (c), and Hering-dark-line version (d) all have negative contours (light-to-dark from shadow to non-shadow), and all three fail to show the face in (b). So it may be the dark line’s negative contour bordering the shadow in (d) that diminishes shape-from-shadow perception – a ‘border-polarity’ hypothesis.

10. Alternative Theory Alternatively, dark lines in (a) and (b) may impair shape-from-shadow perception because a line has two contours rather than one (c) – a ‘number-of-contours’ account. c a b The number-of-contours account is based on an ‘axis’ theory of a line (Kennedy, 1993).

11. Kennedy & Bai (2000): Polarity vs. Number-of-contours c a b Border-polarity predicts the face in (a) should be easy to see, but number-of-contours predicts otherwise. Results = (a) and (c) were equally easy, only (b) difficult, supporting border-polarity.

12. Demonstrations for K & B (2000) An arm-and-hand is relatively easy to see in gray-line version (a) but not in black-line version (b). It is evident in no-line version (c). So only dark line fails. c b a arm-and-hand (adapted from Kennedy, 1988)

13. figure ground wire or crack figure ground Kennedy & Bai (2004): Testing Belongingness c a b A ‘belongingness’ hypothesis may propose that a dark-to-light shadow border has to belong to both the shadowed and the illuminated areas to allow perception of a continuous face bearing the shadow. The purpose of K & B (2004) is to test this hypothesis.

14. final dots K & B (2004)’s Dotted Shadow for Belongingness Test Result = the dot-grating figure showed the face, possibly because on average the border polarity is positive, despite negative polarity along the final dots.

15. Rationale for K & B (2004) Stereo Test When two areas share a common border, it is said that the common border belongs to the closer area – the figure; it does not belong to the area that is farther away – the ground (e.g., Nakayama et al., 1989). If so, when stereo depth indicates that the bordering elements belong only to a closer, shadow area or a closer, non-shadow area, the inappropriate belongingness should impair shape-from-shadow perception.

16. Stereo Stimuli Only right eye sees smaller dots unpaired dots most shadow

17. Stereo Stimuli (Continued) Only left eye sees smaller dots most shadow unpaired dots

18. same stereo depth binocular percept Stereo Stimuli (Control) Both eyes see smaller dots at the shadow border

19. Stereo Stimuli (Negative Control) Results = in binocular percept, positive images showed the face in shadow, even when stereo depth signaled different belongingness of the border. Negatives did not show the face. The results rejected predictions from belongingness, but supported border-polarity.

20. Conclusions A Hering line added along a shadow border may have impaired shape-from-shadow perception because of negative polarity of its contour bordering the shadow. There are 8 kinds of borders in a visual display that can cause perception of surfaces. These include luminance/spectral, monocular/binocular, and static/moving borders (Kennedy et al., 2003). Among these 8 combinations, only luminance matters for shape-from-shadow: From shadow to non-shadow the border has to be from darker to lighter to allow perception of shapes from shadows.