Introduction - Hierarchical Neural Networks for Image Interpretation - page 5

(a) (b) (c)
Fig. 1.5. Visual illusions: (a) Muller-Lyer illusion [163] (the vertical lines appear to have dif-
ferent lengths); (b) horizontal-vertical illusion (the vertical line appears to be longer than the
horizontal one); (c) Ebbinghaus-Titchener illusion (the central circles appear to have different
sizes).
(a) (b)
Fig. 1.6. Munker-White illusion [224] illustrates contextual effects of brightness perception:
(a) both diagonals have the same brightness; (b) same situation without occlusion.
this interpretation is the simplest one. Illusory contours are perceived between the
inducers, although there is no intensity change. The middle of the figure shows that
virtual contours are also induced at line endings perpendicular to the lines because
occlusions are likely causes of line endings. In the right part of the figure it is shown
that one can even bend the square, if the opening angles of the arc segments are
slightly changed.
Three more visual illusions are shown in Figure 1.5. In the M uller-Lyer illu-
sion [163] (Part (a)), two vertical lines appear to have different lengths, although
they are identical. This perception is caused by the different three-dimensional in-
terpretation of the junctions at the line endings. The left line is interpreted as the
convex edge of two meeting walls, whereas the right line appears to be a concave
corner. Part (b) of the figure shows the horizontal-vertical illusion. The vertical line
appears to be longer than the horizontal one, although both have the same length.
In Part (c), the Ebbinghaus-Titchener illusion is shown. The perceived size of the
central circle depends on the size of the black circles surrounding it.
Contextual effects of brightness perception are illustrated by the Munker-White
illusion [224], shown in Figure 1.6. Two gray diagonals are partially occluded by a
black-and-white pattern of horizontal stripes. The perceived brightness of the diag-
onals is very different, although they have the same reflectance. This illustrates that