Introduction - Hierarchical Neural Networks for Image Interpretation - page 3

(a) (b)
Fig. 1.1. Role of occluding region in recognition of occluded letters: (a) letters 'B' partially
occluded by a black line; (b) same situation, but the occluding line is white (it merges with
the background; recognition is much more difficult) (image from [164]).
(a) (b)
Fig. 1.2. Light-from-above assumption: (a) stimuli in the middle column are perceived as
concave surfaces whereas stimuli on the sides appear to be convex; (b) rotation by 180 ◦
makes convex stimuli concave and vice versa.
ambiguous. Inferring three-dimensional structure from two-dimensional images, for
example, is inherently ambiguous. Many 3D objects correspond to the same image.
The visual system must rely on various depth cues to infer the third dimension.
Another example is the interpretation of spatial changes in intensity. Among their
potential causes are changes in the reflectance of an object's surface (e.g. texture),
inhomogeneous illumination (e.g. at the edge of a shadow) and the discontinuity of
the reflecting surface at the object borders.
Occlusions are a frequent source of ambiguity as well. Our visual system must
guess what occluded object parts look like. This is illustrated in Figure 1.1. We are
able to recognize the letters 'B', which are partially occluded by a black line. If the
occluding line is white, the interpretation is much more challenging, because the
occlusion is not detected and the 'guessing mode' is not employed.
Since the task of interpreting ambiguous stimuli is not well-posed, prior knowl-
edge must be used for visual inference. The human visual system uses many heuris-
tics to resolve ambiguities. One of the assumptions, the system relies on, is that light
comes from above. Figure 1.2 illustrates this fact. Since the curvature of surfaces can
be inferred from shading only up to the ambiguity of a convex or a concave inter-
pretation, the visual system prefers the interpretation that is consistent with a light
source located above the object. This choice is correct most of the time.