Recovering Shading from Color Images
Existing shape-from-shading algorithms assume constant reflectance
across the shaded surface. Multi-colored surfaces are excluded
because both shading and reflectance affect the measured image
intensity. Given a standard RGB color image, we describe a method of
eliminating the reflectance effects in order to calculate a shading
field that depends only on the relative positions of the illuminant
and surface. Of course, shading recovery is closely tied to lightness
recovery and our method follows from the work of
Land [10, 9],
Horn  and Blake . In
the luminance image, R+G+B, shading and reflectance are confounded.
Reflectance changes are located and removed from the luminance image
by thresholding the gradient of its logarithm at locations of abrupt
chromaticity change. Thresholding can lead to gradient fields which
are not conservative (do not have zero curl everywhere and are not
integrable) and therefore do not represent realizable shading fields.
By applying a new curl-correction technique at the thresholded
locations, the thresholding is improved and the gradient fields are
forced to be conservative. The resulting Poisson equation is solved
directly by the Fourier transform method. Experiments with real
images are presented.
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