Perceptual Coherence : Hearing and Seeing

300 Perceptual Coherence
Shape World
To achieve a realistic world model, we need to include three-dimensional
objects that create shadows, indirect reflections, and specular reflections.
The relationships between the surfaces and reflectances are complex, and
there are two simplifications that are commonly employed (L. T. Maloney,
1999).
1. Geometry-reflectance separability. A change in viewing conditions is
assumed merely to scale by a constant the surface reflectance function.
2. Diffuse-specular superposition. Some surfaces do not satisfy the
geometric-reflectance separability assumption when considered in
terms of the sum of the specular and spectral reflectance. However,
the separability assumption may be satisfied if the specular and
spectral (diffuse) reflectances are considered separately. If this is true,
there will be different geometric functions for the two types of reflectances,
and the light reaching the observer is some mix of the two.
Flat World
To simplify the computational problems in recovering the color of objects,
some models further assume what L. T. Maloney (1999) termed the flatworld
environment. The flat-world model reduces a three-dimensional
world to a flat two-dimensional plane. The observer is viewing a world
painted onto a flat sheet with all areas equidistant from the observer. The
scene is illuminated by one source; all the light reaching the viewer comes
from the direct body reflection of the pigments. There are no specular reflections
and no indirect reflections from other objects. Moreover, due to
the flat-field assumption, there are no shadows created by obscuring objects.
Stimuli that create these assumptions are colloquially termed Mondrian
stimuli: They are coplanar patchwork patterns of different hues
resembling paintings by the Dutch artist Piet Mondrian (figure 7.2). Troost
(1998) was skeptical that such Mondrian patterns reach the level of complexity
that leads observers to directly perceive surface color. He suggested
that the hues are perceived as free-floating colors not belonging to any surface.
The observer reasons about what the colors ought to be, rather than
perceiving the surface colors directly.
It should be kept in mind that the above models are essentially pointwise.
They are designed only to depict physical energy processes that can
be explained by characteristics of material at one point. But the visual
world also has surface effects that can only be defined on a finite patch of
surface, for example wood grain or texture. In addition to surface effects,
there are environmental effects that cannot be explained by any description