Adaptation and the color statistics of natural images
Michael A. Webster
We have examined the color distributions that are characteristic of
natural scenes and how visual adaptation to these distributions
influences color appearance. Color distributions were measured by
sampling an array of locations within the scene with a
spectroradiometer or by recording the scene with a digital camera
through an array of narrow-band filters. Individual scenes vary
substantially in mean color, in the range of colors, and in the
directions within color-luminance space along which the colors are
distributed. Chromatic contrasts tend to vary along a relatively
narrow range of bluish to yellowish-green axes, spanned roughly by the
S-cone (tritanopic) axis (typical of scenes with lush vegetation and
little sky) and a unique blue-yellow axis (which was more typical of
arid scenes). Many scenes exhibit strong biases in color direction,
and strong correlations between the component contrasts along the axes
that are thought to characterize post-receptoral color coding in the
retina and geniculate. Adaptation adjusts visual sensitivity according
to the color characteristics of prevailing scenes. We illustrate the
large and selective changes in sensitivity that can result from light
adaptation to the mean color of scenes and from contrast adaptation to
the color and spatial structure of scenes. These adjustments are an
intrinsic part of the visual response to images, and may therefore be
important to consider when relating visual coding to natural scene
statistics.
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